Abstract:
Vertical axis Darrieus type wind turbines are lift based devices offering comparable peak efficiencies to the standard horizontal axis wind turbine. For small scale power generation they have several desirable characteristics. These include potential for direct mechanical power transmission to ground level, low noise, and they require no yaw system as they are omnidirectional. Darrieus turbines however have an inherent drawback. They have poor low speed efficiency, and generally require being driven up to the operating speed using a motor. The aim for this project is to investigate a means for improving the low speed performance of the Darrieus turbine, by the use of what is termed a ’variable pitch’ system. For such a system the angle of each turbine blade is changed to a more aerodynamically desirable position. In this project a blade pitch actuation linkage based on a marine propulsion drive called the Voith-Schneider propeller is investigated. A combined theoretical and experimental approach was taken. A mathematical model was written using MATLAB to calculate time averaged span-wise and through-turbine flow velocities, as well as blade forces. This was used to investigate the effects on turbine performance due to the blade actuation system. A turbine test rig incorporating the variable blade pitch system, as well an eddy current brake with a load cell for power measurement, dissipation, and speed control was designed and constructed. This turbine was tested at the University of Auckland Twisted Flow Wind Tunnel, and compared to the predictions from the mathematical model. There was good agreement between the mathematical model and the experimental performance of the test rig. The variable pitch system removed the poor low speed performance typical of Darrieus turbines, and instead developed high torque at all speeds. At a tip speed ratio of = 1:5 the blade pitch system improved power output of the turbine by as much as 400%, while the peak efficiency increased by 40% from CP = 0:23 to CP = 0:32.
