Passive Loads Control in the Preliminary and Conceptual Design of Wind Turbine Blades

This thesis deals with the development of methodologies for the implementation of passive control strategies in the preliminary and conceptual design process of a wind turbine blade.

Reducing the cost of energy is a key concern for wind energy research and the ultimate goal for both academia and industry. An effective path to achieve this goal is to scale down the increase in total mass of the blades while designing rotors with increasing size and energy yield. In this context, the capability to mitigate loads on the structure during operation becomes an attractive characteristic for the design of modern wind turbine blades.

One of the family of methods for the alleviation of loads on a wind turbine is called passive control, as it relies on the idea of designing a structure that,without any active mechanisms, deforms so as to reduce the unsteady loading generated by turbulent fluctuating wind inflow. The concept behind passive control for wind turbine blades is to produce a structural coupling between flap-wise bending towards the tower and torsion towards feathering. This coupling mitigates loads dynamically on the wind turbine structure due to a decrease in the angle of attack.

Researchers have been fascinated by the possibility to embed a form of control directly into the structural design of a wind turbine blade for decades. A wind turbine rotor that can mitigate loads passively can be considered a cost effective solution because the load mitigation effects allow the employment of lighter components without the addition of actuators and mechanical actively-controlled parts. The load mitigation effects can be also used to stretch the size of the rotor, increasing the energy yield by the machine.

Our contribution to the research in the topic of passive control for wind turbines is articulated as follows. First, we provide a validation of the aero-servo-elastic model used to perform the analysis throughout the work. Once the accuracyof the nonlinear aeroelastic models has been established, we focus on the subtle and complex interactions arising during the design process of passively controlled rotors, due to the mutual effects of aerodynamics, structure, and control. The aim is to provide a fair estimation of the load alleviation potential of different categories of passive control methods.The parametric study approach, where design parameters which trigger favourable structural coupling effects are changed individually, is not enough to ensure that the full potential of passive control is exploited. Even though parametric studies offer an estimation of load mitigation effects, they do not allow any control on whether standard design requirements have been met or not. Furthermore,these type of studies are not suited to convert the reduction in loads into factors that have a direct impact on the cost of energy, such as the decrease in blade mass or the increase in annual energy production.

We overcome these shortcomings by formulating the passively controlled wind turbine blade design process as an optimization problem using a multidisciplinary design optimization framework. From the application studies reported,we demonstrate how the integration of passive control as a design variable can open the path to the preliminary design of wind turbine rotors with not only considerable load alleviation potential, but also with substantially decreased blade mass or increased annual energy production.