Abstract:
A method for controlling a wind turbine having twist bend coupled rotor blades on a rotor mechanically coupled to a generator includes determining a speed of a rotor blade tip of the wind turbine, measuring a current twist distribution and current blade loading, and adjusting a torque of a generator to change the speed of the rotor blade tip to thereby increase an energy capture power coefficient of the wind turbine.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &amp; DEVELOPMENT  
       [0001]     The U.S. Government has certain rights in this invention as provided for by the terms of Contract No. DE-AC36-83CH10093, Subcontract No. ZAM-7-13320-26 awarded by the Department of Energy/Midwest Research Institute, National Renewable Energy Laboratory Division. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates generally to wind turbines, and more particularly to methods and apparatus for increasing energy capture and for controlling twist angles of blades resulting from passive twist bend coupling.  
         [0003]     Recently, wind turbines have received increased attention as environmentally safe and relatively inexpensive alternative energy sources. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.  
         [0004]     Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted within a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid.  
         [0005]     Studies have shown benefits of passive pitch control of rotor blades (i.e., twist-bend coupling, or TBC) to attenuate extreme blade loads. Within a twist-bend coupled section of a rotor blade that is made of laminate material, the laminates of the blade undergo shear fatigue resulting from continuous passive pitching. This fatigue presents a risk to TBC technologies. However, very little, if any, research has been devoted to the study of how laminates used in the manufacturing of rotor blades respond to shear axis fatigue.  
         [0006]     At least one known wind turbine utilizes torsionally stiff blades without twist-bend coupled blades. Only a single tip speed ratio (i.e., rotation rate divided by wind speed) is tracked and used to maintain a maximum power coefficient. More specifically, rotor speed rises with wind speed in such a manner as to maintain an optimized or nearly optimized tip speed ratio over a certain period of time. Tip speed ratio is known in the industry and is generally a limiting factor in blade rotational design. This limit results from aerodynamic noise.  
         [0007]     At least one known wind turbine uses twist-bend-coupled blades that passively pitch to feather a relatively small amount when loaded by aerodynamic loads. Particularly in response to strong wind gusts, this passive pitch tends to balance out asymmetric loads across the rotor disk and reduces system fatigue damage. However, due to passive pitching of the TBC, energy capture below rated wind speed is reduced slightly compared to a non-coupled blade with an identical aerodynamic envelope. More specifically, optimum pitch setting for maximum energy capture varies with wind speed. To partially compensate for the loss of energy capture, at least one known configuration provides a pre-twist (i.e., a twist bias built into the rotor blades), but this bias is most effective at only one wind speed. Energy loss still occurs at other wind speeds. However, pre-twist in the TBC section mitigates power loss at below-power rated wind speeds.  
         [0008]     For large-scale wind turbines, rotor blades require pitch actuation at the blade root to actively adjust the pitch angle or angle-of-attack of the rotor blade. However, during operation, fine pitching of the outboard section of the rotor blade can be achieved through passive pitching by means of a Twist-Bend Coupling (TBC). Passive pitching of a several degrees is achieved in the TBC by means of blade construction and specifically due to laminates orientation and lay-up. For example, in the TBC section the fiber matt material is orientated to allow the blade to passively pitch under specific loading conditions. This pitching reduces aerodynamic lift by passively pitching slightly towards a feathered position and therefore reduces blade loading. TBC is a feature that has been shown to reduce the demand on the active pitch mechanism, which is typically located at the root of the rotor blade. Thus passive TBC can reduce pitching power requirements and associated fatigue and damage to the active pitch axis system, located near the blade root.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0009]     In some aspects, the present invention therefore provides a method for controlling a wind turbine having twist bend coupled rotor blades on a rotor mechanically coupled to a generator. The method includes determining a speed of a rotor blade tip of the wind turbine, measuring a current twist distribution and current blade loading, and adjusting a torque of a generator to change the speed of the rotor blade tip to thereby increase an energy capture power coefficient of the wind turbine.  
         [0010]     Also, some aspects of the present invention provide a twist-bend coupled rotor blade for a wind turbine. The rotor blade includes a leading edge and a trailing edge, a laminate lay-up configured for twist-bend coupling, a passive twist-bend coupling section, one or more shear webs inside the rotor blade, and a twist-bend control system inside the passive twist-bend coupling section.  
         [0011]     In still other aspects, the present invention provides a wind turbine having twist bend coupled rotor blades on a rotor mechanically coupled to a generator. The wind turbine also has a controller that is configured to determine a speed of a tip of a rotor blade, measure a current twist distribution and current blade loading, and adjust a torque of the generator to change the speed of the rotor blade tip to thereby increase an energy capture power coefficient of the wind turbine.  
