Slow or fast rotational speed
The typical modern three bladed wind turbine can be efficient even if the rotor blades only cover, say, 3% of the swept surface of the rotor because these machines generally have a relatively high rotational speed. This way, the blades - so to speak - quickly move to the place where they are needed to pick up the energy in the wind. Thus, it can be seen that the fewer blades a rotor has, the faster it has to rotate to pick up the same amount of power from the wind.
A key consideration in selecting the number of blades is that the stress in the blade root increases with the number of blades for a turbine given solidity. Thus all other things being equal, increasing tip speed ratio entails decreasing the number of blades. So the rotational speed is also of importance.
There is still one thing that should not be forgotten regarding the rotational speed in combination with the environmental issue of noise. Aerodynamic noise is highly influenced by the rotational speed which makes this also an important consideration in selecting the rotational speed.
Constant or variable rotational speed
Most rotors on grid-connected wind turbines operate at a nearly constant rotational speed, determined by the electrical generator and the gearbox. In some turbines, however, the rotor speed is allowed to vary. The choice of whether the rotor speed is fixed or variable may have some impact on the overall design, although generally in a secondary way.
The choice of either fixed or variable rotational speed influences the performance of the windturbine. This can be illustrated quite good by looking at the construction of the power curves for both options.
look power curves
- Variable vs constant speed: general comparison
- Constant speed layout: topology and notes (induction/synchronous generator)
- Variable speed layout (A): topology and notes (synchronous generator)
- Variable speed layout (B): topology and notes (doubly fed generator)
- Graphical representation (A): P(V) curve for variable speed
- Graphical representation (B): P(ω) curve for fixed speed
- Graphical representation (C): P(V) curve for constant and variable speed