Patent Publication Number: US-9422915-B2

Title: Customizing a wind turbine for site-specific conditions

Description:
FIELD OF THE INVENTION 
     The invention relates generally to wind turbines, and more particularly to customizing the design and operation of a wind turbine for site-specific conditions, such as wind loading conditions or noise limits. 
     BACKGROUND OF THE INVENTION 
     A wind turbine blade design is optimized for a given standard design environment including mean wind speed, turbulence, and other factors. Once the blade mold is created, the outer geometry and aerodynamic response of the blade is fixed. Blade design is a balance between power production and turbine loads, and must meet International Electrotechnical Commission requirements for a specific wind class. Molds are expensive and blade designs are standardized and are used for many wind turbines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  shows a function relating coefficient of mechanical power to tip speed ratio (TSR), and indicating an optimum TSR. 
         FIG. 2  compares the function of  FIG. 1  to a second function for a blade designed for a lower TSR. 
         FIG. 3  shows functions relating RPM to wind speed for a standard design operation and for a noise-curtailed operation that reduces maximum RPM. 
         FIG. 4  shows functions relating tip speed ratio to wind speed for a standard design operation and for a noise-curtailed operation consistent with  FIG. 3 . 
         FIG. 5  compares functions relating RPM to wind speed under two environmental conditions for a blade with a first design TSR and a blade with a lower design TSR. 
         FIG. 6  compares functions relating coefficient of power to wind speed under two conditions for a blade with a first design TSR and a blade with a lower design TSR. 
         FIG. 7  is a sectional view of a wind turbine blade with a movable trailing edge flap and a movable vortex generator. 
         FIG. 8  shows probability densities for three wind speed envelopes representing site-specific conditions at respective different sites or at the same site at different times. 
         FIG. 9  shows curves of RPM, mechanical power, coefficient of power, flapwise root bending moment, and coefficient of flapwise bending moment, as functions of wind speed for a blade designed for a first wind speed environment but operating in a lower wind speed environment. 
         FIG. 10  shows curves as in  FIG. 9  for a blade customized with increased lift coefficient for the lower wind speed environment. 
         FIG. 11  compares selected curves from  FIGS. 9 and 10  to show an increase in mechanical power over a significant range of wind speeds for the modified blade of  FIG. 10  compared with the standard blade of  FIG. 9 . 
         FIG. 12  is a sectional view of a wind turbine blade designed to incorporate a flap add-on. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a function  20  relating coefficient of power (Cp) to tip speed ratio (TSR). TSR is the ratio of the blade tip speed to the wind speed. TSR=rotor radius (m)*rotation rate (radians/s)/wind speed (m/s). Coefficient of power is the ratio of power extracted by the rotor for a given available wind power (Cp=P t /P w ), and is a measure of aerodynamic efficiency of the blades and rotor. A blade operates at maximum aerodynamic efficiency when its TSR is maintained at the maximum  21  of the Cp/TSR curve  20 . A blade is engineered for given design TSR through the design of sectional lift coefficients C L  along the span of the blade. Higher lift coefficients result in a lower optimum TSR. Herein the lift coefficient is calculated with respect to the main blade element, not including added flaps. 
     
       
         
           
             
               Section 
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               coefficient 
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               of 
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               lift 
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                 c 
                 l 
               
             
             = 
             
               l 
               
                 
                   1 
                   2 
                 
                 ⁢ 
                 ρ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   υ 
                   2 
                 
                 ⁢ 
                 c 
               
             
           
         
       
     
     where l=lift, ρ=air density, v=wind speed, and c=chord length 
     
       
         
           
             
               Section 
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               lift 
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               l 
             
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               c 
             
           
         
       
       
         
           
             
               Bladewise 
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               coefficient 
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               of 
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               lift 
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                 C 
                 L 
               
             
             = 
             
               
                 
                   L 
                   
                     
                       1 
                       2 
                     
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                       ρυ 
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                     S 
                   
                 
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                 or 
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                   C 
                   L 
                 
               
               = 
               
                 L 
                 qS 
               
             
           
         
       
     
     where L=lift, ρ=air density, v=wind speed, S=blade plan area as viewed perpendicular to the chord lines, and q=dynamic pressure. 
     
