Wind Turbine PrebendWhat is prebend?Prebend refers to wind turbine blades that are not straight, but manufactured with an itentional bend. More specifically, the term normally refers to bending the outboard portion of the blade upwind, as shown in the second row of the image below. Since aerodynamic forces (thrust) bend the blade downwind in operation, prebent blades end up straighter than straight blades in operation. 
AdvantagesPerformancePrebend can increase the energy captured by a wind turbine. A simple way to see this is by examining the area of the rotor normal to the wind. When a straight blade is operating at a high power setting, thrust bends the blade downwind, effectively reducing the area swept by the blade. A prebent blade, on the other hand, increases swept area as the blade straightens out with thrust. That’s not all. Most wind turbine rotors are tilted upward and/or preconed to increase distance from the tower. Both options reduce the area of the rotor normal to the wind. With prebend, less tilt/precone is required to achieve the same tower clearance. Performance may alternatively be increased by reducing tower pass effects. The tower induces a change in windflow that reduces energy capture of a blade as it passes. Increasing distance from the tower reduces this effect. More complicated benefits are also possible. Tip vortices reduce energy capture on the outer portion of the blades. Careful shaping, including prebend, can modify tip vortices to reduce tip loss effects. This benefit may not yet be proven. It’s likely more complicated than the winglet designs on jets because wind turbines operate in more turbulent flows. OverhangAs mentioned previously, prebending blades away from the tower increases tower clearance. This can be used to increase performance, as described in the prior subsection. Alternatively, it can be used to reduce overhang—the distance the rotor is mounted out in front of the tower (see the image below). Less overhang generally reduces loads and cost for the tower, foundation and nacelle. 
LoadsThe tower shadow effect, along with wind shear and the combination of coning and tilt, reduce thrust and torque on a blade passing the tower. Such time-varying loads fatigue the rotor and can excite vibrational modes. This impacts the structural design of blades, increasing cost. Such costs are reduced by bending blades away from the tower. Bent blades also give engineers more flexibility to smooth out load distribution and perhaps reduce material costs further. NoiseRotor noise is generated by air pressure fluctuations near the blades. Anything that modifies blade loads, including tower clearance, impacts noise. Prebend may reduce noise by reducing the tower pass effects and/or modifying the 3D flow near blade tips. Both can create significant noise that limits turbine installation in populated areas. DisadvantagesAnalysisWind turbine manufacturers use various software models to study performance, loads and noise. Many such models, including BEM, are unable to predict the impacts of prebend. In order to do proper design and analysis, manufacturers must invest in new software tools. ManufacturingMore than about 4 M of prebend may require new manufacturing facilities and tools. For example, there may not be enough space to open and close the required blade molds in the production facility. TransportationOf course, significant prebend can increase the total length/width/height “box dimensions” of a blade. This can complicate shipping and even prevent road delivery for onshore wind farms. Low wind speed performancePrebend is typically designed so that the blade is roughly straight around rated power. At low wind speeds, the forward curvature of the blades reduces the swept area of the rotor, which reduces energy capture. This also increases the cut-in wind speed—the lowest wind speed at which the turbine can operate. This means the turbine would spend more time “offline” at low wind speeds. As long as the wind speed over time roughly matches the turbine design targets, the reduced energy at low wind speed would be more than offset by the increased energy at high wind speeds. In other words, prebend would only hurt overall performance if used in a location with unexpectedly low wind over the long haul. Wind turbine models with prebendThe following are popular wind turbines with some amount of prebend.
|