Morphing Blades

Abstract

Tidal turbines experience large load fluctuations due to the unsteady environment and the shear in the tidal flow. Mitigating these fluctuations without affecting the mean load would result in lower capital and operational costs. In this paper we discuss how this could be achieved through blades that passively and elastically adapt their camber and angle of attack to counteract unsteady flow conditions. Firstly, we discuss the underlying principles of unsteady thrust mitigation. We show that complete cancellation of the thrust fluctuations would be possible if every blade section could pitch passively and independently of neighbouring sections. Secondly, we provide proof of principle for two practical implementations through physical experiments and computational fluid dynamics simulations. We consider a blade that is rigid near the leading edge and flexible near the trailing edge. We show that the unsteady load mitigation is proportional to the ratio between the length of the flexible and rigid parts of the blade. For example, for a blade section where the flexibility is concentrated in a hinge at 3/4 of the chord, the amplitude of the fluctuations is 3/4 of the original amplitude. Secondly, we consider a solid, rigid blade with a passive pitch mechanism. We show that, for a 1 MW turbine operating in shear flow, more than 80% of the unsteady loading is mitigated. These results demonstrate the potential effectiveness of morphing blades for mitigating thrust fluctuations on tidal turbines.

Control Strategies for Power Enhancement and Fatigue Damage Mitigation of Wave Cycloidal Rotors

“…Morphing foils, modelled as passively pitching systems, have shown to be a promising technology for peak loading alleviation in tidal turbines [8,17,41,49]. However, to date, passive compliance has not been studied for wave energy devices and, in particular, for wave energy cyclorotors.…”

Design and Demonstration of a Passive Pitch System for Tidal Turbines

“…To alleviate these loads, Viola et al [8] have introduced the concept of passively pitching blades, where the blades of an axial tidal turbine are not mounted rigidly on the turbine structure: instead, an elastic component such as a torsional spring provides a degree of freedom to the blades, which are allowed to pitch around a radial axis. With this arrangement, each blade individually reduces its angle of attack when subject to above-design loads and, conversely, increases its angle of attack when the load reduces, thus opposing changes in the generated loads.…”