An apparatus for generating electricity from water flow comprises a convergent section, a diffuser section and tube for locating within a body of water. The convergent section is connected to a first end of a mixing chamber such that a venturi is defined between the end of the convergent section and the mixing chamber. The diffuser section is connected to a second end of the mixing chamber, the diffuser section configured such that in use the pressure at the exit of the diffuser section is greater than the pressure at the venturi. At least part of the tube is located in the convergent section, such that an annulus is defined between the tube and the convergent section, to form a first flow passage. The tube defines a second flow passage within the tube; and a turbine connectable to a generator is located within the tube.