Different types of vertical axis wind turbines (VAWT) are presented and compared as regards their thermodynamic performances. A blade with a given airfoil selection and length can be fastened to spokes in several ways giving rise to many configurations. Two models are used in the analysis of Darrieus turbines: the single streamtube momentum model and the free wake vortex model. The former is more suitable to carry out an extensive study because of its short CPU time, the latter, which guarantees the best accuracy, can be used to verify the most significant results. The diagrams of the average coefficient of power and of the power for a given speed of the attacking wind are shown. The work-exchange between the wind flow and the machine is investigated by defining an internal efficiency of the wind turbine.