       
         
           
             
               Bladewise 
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               lift 
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               L 
             
             = 
             
               
                 1 
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                 C 
                 L 
               
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               ρ 
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                 υ 
                 2 
               
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               S 
             
           
         
       
     
               Available   ⁢           ⁢   wind   ⁢           ⁢   power   ⁢           ⁢   P     =       1   2     ⁢   A   ⁢           ⁢   ρ   ⁢           ⁢     υ   3             
where A=rotor disk area
 
     A higher TSR design point provides higher power output for a given rotor torque, since mechanical power=torque times rotation rate. However rotor speeds are limited by mechanical loads on the rotor, noise, and generator speed limits. 
       FIG. 2  shows Cp/TSR curves  20 ,  22  for two blades. One blade operates along curve  20  and has a first, relatively higher optimum TSR  21 . Another blade operates along a steeper curve  22  due to higher lift coefficients, and it has a lower TSR  23  at its maximum Cp. The maximum of curve  22  may as high or higher than the maximum of curve  20  at respectively lower/higher tip speed ratios. 
       FIG. 3  shows RPM vs wind speed curves for a given blade in two alternate operational modes of a wind turbine. Under standard operating conditions the rotor reaches maximum RPM at line  26 . When noise limits are imposed, the rotor is limited to a lower maximum RPM  28 . Both operations maintain optimum TSRs along the slope  30  until respective inflection points  27 ,  29  are reached. At wind speeds above the inflection points, TSR is reduced as next shown, lowering the coefficient of power. 
       FIG. 4  shows the effect of the two RPM limits of  FIG. 3  on the TSR of a blade with a first, higher design TSR  21 . In standard operation the design TSR  21  is maintained up to inflection point  27 , after which TSR drops along curve  32 , since wind increases while RPM is constant ( FIG. 3 ). In noise curtailed operation  34 , the inflection point  29  occurs at a lower wind speed because the RPM limit is lower. Turbine performance is reduced in proportion to how far it operates from design TSR. Thus, efficiency drops sooner under noise curtailed operation  29 ,  34  than under standard operation  27 ,  32 . This reduces annual energy production (AEP) at sites with noise limits. 
     The blade design for a given wind turbine model is a compromise for a range of actual site conditions. Blade airfoils are not modified in geometry for specific site conditions. However, environmental and operating conditions vary substantially from site to site. Noise limits at some sites impose permanent or temporary limits on rotor speed, and sites vary in mean wind speed and other wind power parameters. Some sites have more turbulence than others. The present inventor has recognized that for a site with frequent or permanent RPM limits, a blade with higher lift coefficient and lower TSR is more efficient, and increases annual energy production. The inventor further recognized that a blade could be customized for site conditions using add-ons such as flaps and vortex generators. A standard blade may be designed for a relatively high TSR  21  as in  FIG. 2  for maximum power at sites with infrequent or no noise limits and high wind power. For sites with an available blade load margin resulting from noise limits or lower mean wind speed, the lift coefficient of the blade can be increased by add-ons, and the wind turbine controller can use site-specific parameters to maintain an optimum TSR considering the add-ons. 
       FIG. 5  shows RPM vs wind speed for a first blade with a higher design TSR and a second blade with a lower design TSR. The first blade RPM follows slope  30  where constant TSR is maintained and slope  26  where constant RPM is maintained under standard conditions. The second blade RPM follows slope  36  where constant TSR is maintained and slope  28  where constant RPM is maintained. Both blades maintain a reduced RPM  28  under noise limits. However, the inflection point  29 A for the lower TSR curve  36  occurs at a higher wind speed than the inflection point  29  for the higher TSR curve  30  under noise-limited operation. Thus, the lower TSR blade operates at maximum efficiency over a wider range of wind speeds in noise-limited conditions. 
     For sites where noise limits are occasional or periodic, such as nightly noise limits, a lower TSR blade may operate  37  above the noise-limited RPM  28  when noise limits are relaxed. An increase in pitch motion to decrease blade loading reduces the effective power conversion of a lower TSR blade compared to a higher TSR blade at wind speeds above some point  29 B under standard operating conditions. For this reason, and as later shown in  FIG. 11 , a low TSR blade is not ideal for all sites under all conditions. Variable lift embodiments of the invention are described later herein to address this issue. 
       FIG. 6  shows power coefficient curves for three situations: 
       40 —A blade with a first, higher design TSR under standard conditions; 
       42 —The blade with higher design TSR under noise-limited RPM; 
       44 —A blade with lower design TSR under noise limited RPM. 
