On the effects of a skewed flow on the performance of a three-bladed H-Darrieus turbine: experimental and theoretical analyses

Microeolic turbines are considered one of the most promising technologies for an effective diffusion of renewable energy sources in new installation contexts, e.g. the urban environment. In these new installations, however, the working conditions can be far from the nominal one. In particular, the turbine functioning can be greatly affected by accidental misalignments between the oncoming flow and the axis of the rotor. In these conditions, the performance of conventional horizontal-axis wind turbines is constantly decreased; on the other hand, recent studies suggested that H-Darrieus turbines can improve their performance in a wide range of skew angles. In this work, the main results of an extensive experimental campaign in a wind tunnel on a full-scale model of an H-Darrieus with three straight blades are presented and discussed. In particular, the effects on the power output of the rotor of a skew angle of the flow up to 40° were investigated, focusing on both the contribution of the blades' aerodynamics and on the parasitic component coming from the supporting struts. The results were also compared with the theoretical predictions obtained with both a literature model and a specifically developed model for the performance estimation in skewed flow; notable agreement was found between theory and experiments, especially when the new code was applied. The analyses confirmed that H-Darrieus turbines in skewed flow can increase their power coefficient, although the performance enhancement is reduced in case of rotors with a high height/diameter ratio. Moreover, the benefits in these functioning conditions can be maximized by means of airfoil-shaped struts, which work like an equivalent horizontal-axis turbine.