VAWTs tend to come in two main configurations: the Savonius and the Darrieus turbines. In the case of the former, power is generated using momentum transfer (a drag device) and, in the latter, using aerodynamic forces (the lift force on an airfoil). The Savonius in characterized by its high torque, low speed and low efficiency (less than half the Betz limit). The Darrieus rotor is characterized by its high speed and high efficiency (approaching the Betz limit