Aerodynamics and Vibration Analysis of Vertical Axis Wind Turbines and Their Applications in Wind Turbine Design

對於垂直軸風力發電機空氣動力學和震動分析及應用於風力發電機設計

Student thesis: Doctoral Thesis

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Award date11 Apr 2017

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Abstract

Recent applications of vertical-axis wind turbines (VAWTs) have increasingly contributed to the wind power industry due to their effectiveness in urban areas with high turbulence intensities. The performance of a VAWT (i.e., its power output) can be represented by a curve that shows the ratio of its coefficient of power and its blade speed (i.e., CP-λ curve), which many researchers have made great efforts to determine by means of experimental and numerical methods. However, details about methods for the measurement of CP-λ curves in wind tunnel tests are not usually reported. The first objective of this study is to develop a fast, cost-effective and reliable method for measuring this CP-λ curve for a given VAWT. A comprehensive parametric study with the newly developed measurement method was conducted to determine the ‘optimal’ configuration of a straight-bladed VAWT. The author has the experience to develop a large-scale 50-m-tall straight-bladed VAWT and to justify its significant utility in harvesting wind energy. However, substantial vibration of the support tower was observed during operation. To provide a practical guideline for the safe and robust design of the support tower for a VAWT, experimental and numerical investigations were conducted in this study to understand the vibration behaviour of this kind of structure with consideration of soil-structure interaction.

To study the ‘optimal’ configuration of a VAWT for power output, a series of parametric studies were conducted in the wind tunnel facility of the City University of Hong Kong (CityU) and validated with a computational fluid dynamics model. The results show that the power output of the VAWT prototype is sensitive to various controlling factors, including the number of blades (solidity ratio) and the turbulence intensity. Based on the results of the analysis, the ‘optimal’ configuration of a VAWT is discussed. It must be pointed out that a study of the effects of turbulence intensity on a VAWT’s power output is essential to show the feasibility of the development of rooftop wind farms in urban areas, where the turbulence intensity is relatively high and the land supply is limited. Furthermore, the use of rooftop wind farms can significantly reduce the power lost during long-distance transmission from an offshore wind farm.

To study VAWT support towers, laboratory testing and numerical modelling were conducted in this study. The results reveal that soil-structure interaction has significant effects on the stiffness of the support tower system. To secure the sound overall structural performance of the wind tower, the dynamic interaction of the supporting soil, the foundation type and the superstructure must be considered together. The foundation can be designed and fine-tuned in accordance with the site’s soil conditions and the desired operational rotational speed of the VAWT. In addition, a series of design charts were developed to assist engineers in designing support towers specifically for VAWTs.