Abstract: A rotor apparatus for extracting energy from bidirectional fluid flows comprises a first rotor (7) mounted for rotation about an axis of rotation (4) in a first direction of rotation, the first rotor (7) having at least one helical blade (2) with a pitch that decreases in a direction along the axis of rotation (4); and a second rotor (8) mounted for rotation about the same axis of rotation (4) in an opposite direction of rotation and having at least one helical blade (2) with a pitch that increases in the same direction along the axis of rotation (4), wherein fluid exiting the first rotor (7) is passed to the second rotor (8).

Abstract: A rotor apparatus for extracting energy from bidirectional fluid flows comprises a first rotor (7) mounted for rotation about an axis of rotation (4) in a first direction of rotation, the first rotor (7) having at least one helical blade (2) with a pitch that decreases in a direction along the axis of rotation (4); and a second rotor (8) mounted for rotation about the same axis of rotation (4) in an opposite direction of rotation and having at least one helical blade (2) with a pitch that increases in the same direction along the axis of rotation (4), wherein fluid exiting the first rotor (7) is passed to the second rotor (8).