HYBRI – A combined Savonius-Darrieus wind turbine: Performances and flow fields

In this paper, wind tunnel measurements on a model of a vertical axis wind turbine (VAWT) are reported. The turbine is a novel hybrid Savonius-Darrieus combined rotor which aims optimizing performances in medium-low wind regimes, by using a careful design of the shape, size and relative positions of Savonius and Darrieus blades. To this end, a dynamically scaled turbine model is tested in wind tunnel to derive instantaneous and averaged velocity fields by means of Particle Image Velocimetry (PIV), which allows deriving wakes and specific fluid flow phenomena on each single configuration (Savonius or Darrieus) and interactions on the combined geometry. These results are coupled with electrical measurements to determine global performances, efficiency and best working conditions for each separate turbine and for the combined turbine. Data are also compared with results obtained by other authors in previously reported combined hybrid configurations. The proposed system is able to work with good performances (power coefficient equal or slightly lower than 0.2), on an extended range of operative conditions, covering those of each component alone, i.e. for tip speed ratios between 0.5 and 4, in comparison to the ranges 0.5 ÷ 1 and 1.7 ÷ 4 of the used Savonius and Darrieus components. Motivations for the increased performances and working ranges reached by the proposed combined turbine, especially in the low tip speed ratio regime, are given as derived from detailed PIV velocity measurements.