Load Alleviation by Morphing Wind Turbine Blades under Uniform, Gust and Turbulent Inflow

Abstract

Morphing wind turbine blades promise to improve wind turbine performance by allowing blade geometry to adapt to changing winds, alleviating the fatigue loads that impact blade life. The net benefit, however, found for a morphing blade comes at a tradeoff for lost power production and depends strongly on the inflow condition it experiences, with the most benefits in load alleviation found in turbulent and gust wind fields. The concept of a morphing blade was then split into passive and active concepts, which differ in how they respond to wind loads. The passive blade utilized bend twist coupling, which responds to present loads through a composite fiber layup that morphs in the face of wind loads, while the active blade utilized flaps which actuate in response to feedback measured from the wind. The study compared the different morphing blades against a conventional blade from the NREL 5MW reference turbine on a three bladed horizontal-axis wind turbine. All three blades underwent three different wind conditions: steady uniform wind sweep from 4 to 20 m/s, a deterministic 1-cosine gust, and IEC Kaimal turbulence at 12 m/s. All cases were modelled off the NREL 5 MW reference turbine using the aeroelastic solver OpenFAST v4.2.1, with BeamDyn structural models. Under steady sheared wind, the active blade and passive blade removed up to 55% and 20% of the blade root flapwise damage equivalent load respectively. Under turbulent winds, the active blade added to the blade root flapwise damage equivalent load by 2.3%. Despite this increase, the passive blade retained its load alleviation with a reduced flapwise fatigue load by 21.8%. The tradeoff for these benefits in load alleviation only cost between 2-3% of annual energy production against the conventional blade in both the active and passive blades. The benefits of morphing are therefore dependent on the wind condition and the type of blade, with the passive blade performing consistently across all three conditions due to its load driven morphing which does not require azimuth scheduling or controller feedback to actuate.