         [0012]     In yet other aspects, the present invention provides a wind turbine having a generator, a rotor mechanically coupled to the generator, and twist-bend coupled rotor blades on the rotor. The blades include a leading edge and a trailing edge, a laminate lay-up configured for twist-bend coupling, a passive twist-bend coupling section, one or more shear webs inside the rotor blade, and a twist-bend control system inside the passive twist-bend coupling section.  
         [0013]     It will be appreciated that configurations of the present invention not only provide wind turbines with greater energy capture, but also mitigate risks associated with shear fatigue of laminates used in twist bend coupled rotor blades. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a drawing of an exemplary configuration of a wind turbine.  
         [0015]      FIG. 2  is a cut-away perspective view of a nacelle of the exemplary wind turbine configuration shown in  FIG. 1 .  
         [0016]      FIG. 3  is a block diagram of an exemplary configuration of a control system for the wind turbine configuration shown in  FIG. 1 .  
         [0017]      FIG. 4  is a perspective view of a rotor blade having a passive twist-bend coupled section and two shear webs.  
         [0018]      FIG. 5  is a cross-sectional view of the rotor blade at line A-A of  FIG. 4 .  
         [0019]      FIG. 6  is a sectional perspective view of the twist-bend coupled section of the rotor blade shown in  FIG. 4 , with certain interior portions indicated by dashed lines. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     In some configurations and referring to  FIG. 1 , a wind turbine  100  comprises a nacelle  102  housing a generator (not shown in  FIG. 1 ). Nacelle  102  is mounted atop a tall tower  104 , only a portion of which is shown in  FIG. 1 . Wind turbine  100  also comprises a rotor  106  that includes one or more rotor blades  108  attached to a rotating hub  110 . Although wind turbine  100  illustrated in  FIG. 1  includes three rotor blades  108 , there are no specific limits on the number of rotor blades  108  required by the present invention.  
         [0021]     In some configurations and referring to  FIG. 2 , various components are housed in nacelle  102  atop tower  104  of wind turbine  100 . The height of tower  104  is selected based upon factors and conditions known in the art. In some configurations, one or more microcontrollers within control panel  112  comprise a control system are used for overall system monitoring and control including pitch and speed regulation, high-speed shaft and yaw brake application, yaw and pump motor application and fault monitoring. Alternative distributed or centralized control architectures are used in some configurations.  
         [0022]     In some configurations, the control system provides control signals to a variable blade pitch drive  114  to control the pitch of blades  108  (not shown in  FIG. 2 ) that drive hub  110  as a result of wind. In some configurations, hub  110  receives three blades  108 , but other configurations can utilize any number of blades. In some configurations, the pitches of blades  108  are individually controlled by blade pitch drive  114 . Hub  110  and blades  108  together comprise wind turbine rotor  106 .  
         [0023]     The drive train of the wind turbine includes a main rotor shaft  116  (also referred to as a “low speed shaft”) connected to hub  110  and supported by a main bearing  130  and, at an opposite end of shaft  116 , to a gear box  118 . Gear box  118 , in some configurations, utilizes a dual path geometry to drive an enclosed high speed shaft. The high speed shaft (not shown in  FIG. 2 ) is used to drive generator  120 , which is mounted on main frame  132 . In some configurations, rotor torque is transmitted via coupling  122 . Generator  120  may be of any suitable type, for example, a wound rotor induction generator.  
         [0024]     Yaw drive  124  and yaw deck  126  provide a yaw orientation system for wind turbine  100 . Wind vane  128  provides information for the yaw orientation system, including measured instantaneous wind direction and wind speed at the wind turbine. In some configurations, the yaw system is mounted on a flange provided atop tower  104 .  
         [0025]     In some configurations and referring to  FIG. 3 , a control system  300  for wind turbine  100  includes a bus  302  or other communications device to communicate information. Processor(s)  304  are coupled to bus  302  to process information, including information from sensors configured to measure displacements or moments. Control system  300  further includes random access memory (RAM)  306  and/or other storage device(s)  308 . RAM  306  and storage device(s)  308  are coupled to bus  302  to store and transfer information and instructions to be executed by processor(s)  304 . RAM  306  (and also storage device(s)  308 , if required) can also be used to store temporary variables or other intermediate information during execution of instructions by processor(s)  304 . Control system  300  can also include read only memory (ROM) and or other static storage device  310 , which is coupled to bus  302  to store and provide static (i.e., non-changing) information and instructions to processor(s)  304 . Input/output device(s)  312  can include any device known in the art to provide input data to control system  300  and to provide yaw control and pitch control outputs. Instructions are provided to memory from a storage device, such as magnetic disk, a read-only memory (ROM) integrated circuit, CD-ROM, DVD, via a remote connection that is either wired or wireless providing access to one or more electronically-accessible media, etc. In some embodiments, hard-wired circuitry can be used in place of or in combination with software instructions. Thus, execution of sequences of instructions is not limited to any specific combination of hardware circuitry and software instructions. Sensor interface  314  is an interface that allows control system  300  to communicate with one or more sensors. Sensor interface  314  can be or can comprise, for example, one or more analog-to-digital converters that convert analog signals into digital signals that can be used by processor(s)  304 .  
