I’ve received a question regarding material selection for wind turbines blades. The reader asked why there is a predominance in the use of composite materials for the blades instead of wood, steel and aluminium and other materials used in the first glorious, pioneering years of wind energy.
Please note that I’m by no mean an expert so the only intention of this post is to give a very general introduction to the subject. This is a very broad topic involving different engineering branches.
In general the 2 design drivers are weight and stiffness.
A blade should be as light as possible for a variety of reasons:
- To lower gravity induced fatigue loads
- To be easily transported and installed
- To have a better performance
However, it should also be stiff (that is, rigid) for several other reasons:
- To withstand loads (both wind loads and gravity loads). Wind loads are function of wind speed and length of the blade, and increase from the root to the tip of the blade. Gravity loads are function of the material density.
- To prevent collision between the blade and the tower under extreme wind
- To prevent instability (local or global buckling) maintaining its shape
For these reasons blade designers try to minimize the mass for assigned stiffness levels – it is to find a balance between aerodynamic and structural requirements.
So we want less weight (that is lower density) and more stiffness.
Stiffness is expressed by the Young’s modulus of the material – basically the relationship between force and deformation. In general blades are very flexible, stronger in the flapwise direction and weaker in the edgewise direction.
And here is the reason for the use of composite materials. For a given Young Modulus, the material with the lower density is the composite (resin plus glass fiber).
You can see graphically this relationship in a type of graphic called “Ashby Plot” (I attach a version stolen online from a document of the University of Cagliari.