CFD simulation and experiments on optimization design of vertical axis wind turbines (VAWTs)

Abstract

Unlike horizontal-axis wind turbines (HAWTs), vertical-axis wind turbines (VAWTs) are still not popular for wind power generation in large scale although a lot of researches have been conducted in an effort to improve the performance of the VAWTs. The VAWTs can take wind from any direction, and they also have been proved to be a promising candidate for power generation in urban environment. However, the current fact is that the Savonius rotor (drag-type VAWT) has a good starting ability, but low power coefficient (CP), and the Darrieus rotor (lift-type VAWT) has a high power coefficient, but poor reputation of self-starting, which highly limits the utilization of the VAWTs in an open area or urban area where the more complex wind environment exists. Thus, the objective of this thesis is to improve the starting ability and the CP of the VAWTs, especially for the Darrieus rotor, through the computational fluid dynamic (CFD) method and experiments. In order to improve the starting ability of the lift-type VAWTs, three methods are proposed and studied in this thesis. These three methods are the usage of two-stage Savonius rotors, an opening at the blade tip and the two sets of blades. Wind tunnel test was conducted to study the two-stage Savonius rotors with two and three blades (first method) which improve the starting ability of Darrieus rotor. The experimental tests of the Savonius rotor find that the two-stage rotors can be a good assistant to improve the self-starting ability of the Darrieus rotors, especially the two-stage rotor with three blades due to the fact that the CTs of the two-stage rotor with three blades is higher than that of the two-stage rotor with two blades. It also demonstrates that although the two-stage rotors produce lower power than that of the one-stage rotor, the two-stage rotors have fairly higher and smoother static torque coefficient (CTs) than that of the one-stage rotor. This makes the Darrieus rotor has the better starting ability. A novel method is proposed as the second method to improve the starting ability of the Darrieus rotor. This novel method is the Darrieus rotor with opening at the blade tips which has not been studied and mentioned in public literatures. The finding shows that the opening at the blade tip can improve the starting ability of the Darrieus rotor. It is also found that the CP loss of the Darrieus rotor with an opening is less than the hybrid Savonius and Darrieus rotor. This new design is a promising device for the standing alone system. The third method proposed to improve the starting ability of the Darrieus rotor is the Darriesu rotors with two sets of blades. This method has been rarely studied. The effect of the distance between two sets of blades, and the effect of the offset angle between two sets of blades have been evaluated. It is found that the CP of the Darrieus rotors with two sets of blades is generally lower than that of the rotor with one set of blades. This conclusion is different from the statement in literature. In addition, a remarkable finding is that the Darrieus rotors with two sets of blades do increase the CTs which make the rotors start easily. In order to improve the CP of the Darrieus rotor, a novel airfoil selection method based on an automatic CFD process and design of experiment has been established in this thesis. By using this novel method, not only can the impact weight of each factor be found, but also an optimal airfoil has been found to improve the CP of the Darrieus rotors. This airfoil selection method not only extended the Darrieus rotor's airfoil selection from one-factor to three factors, but also laid down the fundamentals of the parametric airfoil design process for Darrieus rotors, which is the original intention of the author for doing this project. An optimal airfoil was found. It can produce 9.7% more energy than that of the NACA 0015 airfoil which is widely used before. In the end, a real 1.5 kW Darrieus rotor turbine with an opening at the blade tip and optimal airfoil gained from the novel airfoil optimal design process has been built for the future testing. Five construction processes of this 1.5kW Darrieus rotor are summarized. An axial flux permanent-magnet synchronous generator (AFPMSG) and a magnetic bearing are integrated into this 1.5kW Darrieus rotor to reduce the starting torque of this system.