The present invention relates to a method of increasing the output of a wind turbine, wherein one or more flow surfaces are placed at a wind turbine. At lower wind speeds, the flow surfaces are set to guide the wind up towards and/or out from the rotor of the wind turbine and hence use the venturi effect in the rotor to advantage. At higher wind speeds, the flow surfaces are completely or partially taken out of the flow of the wind. Moreover, the flow surfaces can be adjusted around the wind turbine based on measurements of the speed of the wind and direction of the wind and/or the power signal of the wind turbine. The invention further relates to a system comprising one or more flow surfaces and means for adjusting same to the effect that the output of a wind turbine can be maximized by using the venturi effect to advantage as described above.