Wind Energy

43
Aerodynamic Behaviour of a New Type
of Slow-Running VAWT
J.-L. Menet
43.1 Introduction
Wind turbines are dimensioned for a nominal running point, i.e. for a given
wind velocity. In most cases, principally because of their higher efficiency, twoor
three-bladed fast running wind turbines are preferred [1].
Although it has undeniably high performances, this type of wind-powered
engine is not necessarily the one which enables the extraction of the maximal
energy from a wind site. Using a special method, namely the L–σ criterion,
Menet et al. [2] have shown that a slow-running vertical axis wind turbine
(VAWT), such as the Savonius rotor [3], can extract more energy than fast
running wind machines.
This idea seems to be in contradiction to the general literature in the field:
the Savonius rotors have an aerodynamic behaviour where the characteristics
of a drag device dominate, which clearly induces a low efficiency. In fact,
using the same intercepted front width of wind L and the same value σ of
the maximal mechanical stress on the paddles or the blades, the reference [2]
clearly indicates that the delivered power of a Savonius rotor is superior to
the one of any fast-running horizontal axis wind turbine. Besides, because of
its high starting torque, a Savonius rotor can theoretically produce energy
at low wind velocities, and because of its low angular velocity, it can deliver
electricity under high wind velocities, when fast running wind turbines must
generally be stopped [1, 4].
The main disadvantage of the Savonius rotor is the great instability of
the mechanical torque because the flow inside the rotor is non-stationary.
Nevertheless, the advantages of such a wind turbine are numerous [4]. For a
few years, many studies have led to raise performances of these machines [5–7].
Here is the purpose of our contribution to the study.