A device for recovering kinetic energy, eg from tidal or river currents or from wind, comprises a rotatable frame 1 supporting hydrofoils or aerofoils. The foils have a leading edge and a trailing edge and are arranged to maintain their leading edge/ trailing edge orientation with respect to a direction of fluid flow and to change their profiles twice per rotation of the frame. The forces generated as fluid passes over the foils act on the frame to cause it to rotate. The foils can be oriented through gearing, each foil being rotated about an axis A. Alternatively, the profile of a foil 50 may be adjusted by inflating a membrane 55 surrounding a central plate 51, 52; by transferring fluid from one side of the central plate to the other the foil changes shape at the top and at the bottom of each rotational cycle. In a modification the profile is changed by a series of parallel semi-cylindrical tubes rotated in unison inside the membrane 55. The device may be fully submerged.
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