First things first. In this post I will have a look at the basic structural requirements of a wind turbine blade, along with the most important material properties one has to keep in mind when selecting blade materials.
A wind turbine generates power by using the energy in the airflow to rotate its impeller, which axis is connected to a generator. The rotor starts rotating because the wind exerts a force on the blade, which is known as the lift force. On the other hand, it also exerts a drag force in the direction of the axis of the turbine, taking away part of the convertible power. With a smart blade shape this effect can be reduced and we all know the lift force is enough to lift a whole airplane up in the sky. Based on the same principle, the form of a wind turbine blade will be more or less the same as one of a wing of an airplane. Yet, there are some changes needed due to some particularities in the case of a wind turbine.
General blade shape
First of all the total length of the blade is an important geometrical parameter to take in consideration. It determines the “swept area” (the area covered by the rotating blade) and thus the area in which it can extract energy from the wind. The section of the blade looks more or less like the wing of an airplane which is optimised to create a great lift force and reduce drag force. An important difference with airplanes is that the blades perform a rotational movement instead of staying at the same position in the the air flow. This means that, in the view point of the blades, the wind is blowing from another angle. This is why they use the term “apparent wind” to denote the air flow under this angle. The blade must be turned in the right way to the airflow to have a good angle of attack. Because the tip of the blade is moving faster than the blade closer to the axis, its apparent wind angle is larger and so a larger twist of the blades is needed at the tips. Another result of the higher tip speed is that it generates more lift (lift is proportional to the square of the wind velocity). To load the blade uniformly it should be narrower at the tips than near the root.
The structural design of a component always starts with identifying the determining forces. In the case of a turbine blade bending is the most important load because of the lift forces and to a lesser extent the weight of the blade. In engineering it’s well known that the material in the middle of a bending beam doesn’t carry much tension and stress, whereas the outside layers take up a very big deal of it. That’s why we can leave some of the material in the middle resulting in two thin layers at the top and the bottom, connected with a vertical part. Everyone knows this as the I-beam, widely used in construction. A similar idea is used for a wind blade: unidirectional fibres are used as the outside layers (called “spar caps”) and they are connected by a shear web comprised of diagonal fibres.
Of course we still have to keep the aerodynamic blade form which creates the necessary lift force. This can be done by creating a shell, mostly with a sandwich structure (thin but strong outer layers with a lightweight core). This way, the shell itself doesn’t weight too much but is still stiff enough not to bend. In addition, diagonal fibres can be used to provide resistance against torsional load as well.
The root is the most loaded part of the blade – the bending moment is the biggest because the leverage is largest. It should also be able to be connected to the rotor axis, and is preferably able to pitch so the blade can be positioned best to the air speed at every moment.
Important material properties
One of the most common reasons of failure in any type of machine is fatigue. It’s failure because one or more components have been loaded with a periodically changing stress. This can initiate cracks in the material which will then grow under working tension. It’s quite obvious a wind turbine blade is prone to this kind of problems – since the blade is rotating and thus the direction of the weight is changing periodically compared to the blade. As the wind is everything but continuous, wind blade load is very variable. Fatigue will thus be one of the main material properties to keep in mind.
Beside the fatigue resistance, the blade may not break. It thus must be strong enough. It shouldn’t be too flexible neither. It should be stiff because of two reasons. First of all the blades may not bend till they hit the tower (placing the blades downwind is no option because the tower is creating a serious distortion of the wind flow). The second reason is that the blades should be kept from resonating (vibrating with high deflection). If one makes a light and stiff blade, the natural frequency will be well above the frequency the blades are passing the tower, thus avoiding the risk of the dangerous resonance.
All this are very general design guidelines, which apply to large scale wind turbines. Next time I will discuss how small-scale wind turbine blade design differs.
