Morphing Blades for Load Alleviation of Tidal Turbines

MORPHING BLADES FOR LOAD ALLEVIATION OF TIDAL TURBINES

Abel Arredondo-Galeanaa, Anna Young, Ignazio Maria Viola

School of Engineering, Institute for Energy Systems, University of Edinburgh

Department of Mechanical Engineering, University of Bath 

The use of passive unsteady load mitigation technology for wind and tidalturbines, such as bend-twist coupling, is typically limited to low frequencyfluctuations and is not suitable to large blades, due to structural rigidityrequirements. Active control systems, such as actuated flaps, can respond tohigher frequencies than whole-blade passive devices due to their smaller size. However, active systems may reduce turbine reliability. Hence there is a needto develop a high-frequency passive technology if turbines are to survive in theharsh marine environment.Here we show analytically and with CFD that the unsteady loads of a turbinecan be completely cancelled with a chordwise flexible blade. Additionally, wedemonstrate that when the blade is rigid near the leading edge and flexibleonly near the trailing edge, the unsteady load mitigation is proportional to thelength of the flexible portion compared to the blade chord.We verify this relationship between flexible portion length and load reductionexperimentally with a blade that has a morphing trailing-edge. The morphingtrailing-edge extends 25% of the chord of the blade and it allows unsteady loadmitigation by about 25%, without any variation in the mean load - thus thereis no penalty in terms of power extraction. The mechanism has been tested atlow and high reduced frequencies showing satisfactory results at both regimes.