         [0026]     In some configurations of the present invention, a bend-twist-coupled blade is provided that changes its aerodynamic twist as it is loaded. For example and referring to  FIG. 1  and  FIG. 4 , a rotor blade  108  is provided having a passive TBC section  402  and one or more shear webs (for example, two shear webs  404  and  406 ).  FIG. 5  shows a section A-A of blade  108  in greater detail. An optimum pitch setting for maximum energy capture varies in wind turbines  100  having blades  108  with twist bend coupling. However, to avoid loss of energy capture, the speed of a blade tip  136  is tracked and varied for maximum or at least favorable power coefficient by adjusting rotor  106  speed (i.e., rotation rate). In some configurations, this adjustment is made by using optical sensors  138  or any other suitable sensors to measure tip  136  speed as rotor  106  rotates. In some configurations, hub rotational speed is known from an encoder on a high speed shaft connected to the aft end of the generator, and blade length, which is known, is used to determine tip speed. This tip speed data is received by control system  300 , which utilizes a table or equation that relates generator  120  torque to an optimum or at least favorable tip speed ratio for the current twist distribution occurring at the current blade loading, both of which are also sensed by suitable sensors (not shown). The equation or table can be empirically determined or calculated using known physical laws. Control system  300  controls generator  120  torque in accordance to the equation or table to produce a rotor  106  speed that provides the optimum or at least a favorable power coefficient. This technique can be used to augment a below-rated pitch schedule or used alone to restore energy capture to levels closer to the entitlement associated with an uncoupled blade.  
         [0027]     In some configurations, twist angles of blades resulting from passive twist bend coupling design are reduced, limited and/or controlled. The reduction, limitation and/or control can be applied in conjunction with a rotor blade  108  with laminates lay-up  402  designed for TBC. For example, referring to a rotor blade  108 , some configurations of the present invention include a passive TBC section  402  and two shear webs  404  and  406  shown in  FIG. 4 . Also identified for reference in  FIG. 4  are leading edge  410  and trailing edge  408  of rotor blade  108 . In some configurations and referring to  FIG. 5 , which shows section A-A of  FIG. 4  in greater detail, dampeners  502  are provided to attenuate twist angle bending motion. One example of a suitable dampener  502  is a shock absorber. Some configurations of the present invention have limiters  504  that are configured to limit twist angle bending. One example of a suitable limiter  504  is a cylinder with limited travel. Still other configurations provide actuators  506  that actively control the passive twist angle bending. Some examples of actuators  506  include electrically or hydraulically driven jackscrews and pneumatic or hydraulic cylinders. Yet other configurations provide various combinations of dampeners  502 , limiters  504 , and/or actuators  506 , as are shown in  FIG. 5 . For example, by combining a shock absorber  502  with a cylinder  504  with limited travel, a dampening limiter is realized in some configurations of the present invention. Limiters  504 , actuators  506 , and particularly dampeners  502  may also benefit blade performance in some configurations of the present invention by reducing tip flutter resulting from Vonkommon vortex shedding. In various configurations, limiters  504 , actuators  506  and/or dampeners  502  are attached to blade spars. As used herein, the term “Twist-Bend Control System” (TCS) is used to refer to a system that includes one or any combination of dampening, limiting, and actuating features.  
         [0028]     A sectional perspective view of a TBC section  402  of a rotor blade  108  is shown in  FIG. 6 . As a result of the passive laminate lay-up configuration of TBC section  402 , a moment force F producing a twist towards a feathering position rotor blade  108  (of which TBC section  402  is a part) is created near an outboard section  602  of coupling  402 . Also, in some configurations, the TCS comprises one or more TCS arms  604 . TCS arms  604  include at least one of a linearly actuated dampener, a limiter, or an actuator, not separately shown in  FIG. 6 , but examples of which appear in  FIG. 5 . Each TCS arm  604  is oriented parallel to an axis opposing passive twist moment F resulting from the TBC.  FIG. 6 , for example, shows four TCS arms  604  orientated diagonally from a base of one shear web  508  to a top on another shear web  508  and also spanning outward to approximately align with a twisting pitch moment force F of the TBC. In addition, TCS arms  604  are secured to rotor blade  108  near a top or bottom of shear webs  508 , although different mounting points from those shown here can be used in other configurations.  
         [0029]     It will be observed that configurations of the present invention not only provide wind turbines with greater energy capture, but also mitigate risks associated with shear fatigue of laminates used in twist bend coupled rotor blades.  
         [0030]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.