     When noise limits are in effect, the blade with lower design TSR is more efficient than the blade with higher design TSR at all wind speeds above the noise-limited RPM inflection point  29  of  FIG. 5  up to maximum power  45  with the lower design TSR. 
       FIG. 7  is a sectional view of a wind turbine blade airfoil  46  with a pressure side PS, suction side SS, leading edge LE, trailing edge TE 1 , and chord line  48 . A flap  49  may be provided to modify the camber and/or lengthen the effective chord length of the blade, extending it to a new trailing edge TE 2 . The chord length  47  of the main blade element  46  is used to calculate coefficients of lift herein both before and after modification. A fixed-position flap may be provided to modify the blade for conditions of a given site, or a movable flap may be provided to adjust for changing conditions. Mechanisms for fixed and movable flaps are known, and are not detailed here. The flap may be configured to increase  49 A or decrease  49 B the lift coefficient of the blade relative to the unmodified blade, responsive to site specific conditions to improve or maximize annual energy production within available blade load margins. If the flap is movable, it may be managed actively by a controller  54  informed by sensors  56 , such as blade strain sensors, wind sensors on the blade or tower, and/or input from an on-site weather station. Alternately, depending on cost/benefit, a fixed-position flap may be provided. 
     Vortex generators (VG)  50  may mounted on a track  52  that provides movable positioning  51  of the VGs on the suction side SS. The track may be surface-mounted or it may be installed flush during original manufacture. The VGs may be moved manually, for example using bolts, pins, or spring latches, or they may be actively controlled by a controller  54 , for example by electric motors or hydraulic pistons. They may be moved forward to increase the lift coefficient and backward to reduce it responsive to a site-specific condition to maximize annual energy production within available blade load margins. 
       FIG. 8  shows examples of probability distributions of wind speeds S 1 , S 2 , S 3  at three different sites, or at the same site in different seasons or times. One type of site specific condition is a mean wind speed for a given site using a Weibull distribution with a shape factor of 2. Wind loading conditions may include such wind speed distributions and may further include parameters for fatigue and extreme loads due to turbulence and peak gusts. These factors result in different fatigue loads for different sites. An Annual Energy Production (AEP) for a wind turbine can be determined relative to such a defined wind loading condition. A wind turbine is certified for a given wind distribution by the International Electrotechnical Commission (IEC). A turbine that is certified for a mean wind speed of 10 m/s can only be installed at sites with mean wind speeds up to 10 m/s. When installed at a site with lower wind speeds, there is a blade load margin on that turbine. An embodiment of the invention uses aerodynamic add-ons to increase the load on the turbine within this blade load margin, for example to fill the blade load margin, and increase annual energy production of the turbine. This allows one blade mold to provide blades optimized for each site according to wind loading conditions at each site. 
     It is non-obvious that increasing the blade coefficient of lift for a lower mean wind environment will result in increased annual energy production, because increasing the coefficient of lift does not inherently increase the coefficient of power in lower winds. This is seen in  FIG. 6 , in which the coefficient of power is the same for the higher and lower design TSR at wind speeds up to inflection point  29 . However, the lower design TSR reaches maximum RPM at a higher wind speed  29 A, so it spends a higher proportion of time on the optimum TSR curve  36 . 
       FIG. 9  shows curves of RPM, mechanical power, coefficient of power, root bending moment, and coefficient of bending moment as functions of wind speed for a blade with a higher TSR designed for a standard wind speed environment, but operating in a lower wind speed environment. Herein, “bending moment” means “flapwise” bending moment, which is bending in a direction normal to the chord line of a blade due to lift and turbulence. There is a large constant RPM region  56  between the wind speed at which maximum RPM is reached and the wind speed of maximum mechanical power. This blade has a relatively limited maximum load A in this environment. 
       FIG. 10  shows curves of RPM, mechanical power, coefficient of power, root bending moment, and coefficient of bending moment as functions of wind speed for a blade with increased lift coefficient operating in the lower wind speed environment. There is a relatively small constant speed region  57  between reaching maximum RPM and maximum power. If the blade accommodates a higher maximum load B, then there is a load margin that allows increasing lift with add-ons. In this situation, the coefficient of power remains optimum up to higher wind speed (max RPM). This increases annual energy production as shown in Table 1 below, which uses engineering simulations for a mean wind speed of 7.5 m/s. This increases annual energy production 0.45%, or about 50 MWh per turbine yearly. Such an improvement is considered very significant in this highly competitive industry. In the table below, all values are at the given maximum RPM. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 3.0-101  
                 3.0-101 
                 Change 
               
