**6. Conclusions**

In this study, different techniques and data types were combined together to prove that two main physical phenomena concur in defining the performance increase (testified by unique experimental wind tunnel data reproducing realistic turbulence features) of small Darrieus turbines in turbulent flows.

The first one, assessed by means of detailed CFD simulations of the rotor, is the higher energy content in the turbulent wind, which induces a slight power surplus. This is, however, limited by the fact that the turbine constantly operates at a non-optimal tip-speed ratio, depending on the flow macrostructures that enter the turbine instant by instant. Overall, an equivalent wind speed can be defined.

The second one—which is thought of major relevance in case of small turbines—is the improved response of the airfoils in terms of delayed stall angle and increased lift-to-drag ratio. This second phenomenon has been verified with dedicated experimental tests in the wind tunnel, which also allowed for the calibration of a CFD tool to virtually replicate the polars.

Finally, the two elements have been combined into a state-of-the-art BEM code, which was able —despite its simplicity—to nicely predict the turbine behavior, thus suggesting that the two highlighted phenomena are really playing a major role in defining the aerodynamic behavior and energy conversion capability of small Darrieus vertical axis wind turbines in turbulent flows.

Future work will be devoted to providing an on-field validation to prove the feasibility of small Darrieus VAWTs in turbulent sites. In particular, gaining a better understanding of the discussed phenomena could lead in the near future to design strategies for small rotors specifically tailored to maximize the performance in the turbulent flows that are typical, for example, of the urban environment.

**Author Contributions:** Conceptualization: A.B. and F.B.; Methodology: A.B., F.B. and M.Z.; Software: A.B. and F.B.; Experiments: A.C.M., T.D.T. and G.B.; Validation: A.C.M.; Formal Analysis: A.B.; Investigation: A.B., F.B. and M.Z.; Resources: M.C.R., G.B. and G.F.; Data Curation: A.C.M., M.Z. and F.B.; Writing—Original Draft Preparation: M.Z., F.B. and A.B.; Writing—Review and Editing: T.D.T.; Visualization: F.B.; Supervision: T.D.T., M.C.R., G.B., G.F. and A.B.; Project Administration: M.C.R., G.B. and G.F.; Funding Acquisition: T.D.T., M.C.R. and G.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** The experimental campaigns are part of the European Innovative Training Network (ITN) AEOLUS4FUTURE "Efficient Harvesting of the Wind Energy". The project is funded by the Horizon 2020 Research and Innovation program under the Marie Skłodowska-Curie grant agreement no. 643167.

**Acknowledgments:** Thanks are due to all the staff of the CRIACIV wind tunnel in Prato for the support during the wind tunnel tests of the turbine. The authors would like also to acknowledge the VUB Bachelor student Julen Echeverria for the help in the construction of the airfoil model for polars in turbulence.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


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