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Small scale horizontal axis wind turbines (HAWTs) are becoming increasingly popular yet they have received much less research attention than their large scale counter parts. Unlike large scale rotors they solely rely on their aerodynamic torque for accelerating the blade from rest to full operational speed while being subjected to a number of torque reducing issues that large turbines do not experience. In this study, Computational Fluid Dynamics (CFD) has been utilised to simulate turbine starting sequences. A newly developed method which uses CFD to model a fully transient turbine start-up has been evaluated. The chosen approach overcomes the assumptions of, currently employed, semi-empirical quasi-steady start-up methods. It has been shown that the quasi-steady approach is of acceptable accuracy in predicting starting sequences when compared to the fully transient method. New techniques have been developed to investigate the flow features and local blade torque characteristics which have subsequently been quantified with respect to their relevance on turbine starting. The level of detail of the present study goes far beyond that of existing experimental or computational literature on turbine starting. Following studies systematically investigated the effect of turbine scale and rotor geometry over a range of wind speeds using the National Renewable Energy Laboratory (NREL) Phase VI rotor as reference turbine. This analysis is the first of its kind which address the individual effect of blade pitch and thickness as well as their interdependence on the rotors performance at different operational Reynolds numbers. As a result of these studies, it has been shown, that the annual energy yield of turbines which frequently restart due to a turbulent flow environment, can be improved by increasing blade pitch and reducing blade thickness. It has been demonstrated that rotors with a small diameter are more resistant to energy yield reductions caused by gusty environments than larger rotors. |
