Numerical and Experimental Investigations of Darrieus Wind Turbine Start-up and Operation

Abstract

The performance of small, H-Darrieus vertical axis wind turbines has been investigated numerically and experimentally with particular attention paid to turbine performance at low tip speed ratios (low Reynolds number) and to turbine self-starting. Comprehensive wind tunnel measurements have been performed to provide accurate aerofoil data at low Reynolds numbers and high angles of attack; a unique requirement for vertical axis wind turbine (VAWT) starting studies. Two-dimensional CFD models and blade element momentum (BEM) models were created and assessed to provide new insight into turbine performance for different wind conditions and into different turbine geometries in order to guide the design of the experimental investigation.
The experimental and numerical studies have demonstrated that design parameters including turbine solidity, blade profile, blade pitch angle and blade surface roughness have strong influences on turbine performance and turbine self-starting capability. Although other authors have conducted numerical studies of the effect of these parameters, this work represents the first experimental validation for turbine performance at low tip speed ratios. In contrast to some previous studies it is shown that there is no advantage to be gained from the use of cambered blades and that symmetrical blades set at small negative incidence provide the best design solution. It is also shown that increasing the turbine’s solidity can significantly improve self-starting capability and that for a given solidity, increasing the rotor radius with a corresponding increase of blade chord improves performance further. However, these starting performance gains are achieved at the expense of a small loss of peak power output.
In addition, bio-inspired blades with tubercle leading edges are demonstrated to be able to significantly improve the turbine self-starting capability by introducing a more gradual stall characteristic. These results are the only reported measurements of the effect of tubercle leading edges on vertical axis wind turbines.
Finally, a novel, real-time on-board pressure measurement system was developed and employed to examine the instantaneous blade pressure distribution and its variation when the turbine is rotating. The complex flow physics including dynamic stall, laminar separation and flow curvature were successfully recorded and provide unique, unsteady data to increase our knowledge and understanding of the transient aerodynamics of the H-Darrieus wind turbine.
The experimental results were also compared with the available CFD and BEM predictions. It is demonstrated that BEM based approaches are highly sensitive to the quality of the aerofoil data that is provided as input to the model.
This thesis provides validation of previous work on the question of whether H-Darrieus wind turbines can start without external assistance and in the light of this research a set of revised design rules are proposed to achieve self-starting turbines.