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Comparison of different numerical approaches to the study of the H-Darrieus turbines start-up A. Rossetti * , G. Pavesi University of Padua, Department of Mechanical Engineering, Via Venezia 1, 35131 Padova (PD), Italy a r t i c l e i n f o Article history: Received 2 December 2011 Accepted 7 June 2012 Available online 16 July 2012 Keywords: VAWT Darrieus Self-start 3D effects CFD BEM a b s t r a c t Self-start capability is an important feature of wind turbines. It allows to obtain simpler and cheaper turbines not actively controlled. Different approaches to describe the self-start of an H-blade Darrieus rotor are presented and compared in the present work. The Blade Element Momentum (BEM) approach was compared with two and three-d imension al CFD simula tions. The tipespeed ratio versus power coef cient curves and the evolution of the trust forces over a blade revolution highlighted the limits and the strengths of each approach. The BEM model showed remarkable limits to describe to describe the self-start behaviour of the tested geometry. The principal limits of the BEM approach can be ascribed to the absence of well documented aerofoil databases for low Reynolds number and the inadequate modelling of dynamics effects. The 2D simula tion allowed to highligh t the unste ady features of the ow elds, and the presence of a complex vortices pattern which interact with the blade. Furthermore the comparison between 2D and 3D data demonstrated the importance of 3D effects such as secondary ows and tip effects. These effects were proved to have a positive effect on start-up, increasing the torque characteristic for tipespeed ratio of 1. The start-up capability of H-Darrieus appears to be inuenced by many differen t factor s, which include secondary ows, three-dime nsional aerodynamic effects and the nite aspect-ra tio of the blades. 2012 Elsevier Ltd. All rights reserved. 1. Introduction The increasing energy cost and the presence of support policy towards rene wabl e ener gy are feedi ng the interest to deve lop smal l power wind turbines suitable for urban and suburban applications. The soft sound emissions, due to the lower optimum tipespeed ratios compared with the three blades HAWT, the insensitivity to the wind direction, and the lack of need of passive or active yawl control are attractive aspects. Moreover, the easy accessibility of the electri cal and mecha nica l part of the VAWT decr eases the main tenan ce costs of the turbi ne. Notwiths tandi ng, the highe r perfo rma nces of the HAWT and the mor e complex struc tur al de sign of the VA WT , the Da rr ieus turbines presents many advantages when small applications in urban agglomerations are considered. The abili ty of self -star t is an impo rtant qualit y/pr oper ty for small turbines, to obtain simpler and cheaper actively controlled turbines. The need of the turbine assisted launch causes energy expense, could impact the turbine overall production and increases the control system complexi ty [1]. The self-start capability of xed pitc h Darri eus turb ine is not complet ely document ed yet and conicting data and conclusions can be found. Baker [2] identied the presence of a negative torque band, on a l range from 0.5 to 2, depending on the blade geometry consistently with the experimental data reported in literature [3] . Baker suggested the use of inc lin ed or wra pped bladesand a series of de sir abl e aer ofo il sec tio n featu re to aid self -star ting. Other autho rs [4,5] have poin ted out the effect of the blade Reynolds number on the self-start behaviour, demonstrating the positive effects of high Re numbers. This has encouraged the construction of turbines with long chord blades that is with high solidity. The capability of high solidity Darrieus to self-start is consistent with some producers claims [6], but the excessive increase i n solidity leads to the decrease of the maxi mum power coef cient. For the Troposkien blade turbine, the optimum solidity was calculated by Strickland to be about 0.3, on the base of Blade Element Momentum analyses [7] and conrmed by wind tunnel experimental tests conducted by Sandia National Labora- tories [8]. Recent pap ers demons tr ated the cap abi li ty of sel f- starting of an H-blade, low solidity Darrieus in open section wind tunnel at low Re [9,10]. The papers highlighted the dif culty of predicting self-start using analytical models, due to the uncertainly of aerodynamic database at low Re and to the presence of complex * Corresponding author. Tel.: þ39 049 827 7474; fax: þ39 049 827 6785. E-mail address: [email protected] t (A. Rossetti). Contents lists available at SciVerse ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene 0960-1481/$ e see front matter 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2012.06.025 Renewable Energy 50 (2013) 7e19

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