Wind tunnel testing of small Vertical-Axis Wind Turbines for urban areas

The implementation of small vertical-axis wind turbines (VAWTs) in urban environments is being studied by the scientific community to complement large wind farms in wind energy generation. The technology is, however, far from maturity due to the complexity of urban flows and the lack of knowledge in the field. This thesis focuses on turbulence, one of the main characteristics of urban flows, and its influence on VAWT performance. Its objective is to generate turbulent wind conditions inside a wind tunnel and testing a VAWT to determine how turbulence intensity (Iu) and integral length scale (Lux) affect its operation. The first part of the research was devoted to obtaining highly-turbulent wind profiles in the wind tunnel with the use of different configurations of square grids. Gathering experience from previous studies and literature, a careful study and validation of this technique was done, in order to obtain uniform wind conditions with the adequate values of turbulence intensity and length scales to model the urban flows. Then, a H-Darrieus VAWT prototype was tested under these turbulent conditions in two different wind tunnels (VUB in Belgium, and CRIACIV in Italy). Those campaigns allowed to evaluate the crossed effect of Iu, Lux and Reynolds numbers on the VAWT performance, and quantify the effect of wind tunnel blockage in the measurements. Further tests with the same set-up included the study of the near wake of the VAWT and the aerodynamic characteristics of the individual blades, always focusing on the effect of incoming turbulence. Finally, a short campaign was done at the large VKI L1-B wind tunnel to observe the effect of a shear flow with high levels of Lux (impossible to model in smaller wind tunnels). The tests on the VAWT revealed an important increase of performance for low to moderate Iu values (5-15%) in low chord-based Reynolds numbers (Rec < 100000), while the effect of Lux was negligible. Iu also proved to cause a positive effect by providing faster wake recovery and by mitigating the negative effect of the turbine shaft. The study of the effect of turbulence on the aerodynamic coefficients of different typical VAWT aerofoils showed that turbulence considerably delays the stall in the blades, which was linked the increase of turbine performance. The tests at VKI proved that this positive effect of turbulence is also evident in shear flows. The possibility of testing in three different wind tunnels strengthens the results of this study, which have been already used in combination with CFD simulations, and present good agreement with literature. The findings of this thesis contribute to expand the knowledge of how turbulent flows interact with VAWTs, and provide a useful insight for the optimization and combination of several VAWTs inside urban environments.