Load Alleviation of Wind Turbine Blades using Trailing Edge Flaps

Abstract

Horizontal axis wind turbines are an attractive renewable energy source because of their low carbon foot print. The key metric determining the success of wind energy industry is the Cost of Energy (CoE). The enlarging of rotor size over the years for the purpose of better performance has also resulted in proportional increase of the CoE. Hence improving the lifetime and lowering the maintenance costs of the large rotors are vital requirements and they can be achieved through load alleviation methods. The loads on the wind turbine blades can be reduced either through active or passive morphing methods. Today, rotor control is mostly limited to blade pitch control and rotational speed control. Of late, though, introduction of a trailing edge flap and its deflection as a method to reduce fatigue loads in the wind turbine blades is increasingly being studied. In related research on main rotors in helicopters, multiple trailing edge flaps on the rotor blade have been shown to reduce hub vibratory loads. The current work was aimed to study the effect of adaptive trailing edge flaps on the root flap bending moment fatigue and on the rotor hub vibratory loads. The NREL 5MW Onshore wind turbine was taken as the baseline model. For the morphed rotor blade, the rear 20 % of the airfoil chord length was deflected to angles from -10° to 10 ° forming the flaps. For predicting aerodynamic loads, the underlying look-up tables of airfoil polars were needed to be modified to take into account the flapped airfoil characteristics. The polars of the airfoils with deflected flaps was determined using CFD. Wind turbine simulations were carried out using NREL’s FAST software for the available land based 5MW turbine settings with required modifications to accommodate the trailing edge flap sections. TurbSim and MLife are the other NREL tools that were used to support the simulation and post processing. The work also included further analysis to determine the effects of trailing edge flaps at locations along the span of the blade.