An ORC apparatus for generation of electric 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; - an electric generator, the expansion turbine being connected to the electric generator; - 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; characterised in that the expansion turbine is of the radial-outflow type wherein, in a way between an inlet and an outlet of the expansion turbine, the working fluid flow moves away, while expanding, from a rotation axis (X-X) of said expansion turbine; wherein the expansion turbine comprises a fixed box having an axial inlet and a radially peripheral outlet, only one rotor disc, mounted in the fixed box and rotating about a rotation axis (X-X), 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 fixed box, facing the rotor disc and disposed around the rotation axis (X-X); wherein the expansion turbine comprises a baffle fixedly mounted on the fixed box at the axial inlet and adapted to radially deviate the axial flow towards the first series of stator blades; wherein the expansion turbine is a multistage turbine; wherein the expansion turbine comprises 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; wherein the baffle has a convex surface facing the axial inlet; wherein the baffle carries the first series of stator blades at a radially peripheral portion thereof; wherein the front face of the rotor disc and the face of the fixed box carrying the stator blades (24a, 24b, 24c) diverge from each other on moving away from the rotation axis (X-X) and the radially outermost blades have a blade height greater than that of the radially innermost blades; wherein the fixed box is formed with a front box half of circular shape and a rear box half joined together by bolts; wherein a sleeve emerges in cantilevered fashion from the rear box half; wherein in an inner volume delimited by the front and rear box halves the rotor disc is housed which is rigidly constrained to a shaft in turn rotatably supported in the sleeve by means of bearings so that it is free to rotate around the rotation axis (X-X). - 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; - an electric generator, the expansion turbine being connected to the electric generator; - 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;