Optimum geometry of seashell-shaped wind turbine rotor: Maximizing output power and minimizing thrust

Inspired by seashells available in nature, a horizontal-axis wind turbine was developed for urban usage, based on Archimedean conical-spiral. It uses both lift and drag to harvest the kinetic energy from the wind by redirecting the flow 90∘ relative to its original direction. The spiral wind turbine in its basic design has been studied in previous research but the optimal geometrical parameters that control the conical-spiral shape have not been identified yet. This paper aims to identify an optimal seashell-shaped rotor configuration, delivering the highest output power under thrust constraints through a systematic multi-objective optimization process. Three relevant geometrical parameters (1) the spiral maximum rotation angle θmax, (2) the spiral initial stretch, z0, (3) the shell opening angle, γ, are varied in order to maximize the power coefficient (CP) and minimize the thrust coefficient (CT) of the wind turbine. The spiral maximum radius (rmax) has been kept constant to provide the same input power for all geometries. The optimization was accomplished using the Non-dominated Sorting Genetic Algorithm (NSGA-II) coupled with Computational Fluid Dynamics (CFD) for the evaluation of the fitness of the individuals. The CFD models employed were validated against results available in the literature for a 1/10 scaled model of the prototype of 0.5 kW-class horizontal-axis spiral wind turbine. A total of 270 simulations over 9 generations were executed to obtain a Pareto front for the 3-bladed wind turbine. Among all the design variables considered, the shell opening angle (γ) and the spiral initial stretch (z0) were notably the most sensitive parameters to the power output and the axial thrust, respectively. The results revealed that it is favorable to decrease the spiral maximum rotation angle and to increase both spiral initial stretch and shell opening angle for this kind of turbines to achieve a good trade-off in between both objectives. In comparison with the reference rotor, the optimized turbine increased the power coefficient by 14% with a CP= 0.305 and reduced the thrust coefficient by 9.6% with a CT= 0.719 at the design operating condition (λ= 2.2).