The focus of the present research is performance enhancement of a vertical axis, Darrieus type, wind turbine. The academic and industry interest in VAWTs is increasing due to its suitability to urban areas, characterized by high turbulence and low wind speeds. The paper describes experimental work performed on a GOE-222 asymmetrical airfoil intended to be used in a straight bladed Darrieus VAWT. Airfoil characteristics were measured in a wide range of incidence angles and Reynolds numbers, relevant for the operation of a small to medium size wind turbine. A verity of passive flow control (passive porosity, surface roughness) and active flow control techniques (boundary layer suction, pulsing suction) were tested in order to evaluate their effects on the airfoil performance. The measured effects of flow control on the 2D airfoil are integrated into a modified version of a Double Multiple Stream-tube model in order to predict the effects on the performance and efficiency of the turbine. It was found that the improvement of 2D airfoil characteristics can be translated into higher total turbine performances. By the use of AFC it was possible to increase the VAWT maximum efficiency. When AFC optimization included the azimuth and Reynolds number conditioning, the effects could be greatly increased while consuming less energy.