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Wind Energy Conversion Systems
Published in Radian Belu, Fundamentals and Source Characteristics of Renewable Energy Systems, 2019
The modern wind turbine is a sophisticated system with an aerodynamically designed rotor, powertrain, control unit, and power electronics interface and an efficient power generation, transmission and regulation components. Size of such wind turbines ranges from a few watts to several megawatts, the current trends for larger systems. The wind turbines may be grouped into wind farms, feeding power to utility, with its own transformers, transmission lines and substations. Standalone wind energy conversion systems (WECS) catering the needs of smaller communities or individual users are also quite common. As wind is an intermittent and fluctuating phenomenon, hybrid systems with energy storage units are also popular, especially in remote areas. For efficient and reliable performance of a WECS, all its components are to be carefully designed, crafted and integrated. Wind energy conversion systems can be divided into two brad types: (1) those which depend on aerodynamic drag and (2) those which depend on aerodynamic lift. The early vertical axis wind wheels utilized the drag principle. Savonius rotors are the most common of the last types of wind wheels. Drag devices, however, have a very low power coefficient, with a CPmax of around 0.16. Modern wind turbines are predominately based on the aerodynamic lift. Lift devices use airfoils (blades) that interact with the incoming wind. The force resulting from the airfoils body intercepting the air flow does not consist only of a drag force component in the direction of the flow but also of a force component that is perpendicular to the drag: the lift forces. The lift force is a multiple of the drag force and therefore the relevant driving power of the rotor. By definition, it is perpendicular to the direction of the air flow that is intercepted by the rotor blade, and via the leverage of the rotor, it causes the necessary driving torque. Wind turbines using the aerodynamic lift can be further divided according to the orientation of the spin axis into horizontal axis and vertical axis type turbines, the so-called Darrieus wind turbines. The Darrieus wind turbine offers an advantage over the horizontal axis wind turbine because of the structural simplicity due to the independence with respect to the wind direction. This feature makes the control system unnecessary to direct the rotor.
Performance assessment of lift-based turbine for small-scale power generation in water pipelines using OpenFOAM
Published in Engineering Applications of Computational Fluid Mechanics, 2022
Ghada Diab, Mohamed Elhakeem, Ahmed M.A. Sattar
Pipe turbines are classified according to their axis of rotation, into vertical and horizontal axis turbines. Vertical axis turbines have the advantages of operating independent of incident flow direction and achieving higher efficiency at lower speed. Based on the driving force, vertical axis turbines are either drag or lift based. A few in-pipe turbines of drag type have been introduced and studied (e.g. Chen et al., 2013; Hasanzadeh et al., 2021; Jiyun et al., 2018; Ma et al., 2018; Samora et al., 2016). However, these studies showed that the power output (and hence power coefficient) of drag pipe turbines is relatively low, with significant corresponding pressure losses. For instance, a pressure head loss of 5 m was reported by Ma et al. (2018) with a 250 mm turbine, yielding only 480 W, while for a smaller diameter of 100 mm, Hasanzadeh et al. (2021) optimized a drag-based turbine to produce 200 W for the same pressure head loss of 5 m. Drag turbines generally require more blades to build sufficient flow resistance, and this has negative consequences on performance and results in more flow area blockage. On the other hand, lift-based turbines, developed in 1926 and known as the Darrieus wind turbine, have lower obstruction to flow than drag-based turbines, offering generally better starting capabilities and a higher power coefficient.