Increasing the power efficiency of wind turbine rotors is still a challenging task for many designers despite the recent aerodynamics knowledge input. In fact, the effected changes in rotor design from the formerly over-sized dimensioned turbines to the present slender turbines of higher power output is the best illustration of this aerodynamic progress. Therefore, to harness more energy, there will be need to increase the rotor size thereby increasing the turbine capacity. Unfortunately, extending the rotor length requires the development of new lightweight materials and causes logistic problems associated with transportation, and the construction and erection the rotor blades. More so, matters concerning public acceptance inhibits the size of onshore wind turbines. To counteract this, there is need for optimal aerodynamic design solutions. To this regard, researchers have developed robust airfoils with a high lift to drag ratio and a low noise level, combined with optimized blade plan forms to enhance aerodynamic performance. Regardless, the aerodynamic performance near the blade root is less studied, and a generally poor aerodynamic performance in the blade root region is considered a problem that cannot be directly solved with conventional design techniques.
In a recent research collaborations between scientists at Technical University of Denmark and Yangzhou University in China and led by professor Wei Jun Zhu, the team successfully enhanced the performance of horizontal axis wind turbines. Their aim was to introduce a cylindrical disk in front of the rotor in order to lead the incoming flow from the inner part to the outer part of the rotor blades. In return, they hoped that this would increase the power output, since the kinetic energy is mainly captured at the outer part of the blades, where the relative wind speed is high. Their work is now published in the research journal, Energy.
To assess the impact of their novel design idea, the researchers employed a hybrid numerical technique, based on solving the Reynolds-averaged Navier-Stokes equations, to determine the aerodynamic performance. They then used an in-house developed EllipSys3D code to represent the upstream cylindrical disc. Eventually, the research team assessed the impact of the disc on the rotor performance by systematically changing the size of the circular disc and its axial distance to the rotor.
The authors of this paper found out that the resulting maximum gain in relative power was around 1.5%. The team also noted that the power was found to increase in most of the simulations, as long as the disc size was not too large, where for the latter case, it started blocking the flow though the effective blade elements at the outer part.
The study reported by Wei Jun Zhu and colleagues presented a thorough numerical investigation on a novel horizontal axis wind turbine rotor system in which a circular disc has been added in front of the main rotor. The results have indicated that additional energy can be captured by placing a circular disc with a suitable diameter upstream of the rotor plane. This being the first numerical attempt to make proof of the concept, it is expected that it will provide a basis for future works that may further optimize the shape of the disc.
Wei Jun Zhu, Wen Zhong Shen, Jens Nørkær Sørensen, Hua Yang. Verification of a novel innovative blade root design for wind turbines using a hybrid numerical method. Energy, volume 141 (2017) pages 1661-1670.
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