Abstract
Low order models based on the Blade Element Momentum (BEM) theory exhibit modeling issues in the performance prediction of Vertical Axis Wind Turbines (VAWT) compared to Computational Fluid Dynamics, despite the widespread engineering practice of such methods. The present study shows that the capability of BEM codes applied to VAWTs can be greatly improved by implementing a novel three-dimensional set of high-order corrections and demonstrates this by comparing the BEM predictions against wind-tunnel experiments conducted on three small-scale VAWT models featuring different rotor design (H-shaped and Troposkein), blade profile (NACA0021 and DU-06-W200), and Reynolds number (from 0.8×105 to 2.5×105). Though based on the conventional Double Multiple Stream Tube (DMST) model, the here-presented in-house BEM code incorporates several two-dimensional and three-dimensional corrections including: accurate extended polar data, flow curvature, dynamic stall, a spanwise-distributed formulation of the tip losses, a fully 3D approach in the modeling of rotors featuring general shape (such as but not only, the Troposkein one), and accounting for the passive effects of supporting struts and pole. The detailed comparison with experimental data of the same models, tested in the large-scale wind tunnel of the Politecnico di Milano, suggests the very good predictive capability of the code in terms of power exchange, torque coefficient, and loads, on both time-mean and time-resolved basis. The peculiar formulation of the code allows including in a straightforward way the usual spanwise non-uniformity of the incoming wind and the effects of skew, thus allowing predicting the turbine operation in a realistic open-field in presence of the environmental boundary layer. A systematic study on the operation of VAWTs in multiple environments, such as in coastal regions or off-shore, and highlighting the sensitivity of VAWT performance to blade profile selection, rotor shape and size, wind shear, and rotor tilt concludes the paper.
Highlights
The history of the Vertical Axis Wind Turbine technology is millennial
The Darrieus turbine was later object of intense research, mainly carried out by the Sandia Lab group [1,2,3], in the second half of the 20th Century; when modern lift-driven wind turbines evolved from concepts to technology, Vertical Axis Wind Turbines (VAWT) eventually lost the competition against Horizontal Axis Wind Turbine (HAWT)
This book clarifies the motivations for the difficult development of VAWT technology, that lie in the flow complexity and aerodynamic forcing of a machine which features unsteady operation by design
Summary
The history of the Vertical Axis Wind Turbine technology is millennial. The very first machines, conceived to extract energy from the wind thousands of years ago, featured the axis of rotation normal to the wind direction and they were based on drag (see, e.g, the Panemone turbine). The development of VAWT technology has seen a significant acceleration, and recent works have enriched the experimental database on prototypes at laboratory scale [5,6,7,8,9,10,11,12,13], have improved the modeling techniques [14,15,16,17,18,19,20,21,22,23], and have investigated issues related to the machine operation [21,24,25,26,27,28] This impulse has improved the VAWT concept, making it progressively competitive against the conventional HAWT.
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