The invention relates to an installation for obtaining mechanical energy from solar thermal energy, combining a conventional solar thermal installation connected to heat exchangers housed inside an externally-insulated vertical shell having an upwardly decreasing cross-section, in which an ascending air flow is generated in order to move a turbine housed at the upper end of the shell. In order to make better use of the wind at the installation a retractable orientable screen can also be installed at the point of the nozzle at which the external air current enters the shell, said screen being automatically oriented, by means of sensors, so as to maximise the flow of air entering the system using the natural currents.