Study of Design Load Cases for Multi-Megawatt Onshore Vertical Axis Wind Turbines

Abstract

In the last years, large scale Vertical Axis Wind Turbines (VAWTs) are re-gaining a general interest in the wind energy sector as an alternative to Horizontal Axis Wind Turbines (HAWTs). However, the existing standards and guidelines for wind turbine certification do not include specific requirements for VAWT rotors. This thesis aims to study the adequacy and applicability of the IEC 61400-1 ed.3, 2005 International Standard minimum design requirements, when it is applied on multi-megawatt VAWTs. This is accomplished by simulating a VAWT aeroelastic model under various Design Load Cases (DLCs) and analysing the results. The motivation comes from the recent years scientific research and developments which revealed that a VAWT can be beneficial in some types of applications in comparison with a HAWT. As a result, a minimum engineering integrity proof for VAWTs is needed before the installation and operation phase as in HAWT. The work is split into two main phases. The first copes with the construction of a reliable large scale VAWT aeroelastic model which serves as the basis for the analysis. A 5 MW 2-bladed VAWT aeroelastic model is built based on the “DeepWind” concept. Since blade instabilities were reported the "Sandia 34 m, 500 kW VAWT test bed" is up-scaled and used as guidance for the modification of the blade structural properties. In addition, the selection and validation of an aeroelastic code able to compute the VAWT aerodynamics and loads under different environmental conditions and wind turbine states is investigated. The study has been conducted through a special project by the author as part of this thesis. It was meant to study and apply an aeroelastic code by constructing a VAWT aeroelastic model. The Sandia 34 m, 500 kW VAWT was modelled and simulated with HAWC2 aeroelastic code, showing good agreement with experimental data published by Sandia National Laboratories. In the second part is studied the adequacy and applicability of the aforementioned international standard on VAWTs and recommendations are made by simulating several DLCs using HAWC2 code. Ultimate and fatigue loads during normal operation of the wind turbine are extracted and sensitivity analyses of relevant parameters that could influence the results are performed. These include the blade stiffness and structural damping, and the generator slip. Various wind conditions are applied including extreme operating gust, extreme wind shear and extreme wind direction change. The results from normal power production are compared with the “NREL 5 MW HAWT” which has the same rated power as the VAWT model in order to give an insight on the load levels between horizontal and vertical axis turbines. Design situations of parked turbine and emergency shut-down are also investigated. The results (chapters 3 and 4) indicate that the loads emerging from the extreme wind shear and wind direction change conditions are not critical for the VAWT, thus the corresponding design load cases could become optional. The case of parked turbine with locked rotor at specific orientations relatively to the wind direction revealed blade instabilities when the turbine was simulated under 50 year recurrence period wind conditions. The load comparison between the horizontal and the vertical axis wind turbines showed that the loads at blade low root and turbine base bottom are higher in VAWT but it should be noted that the NREL turbine is a 3 bladed configuration, while the developed VAWT model has 2 blades. The case of supported tower with guy wires was analysed and relevant load cases are proposed. In general regarding the loads it is concluded that the inherent variation of the blade aerodynamic forces during every revolution results to highly deterministic loads on the turbine reducing the effect of turbulence. This could lead to lower partial safety factors if could be demonstrated in other VAWT types. Finally, some small modifications on the definitions of the standard are proposed in order to avoid ambiguities when it is applied to VAWTs.EWEMWind EnergyAerospace Engineerin