Wind Generators

Basics, Working Principle

Basics regarding wind generators:

according to the Betz Law (German physicist Albert Betz, 1885-1968) the maximum energy to be harvested, from the wind energy passing a rotor, and transformed into usable energy, is 16/27 (59%) of the energy contained by the wind.
The percentage of this total capacity, which is extracted by the wind generator from the wind, is called coefficient.

The “ideal” wind generator would have a coefficient of 0.59 (=16/27, which means that it extracts 59% of the energy contained by the wind).

Based on the aerodynamic type, there are wind buoyant and resistance rotors.

The wind buoyant rotors have blades formed in a typical wing profile (airplane) and work like a sailboat, which sails close to the wind. As a result of the buoyancy effect, the rotor blade is moving faster than the wind.
An important figure is the tip speed ratio (=the ratio of the peripheral speed of the rotor to the speed of the wind).
The tip speed ratio of common wind generators are presented by the attached graphic.
Real wind buoyant rotors currently achieve tip speed ratios of approx. 8, the best coefficients of up to 0.52 (= 52%).
A tip speed ratio of 8 means that in case of wind speeds of 50 km/h, the tip of the rotor is travelling at 400 km/h!
This is also the cause for known problems: noise, vibrations and complex security systems against an overload during storms.

The resistance rotors use the resistance of a body in the wind. They use slow rotations, the tip speed ratio is always below 1 – they are turning slower than the wind. The coefficient of the current resistance rotors is, however, only 0,19. They usually have a technically simple structure, do not generate noise, are resistant to turbulences and secure regarding overload.