               
                 Parameter 
                 Standard 
                 Lower TSR 
                 (Percent) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Max RPM 
                 16 
                 16 
                      0% 
               
               
                 Max Power 
                 3000 
                 3000 
                      0% 
               
               
                 (kW) 
                   
                   
                   
               
               
                 Max Torque 
                 1927 
                 1927 
                      0% 
               
               
                 (kNm) 
                   
                   
                   
               
               
                 Design TSR 
                 9.86 
                 9.39 
                    −5% 
               
               
                 Noise 
                 108 
                 108 
                      0% 
               
               
                 Production (dB) 
                   
                   
                   
               
               
                 Tip Speed 
                 84.6 
                 84.6 
                      0% 
               
               
                 (m/s) 
                   
                   
                   
               
               
                 Blade Load 
                 6.13 
                 6.43 
                      5% 
               
               
                 (MNm) 
                   
                   
                   
               
               
                 AEP @ 7.5 m/s 
                 11000 
                 11050 
                    0.45% 
               
               
                 (MWh) 
                   
                   
                   
               
               
                 Wind Speed at  
                 8.28 
                 9.01  
                    8.80% 
               
               
                 max RPM 
                   
                   
                   
               
               
                 Wind Speed at 
                 11.31 
                 11.16  
                  −1.30% 
               
               
                 max Power 
                   
                   
                   
               
               
                 Size of 
                 2.73 
                 2.15 
                 −21.20% 
               
               
                 Constant 
                   
                   
                   
               
               
                 Speed Region 
                   
                   
                   
               
               
                 (m/s) 
               
               
                   
               
            
           
         
       
     
       FIG. 11  compares selected curves from  FIGS. 9 and 10  to illustrate the increase in mechanical power throughout a significant range of wind speeds with the modified blade of  FIG. 10  in comparison to the standard blade of  FIG. 9 . This results from reducing the constant speed region ( 57  of  FIG. 10  versus  56  of  FIG. 9 ). This in turn causes the higher lift blade to maintain optimum TSR in higher wind speeds and maintains it closer to the maximum power point, thus increasing the coefficient of power during a substantial proportion of operation time, increasing annual energy production. 
     Through the use of aerodynamic add-ons, rotor loads can be increased to increase power production by customizing blades from the same base mold design for different site conditions. This creates customized aerodynamic configurations for a line of blades to fit load envelopes and noise constraints at different sites and maximize energy production. Add-ons can be configured to increase or decrease the lift coefficient relative to the unmodified blade. For example, trailing edge flaps can be angled toward the suction side SS to reduce lift as shown by  49 B in  FIG. 7 . 
     A site may be evaluated to determine whether annual energy production will increase with a modified coefficient of lift due a site-specific environmental condition such as different mean wind speed or an RPM limit for noise reduction. The following steps may be used, among others: 
     a) Establish a design environmental condition for a wind turbine base blade; 
     b) Engineer the base blade to a coefficient of lift and a corresponding optimum blade tip speed ratio (TSR) that maximizes a first annual energy production of the wind turbine when operating under the design environmental condition; 
     c) Determine a site-specific condition that changes the wind loading conditions compared to the design environmental condition; and 
     d) Provide an add-on device for the base blade that maximizes a second annual energy production of the wind turbine using the modified blade under the site-specific condition by modifying the coefficient of lift and the TSR. 
       FIG. 12  is a sectional view of a wind turbine blade designed to incorporate a flap add-on. It may have a factory trailing edge TE 1  that is shaped to merge with a flap  60 , and is equipped with fastening hardware and a control line  62 . A suitable flap  60  may be added on-site, and may be selected from movable or non-movable add-on flaps based on cost/benefit for each site. A movable flap embodiment may rotate  60 A toward the pressure site to increase lift and/or may rotate  60 B toward the suction side SS to decrease lift. It may be aligned with the chord line  48  or mean camber line for a site with standard design environmental conditions, providing an aligned trailing edge TE 2  in that condition. Flap(s)  49 ,  60  and/or vortex generators  50  may cover part or most of the span of the blade either individually or in combination. 
     Using the method and embodiments described herein, a blade mold may be made that produces blades with optimum aerodynamics for a standard design environmental condition. The aerodynamics of the blade may be economically and effectively customized for each site with add-on devices to increase annual energy production at each site. Furthermore, the selection of a wind turbine model for a given site can take into account the described modifications in order to meet the site AEP goal. Moreover, when a lower rated wind turbine is mandated for a given site due to a limit on the maximum amount of power that the grid can handle, such lower rated wind turbine may be modified in accordance with the present invention to optimize its power production during periods when the wind speed is below that which is necessary to produce peak power. 
     While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.