Development of a multi-MW VAWT for floating offshore wind application

Abstract To fight against climate change and encourage the energy transition, the adoption of renewable energy sources is essential. Among the most widespread in recent years is the wind energy, both onshore and offshore. While the onshore production is currently the most consolidated and reliable solution due to a long experience, the immense potential of offshore energy production is not yet fully exploited. Offshore production would benefit from greater wind availability, allowing for the construction of wind farms with larger turbines and thus ensuring significantly higher productivity compared to onshore. Additionally, offshore production does not consume land and does not generate acoustic pollution, driving research and investments in the offshore sector. In the context of offshore production, Vertical Axis Wind Turbines (VAWTs) seem to promise significant advantages over more consolidated Horizontal Axis Wind Turbines (HAWTs), which are the absolute leaders in the onshore market. VAWTs have a simpler maintenance, better wake aerodynamics, improved stability, allowing potential cost reductions to be addressed. However, many challenges need to be addressed, including upscaling turbines capacity and managing cyclic loads affecting the mooring lines. For this reasons, accurate numerical models to understand the VAWT dynamics are required to spread their diffusion. This thesis aims to model a multi-megawatt VAWT for offshore applications. The design and simulation process was conducted entirely within QBlade, a state-of-the-art software developed by Technical University of Berlin. After a state-of-the-art analysis about the development of floating VAWT projects and demonstrators in Europe, the thesis focus on two aerodynamics models for VAWT: the Double Multiple Streamtube and the Lifting Line Free Vortex Wake. The Double Multiple Stream Tube (DMST) theory, based on the Boundary Element Momentum (BEM) method, allows for quick analyses with reduced computational effort but with lower accuracy. Consequently, it is particularly suitable for the preliminary design and optimization phases of a VAWT. Through the use of DMST, the rotor design of the turbine and the evaluation of performance in terms of power generation, torque, and thrust have been carried out, considering key design parameters such as chord length, airfoil profile, pitch angle, and Reynolds number. Subsequently, to assess the accuracy of the results obtained, simulations were conducted using the Lifting Line Free Vortex Wake (LLFVW) method. By explicitly resolving the wake, LLFVW eliminates dependence on corrections and provides more accurate results but requires high computational time. Finally, the dynamics of a floating VAWT system, consisting of the developed turbine supported by a floating foundation, is presented. The simulation considers both the aerodynamics and hydrodynamics of the system under varying conditions of turbulent wind and wave motion.