Sharpening the blades

Making wind turbines competitive with other sources of power involves enhancing energy generating performance and lowering operational and maintenance costs.

Etana Ferede of the Faculty of Aerospace Engineering (LR) defended his doctoral thesis research on wind turbine blade optimisation targeting these two improvements.

Wind turbines have to react to variable wind conditions, harvesting energy at low speeds and resisting damage at high wind speeds. Ferede explained that wind turbines can be made with smart rotors to control the power generated and the loading, but this comes with added complexity leading to more costs and maintenance.

His research addressed the search for robust passive designs for reduced maintenance, lessening the amount of rotating parts subject to wear. So he looked in more detail at the way blades themselves can be made more adaptive to control the stalling behaviour of the rotor. Progress in composite technologies now allows making blades that respond to structural loads and twist their angle of attack to oncoming air flow, thus regulating the power and torque of the turbine.

To do this he developed a detailed optimisation methodology using variable stiffness laminates to see how twist coupled blades might improve the performance of stall controlled wind turbines. His study included detailed structural and aerodynamic constraints and is sophisticated enough to accurately capture the behaviour of twist coupled blades. His framework first determines the blade loads and deformations using a coarse model, then more detailed structural analysis includes stress and buckling responses.

The stiffness based optimization incorporates the thickness and membrane and bending stiffness of the laminate. In the future more modules can be added to the methodology to determine the laminate fibre paths, their angles and stacking sequences.

He applied the rotor blade optimisation methodology in a steady state, static scenario. A future challenge would be to expand the framework to apply it to more dynamic situations and approximate safety and fatigue issues of reality. For this the whole turbine would have to be modelled.

Ferede’s supervisor, Dr. Gerard van Bussel, Chair in Wind Energy at LR, praised his work, saying his multilevel parametrisation methodology was a big achievement. It reinvigorates the field of wind turbine passive stall control, which has been dormant since the mid-1990s.

Etana Ferede Thesis: Static Aeroelastic Optimization of Composite Wind Turbine Blades Using Variable Stiffness Laminates. PhD supervisor: Dr. G.J.W. van Bussel (Faculty of Aerospace Engineering). Defence date: November 14, 2016

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