A device for converting kinetic energy of a flowing medium to electrical energy includes a rotor for placing in the flowing medium and a generator connected to the rotor. The rotor includes a tube with one or more vanes mounted on the inner side of the tube and extending radially to the centre thereof, wherein the tube is mounted for rotation about a horizontal axis. A length of the tube in horizontal direction amounts here to at least 25% of a diameter of the tube in vertical direction. An outflow part diverging in the flow direction can connect to a rear edge of the tube as seen in flow direction of the medium. The tube can be bearing-mounted in a frame via a central shaft mounted on the inner ends of the vanes. The frame can on the other hand include an outer bearing, for instance a stator tube.