Valve system for controlling the fluid flow in a duct, comprising: a substantially hollow valve body provided with an inlet and an outlet which allow a flow of a fluid in said valve body, said valve body comprising: an obstructer rotatable, about a rotation axis, between: a second position, in which said valve system is closed, wherein said obstructer stops the passage of fluid between the inlet and the outlet of the valve body, and a first position, in which said valve system is partially or totally open, wherein said obstructer allows a passage of fluid between the inlet and the outlet of said valve body; actuation means adapted to rotate said obstructer between said first position and said second position, a rotor shaped substantially as a turbine, placed inside the valve body and provided with a rotation shaft, the rotation shaft being substantially superposed on or parallel to the axis of rotation of the obstructer, said rotor being adapted to be rotated by said fluid flow; characterized in that said valve body further comprises: a housing, arranged on a lateral portion of the valve body, said housing housing said obstructer in said second position; wherein said housing is shaped substantially as a spherical segment, wherein said obstructer has a spherical-segment outer contour which, during opening and closing movements, adheres to an inner edge of the housing of the valve body; wherein, in the first position, the spherical-segment outer contour of the obstructer is upstream of the rotation axis with respect to the direction of the fluid flow; wherein, in the second position, said obstructer is so arranged to fill the housing, leaving a lateral part of the valve body empty, the lateral part being opposite to the housing. an obstructer rotatable, about a rotation axis, between: a second position, in which said valve system is closed, wherein said obstructer stops the passage of fluid between the inlet and the outlet of the valve body, and a first position, in which said valve system is partially or totally open, wherein said obstructer allows a passage of fluid between the inlet and the outlet of said valve body; a second position, in which said valve system is closed, wherein said obstructer stops the passage of fluid between the inlet and the outlet of the valve body, and a first position, in which said valve system is partially or totally open, wherein said obstructer allows a passage of fluid between the inlet and the outlet of said valve body; actuation means adapted to rotate said obstructer between said first position and said second position, a rotor shaped substantially as a turbine, placed inside the valve body and provided with a rotation shaft, the rotation shaft being substantially superposed on or parallel to the axis of rotation of the obstructer, said rotor being adapted to be rotated by said fluid flow; characterized in that said valve body further comprises: a housing, arranged on a lateral portion of the valve body, said housing housing said obstructer in said second position; wherein said housing is shaped substantially as a spherical segment, wherein said obstructer has a spherical-segment outer contour which, during opening and closing movements, adheres to an inner edge of the housing of the valve body; wherein, in the first position, the spherical-segment outer contour of the obstructer is upstream of the rotation axis with respect to the direction of the fluid flow; wherein, in the second position, said obstructer is so arranged to fill the housing, leaving a lateral part of the valve body empty, the lateral part being opposite to the housing. a housing, arranged on a lateral portion of the valve body, said housing housing said obstructer in said second position; wherein said housing is shaped substantially as a spherical segment, wherein said obstructer has a spherical-segment outer contour which, during opening and closing movements, adheres to an inner edge of the housing of the valve body; wherein, in the first position, the spherical-segment outer contour of the obstructer is upstream of the rotation axis with respect to the direction of the fluid flow; wherein, in the second position, said obstructer is so arranged to fill the housing, leaving a lateral part of the valve body empty, the lateral part being opposite to the housing.