Measured and predicted performance of a small wind turbine operating in unsteady flow

Research Doctorate - Doctor of Philosophy (PhD)Small horizontal-axis wind turbines operate in a broad range of wind regimes, which due to their site placement, is often highly turbulent or unsteady. A unique challenge impacting the performance of a small wind turbine, is the use of a passive, free yaw system, where the wind turbine follows changes in wind direction using a tail fin. Controlling all aspects of a free yaw wind turbine in unsteady wind, to maximise energy capture and minimise loading, presents a significant challenge, as the wind turbine's control system must respond quickly and appropriately to changes in aerodynamic loading. No such control system has been designed, due principally to a lack of detailed understanding of the dynamic behaviour of the whole wind turbine in unsteady flow. The work presented in this thesis proposes an accurate dynamic model of a complete small wind turbine operating in unsteady wind, developed from a large volume of experimental data. A two-bladed 5 kW Aerogenesis horizontal-axis wind turbine located at the University of Newcastle, Australia, was instrumented to provide detailed, high-resolution, experimental data during operation in a broad range of wind conditions. The wind turbine is situated in a highly turbulent urban wind environment, with a measured turbulence intensity exceeding 30%. In total, nine different types of sensors were integrated onto the wind turbine, many of which were custom-developed, including an Arduino-based multi-node network and a novel optical recognition system for measuring wind turbine yaw position. The sensors measure the instantaneous blade response to load, blade rotational speed, wind turbine direction, wind speed and direction, and tail fin yaw moment. There are many existing theoretical models to predict the performance of each individual component of a wind turbine, including those for the blades, gearbox, generator, control system, and wind turbine yaw behaviour. Many of these models are used in isolation or are derived from the assumption of steady wind conditions. Furthermore, the accuracy of these models has not been thoroughly evaluated for a wind turbine of this size or class when operating in unsteady flows, due to the lack of detailed experimental data. The experimental data presented in this thesis, shows that unsteady blade element momentum theory, utilising steady-state aerodynamic data, over-predicted the aerodynamic torque produced by the blades in unsteady wind. As a consequence, the rotor speed and power production were also over-estimated. Modifying the quasi-steady lift and drag coefficients to account for turbulence intensity showed excellent agreement between model predictions and experimental results. The unsteady dynamic yaw performance of the wind turbine was measured for two different sized delta-wing tail fins, and predictions from a range of tail fin models were assessed. It was found that for both tail fins, the method which utilised experimentally-estimated pseudo delta-wing tail fin lift and drag data for the tail fin, consistently gave the most accurate predictions. Furthermore, an evaluation of tail fin size both computationally and experimentally showed the detrimental impact that an undersized tail fin has on energy capture in unsteady wind. Results from a computational sensitivity analysis of the area of a delta-wing tail fin, indicate a minimum of 4% of the swept rotor area to produce optimal yaw alignment. The starting performance of a wind turbine is critical to its design and energy yield, especially when operating in unsteady wind, due to the higher frequency of starting events. The speed of the rotor was measured as a function of time, in a broad range of starting conditions, including high yaw misalignment starts, rapid wind deceleration starts, and failed starts. It was determined that the model that incorporates a high angle approximation and includes the effects of yaw misalignment and modified aerodynamic coefficients to account for turbulence intensity, gave the most accurate predictions of starting performance. The maximum difference between measured and simulated starting time was 1.2%. Overall, the work presented in this thesis has successfully developed, and experimentally validated, a complete and accurate unsteady model of a free yawing 5 kW horizontal-axis small wind turbine. In doing so, this work has clearly shown the effects of operating in unsteady wind conditions, extending the existing body of knowledge