Horizontal-axis wind turbine (HAWT)

The horizontal-axis wind turbine (HAWT) is one of the most diffused architectures among traditional wind energy conversion systems (WECS), due to its high aerodynamic efficiency. Several works about HAWTs concern the various fluid dynamic aspects of the rotor blade, in order to improve its efficiency and, therefore, the overall energy production. Different airfoil shapes and design solutions have been characterized and studied, but a limited number of works dwell about the structural analysis of the blade, pointing mainly on its modal characterization. Nevertheless, the internal structure of a rotor blade appears to be an essential aspect of its design, being intended to ensure the resistance of this huge mechanical component during the entire life cycle of the machine.

The numerical investigations are based on the analysis of a Composite Blade for the AOC 15/50 wind turbine made by the Sandia National Laboratories. The same finite element model is applied on a similar rotor blade, obtained from the geometrical data.

In order to develop an optimization strategy for the structural design of a rotor blade, which has been named with the acronym S.O.C.R.A.TE. (Structural Optimization for Composite Rotor Air TurbinE), the present study combines a  structural analysis and a modified genetic algorithm. The purpose of the investigation is to improve the structural characteristics of the blade, reducing both the maximum displacements and the mass of the original AOC 15/50 blade, changing the original layup of the composite materials.

The optimization of the rotor blade concerns the choice of the materials and their positioning in order to improve the structural performance of the blade. The disposal of the materials in the composite layout represents the main design variable of the problem. The skin of the blade includes several layers, each of which is characterized by the adopted material, the orientation of the fibers and the thickness. Therefore, a function that computes the main deformations of the blade and a mass function represent the two objective functions of the problem.

The layout of the composite skin blade has been encoded in a parametric mode and optimized using a genetic algorithmic. The evolutionary process has required the decomposition of the original problem in four successive optimizations for the different layout zones of the blade and each time the result of an optimization has been considered as the initial model for the following one: an architectural criteria has been chosen and the four stages represent the structural components of the blade: the External composite Skin, the Internal Reinforcement, the Spar Flange and the Spar Web.

An evolution of the Optimal Pareto Front can be noticed in every stage of optimization. The movement of the solutions in the direction of minimum values of the objective functions represents an increase in the structural performance of the blade. The improvements can be observed both through the generations in a single step of the optimization and in the sequence of the four optimizations.

The complete optimization process has provided a good improvement of the structural characteristics of the blade in terms of weight (8% reduction) and flapwise deformation (12% reduction), while a slight increase of the edgewise deformation (3%) has been registered. The improvements in blade performance are mainly due to the presence of the more resistant material D155 in every component of the blade. The use of D155 allows a reduction of the total mass, increasing at the same time the structural properties of the blade.