An ORC apparatus for generation of energy by organic Rankine cycle, comprising: - at least one heat exchanger to exchange heat between a high temperature source and an organic working fluid, so as to heat and evaporate said working fluid; - at least one expansion turbine fed with the vaporised working fluid coming out of the heat exchanger, to make a conversion of the thermal energy present in the working fluid into mechanical energy according to a Rankine cycle; - at least one condenser where the working fluid outflowing from said at least one turbine is condensed and sent to at least one pump; the fluid is then fed to said at least one heat exchanger; - an electric generator, the expansion turbine being connected to the electric generator; wherein the expansion turbine comprises: a fixed box having an axial inlet and a radially peripheral outlet, only one rotor disc, mounted in the box and rotating about a rotation axis (X-X), at least a first radial outflow stage comprising at least one first series of rotor blades mounted on a front face of the rotor disc and disposed around the rotation axis (X-X) and at least one first series of stator blades mounted on the box, facing the rotor disc and disposed around the rotation axis (X-X), at least a second radial outflow stage comprising at least one second series of rotor blades (22b, 22c) disposed at a position radially external to the first series of rotor blades and at least one second series of stator blades (24b, 24c) disposed at a position radially external to the first series of stator blades, characterised in that the expansion turbine comprises at least one axial stage fitted on a radially external perimeter of the rotor disc. - at least one heat exchanger to exchange heat between a high temperature source and an organic working fluid, so as to heat and evaporate said working fluid; - at least one expansion turbine fed with the vaporised working fluid coming out of the heat exchanger, to make a conversion of the thermal energy present in the working fluid into mechanical energy according to a Rankine cycle; - at least one condenser where the working fluid outflowing from said at least one turbine is condensed and sent to at least one pump; the fluid is then fed to said at least one heat exchanger; - an electric generator, the expansion turbine being connected to the electric generator; wherein the expansion turbine comprises: a fixed box having an axial inlet and a radially peripheral outlet, only one rotor disc, mounted in the box and rotating about a rotation axis (X-X), at least a first radial outflow stage comprising at least one first series of rotor blades mounted on a front face of the rotor disc and disposed around the rotation axis (X-X) and at least one first series of stator blades mounted on the box, facing the rotor disc and disposed around the rotation axis (X-X), at least a second radial outflow stage comprising at least one second series of rotor blades (22b, 22c) disposed at a position radially external to the first series of rotor blades and at least one second series of stator blades (24b, 24c) disposed at a position radially external to the first series of stator blades, characterised in that the expansion turbine comprises at least one axial stage fitted on a radially external perimeter of the rotor disc. a fixed box having an axial inlet and a radially peripheral outlet, only one rotor disc, mounted in the box and rotating about a rotation axis (X-X), at least a first radial outflow stage comprising at least one first series of rotor blades mounted on a front face of the rotor disc and disposed around the rotation axis (X-X) and at least one first series of stator blades mounted on the box, facing the rotor disc and disposed around the rotation axis (X-X), at least a second radial outflow stage comprising at least one second series of rotor blades (22b, 22c) disposed at a position radially external to the first series of rotor blades and at least one second series of stator blades (24b, 24c) disposed at a position radially external to the first series of stator blades, characterised in that the expansion turbine comprises at least one axial stage fitted on a radially external perimeter of the rotor disc.