Double - stage H-Darrieus wind turbine - rotor aerodynamics

H-Darrieus wind turbines, due to their simple design and relatively low manufacturing costs have recently received much attention particularly for standalone applications. However start-up issues associated with their operation restricted their operation in areas of low average wind speed and encourages engineers to develop novel design. Several design proposed in this way but in most cases design came up with complex sensing mechanisms and mechanical actuators or high cost manufacturing parts. A recent rotor design called double Darrieus rotor proposed as a German patent case bridged these complexities appropriately. The aim of present study is to investigate this innovative design from aerodynamic point of view by means of validated CFD techniques. A flow-driven simulation setup based on 6DOF calculations employed in order to study rotor operation from stand still until peak performance obtained. Results from these precise modeling reveal the superiority of the proposed double-stage design in compare with the original H-Darrieus rotors in terms of start-up behavior and optimum performance

Modal behaviour of vertical axis wind turbine comprising prestressed rotor blades: a finite element analysis

Pre-stressing is a concept used in many engineering structures. In this study prestressing in the form of axial compression stress is proposed in the blade structure of H-Darrieus wind turbine. The study draws a structural comparison between reference and prestressed configurations of turbine rotor with respect to their dynamic vibrational response. Rotordynamics calculations provided by ANSYS Mechanical is used to investigate the effects of turbine rotation on the dynamic response of the system. Rotation speed ranging between 0 to 150 rad/s was examined to cover the whole operating range of commercial instances. The modal analysis ends up with first six mode shapes of both rotor configurations. As a result, the displacement of the proposed configurations reduced effectively. Apparent variations in Campbell diagrams of both cases indicate that prestressed configuration has its resonant frequencies far away from turbine operation speeds and thus remarkably higher safety factor against whirling and probable following failures

Study on start-up characteristics of H-Darrieus vertical axis wind turbines comprising NACA 4-digit series blade airfoils

Installation of H-type vertical axis wind turbines is in many cases limited by the inherent start-up issues associated with this type of turbine. This could be crucial in environments with low wind speed. The aim of this study is to provide an appropriate CFD modeling setup for investigation of the start-up behavior associated with this class of turbines. For this purpose, a series of transient CFD simulations were carried out using ANSYS Fluent. In contrast with the conventional approach, whereby a constant angular velocity is specified for the rotor, in the present work, the turbine was left free to accelerate based on the torque experienced over time. Careful considerations were made regarding turbulence modeling and grid generation, which are key to ensuring accuracy in this investigation. The result of this simulation, in the form of an accelerating time series, demonstrates good agreement with the published experimental data, and the method yields a high level of accuracy, proving its usefulness for similar problems. In another attempt, the validated CFD setup was utilized to evaluate the effects of several geometric attributes of the turbine rotor on the starting characteristics. Symmetric and cambered airfoils of different thicknesses with a wide range of pitch angles were examined. The optimum start-up characteristics were observed with the use of a medium-thickness cambered airfoil, NACA2418, put to use with an outward pitch angle of 1.5°; this configuration decreased the start-up time while retaining the turbine's peak performance