Performance investigations of modified asymmetric blade H-Darrieus VAWT rotors

Abstract

Wind is an important renewable energy resource, which can be extracted for power generation by horizontal and vertical axis wind turbines (VAWTs). VAWTs can be installed in low wind speed regimes for small-scale power generation. However, for better performance in such a scenario, an improved rotor blade configuration is essential. This paper investigates the performances of an asymmetric blade H-Darrieus rotor with a blade span constant relative thickness (RT) and a variable thickness (VT) at different low wind speeds between 4 and 6 m/s. Both static and steady-state performances have been evaluated. Different blade span thickness to chord ratios (t/c) of 30% and 45% (constant RT case) are considered by increasing the blade thickness of the considered base profile NACA 63-415 while keeping the chord length same (5.0 cm). After determining the optimal blade RT, a new blade span variable thickness (VT) H-Darrieus rotor has been designed with a thicker portion (t/c =37.5%) on the top end and a thinner portion (t/c = 22.5%) on the bottom end of the blade, i.e., the thickness of the blade profile throughout the blade span is not the same. The thickness gradually decreases from top to bottom of the blades. The effects of all the thickness modifications have been studied with respect to the performance improvement of the H-Darrieus rotor. It has been shown that constant blade span RT (t/c = 45%) can largely improve the starting characteristics of the asymmetric blade H-Darrieus rotor, whereas blade span VT design can result in better power coefficients of the same in low wind speed conditions. Furthermore, an attempt has also been made to study the fluctuations of steady state torque at the selected wind speeds for understanding the stability and smoothness of operation. Moreover, the fluctuations of power and torque coefficients due to steady turbulence intensity and flow non-uniformity over a constant wind speed have also been demonstrated to connect with real-life applications of the H-Darrieus rotor.