PSI - Issue 48

Muhammad Rizky Arga Wijaya et al. / Procedia Structural Integrity 48 (2023) 41–49 Wijaya et al. / Structural Integrity Procedia 00 (2019) 000–000

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Wind turbine blades work like the concept of an airplane wing by generating lift due to their shape. The more curved side causes low air pressure, while the high-pressure air pushes against the other side of the airfoil. The result is a lift force perpendicular to the direction of airflow. The lift force increases when the blade is rotated at a greater angle to the wind. This is called the angle of attack, as shown in Figure 2 (a). At huge angles of attack, the blade “stalls,” and the lift decreases again, as shown in Figure 2 (b). Therefore, there is an optimum angle of attack to produce maximum lift.

(a)

(b)

(c) Fig. 2. Illustration of wind turbine blade geometries: (a) angle of attack; (b) stall; and (c) apparent wind.

There is a drag force on the propeller. This force is parallel to the wind flow. If the airfoil shape is good, the lift is much greater than the drag, but at high angles of attack, when the propeller is stopped, the drag increases dramatically. Thus, the propeller will achieve the maximum lift/drag ratio at angles less than the maximum lift angle. Since the drag is in the downwind direction, this is not a problem for the wind turbine as the drag will be parallel to the turbine axis, so it will not slow down the rotor. It will only create "thrust." Forces acting parallel to the turbine axis do not tend to accelerate or decelerate the rotor. However, air movement past the blades means the wind is blowing from a different angle. This is so-called the apparent wind, as shown in Figure 2 (c). The apparent wind is stronger than the true wind, but the angle is less favourable. It rotates the angles of lift and drags to reduce the effect of the lift force that pulls the propeller around and increase the effect of the drag force that slows it down. This also means that the lift force contributes to the thrust on the rotor. 3. Material aspect in wind turbine The turbine blade material plays a vital role in wind turbines. Wind turbines are highly dependent on the efficiency of the blades' material, the blades' shape, and the blades' angle. The blade material should have high stiffness, low density, and extended fatigue life features (Thomas and Ramachandra, 2018). While many materials have been used successfully for wind turbine blades, blade designs are primarily based on fibreglass composites. The increasing rotor size of current designs, with its changing design drivers, is leading designers to examine other materials and use them innovatively. Nowadays, many wind turbine manufacturers are taking a considerable interest in composite materials, which many researchers of wind technology see as the materials of the future. The main