After understanding the basic types of wind turbines, we then analyze their working principles. From the perspective of physics, its working principle is: when the wind blows on the wheel plane of the wind turbine, the wind wheel will be thrust by the wind, and then torque will be formed, and the torque will promote the rotation of the wind wheel, thereby converting the wind energy into the mechanical energy of the wind wheel.
Drag and lift
The force that an object receives in the airflow comes from the action of air on it. The force from the airflow on the object can be equally decomposed into two directions that are consistent with the direction of the airflow and perpendicular. The forces decomposed into the two directions are called drag and lift respectively.
As shown in Figure 1, the direction pointed to is the direction of the air flow, and the shaded part represents the cross section of the blade of the wind J machine, and the force from the air flow is F. If the force F is decomposed into two components, the component FD that is the same as the airflow direction is called drag, and the component F that is the same as the airflow direction is called lift.
17Figure 1 – The force of the airflow acting on the object
From the above analysis, it can be seen that the resistance is the component on the same line as the airflow. When the direction of the airflow is perpendicular to the surface of the blade, the blade receives the greatest resistance. Lift is a component perpendicular to the direction of airflow. In aerodynamics, this component can cause the aircraft to fly off the ground, so it is called lift. In practical applications, the lift force may also be a lateral force or a downward force. When the angle between the blades of the object and the direction of the airflow is zero, the lift is the smallest.
Studies have shown that the pressure of air has a certain corresponding relationship with the speed of airflow. The faster the flow rate, the lower the pressure. This phenomenon is called the Bernoulli effect. Next, analyze the Bernoulli effect of the blade as shown in Figure 2. When the angle between its surface and the airflow direction is small, a low pressure area will be formed in the downstream or downwind direction due to the change of the airflow velocity. The lift force acts as “inhalation” or “upward lift” in the vertical direction of the airflow.
17Figure 2 – Bernoulli effect of wind turbine blades
Further research has shown that lift and drag are proportional to the intensity of wind energy. The blades of a wind turbine in the wind rotate the wind wheel and output mechanical power on its shaft under the action of lift, drag, or both.
Airfoil, angle of attack and pitch
In wind power generation systems, most wind turbines use blade designs similar to airplane propellers, and the blades that resemble airplane wings are called airfoil. There are two main types of airfoils: one is an asymmetric airfoil and the other is a symmetric airfoil. Both of these airfoils have a clearly convex upper surface, a rounded head called the leading edge (facing the direction of incoming flow) of the wing, and a pointed or sharp tail called the trailing edge of the wing. Figure 3 shows the approximate cross-sectional shape of the most common asymmetric airfoil.
17Figure 3 – Cross section of common asymmetric airfoil
Since the airfoil of wind turbine blades is mostly not straight, but has a certain curve or protrusion, the chord line is usually used as the guideline for measurement. The angle between the airflow direction and the airfoil alignment is called the angle of attack, which is represented by the Greek letter α (see Figure 1). When the incoming flow is toward the underside of the airfoil, the angle of attack is positive.
In the practice of wind turbine design, obtaining proper lift or drag to push the wind turbine to rotate is the fundamental purpose of airfoil design. For the airfoil as shown in Figure 3, the air flow on the convex part of the upper surface is faster, causing the air pressure on the upper surface to be significantly lower than that on the lower surface, thereby causing upward “suction” on the airfoil object. As shown in Figure 4, the relationship between the lift coefficient and drag coefficient and the airfoil angle of attack is given. Generally, the angle of attack is related to the installation angle of the blade. When the wind wheel rotates, the blades also move relative to the airflow in the direction perpendicular to the airflow, so the actual angle of attack α is different from the angle of attack when the blades are stationary.
17Figure 4 – The relationship between lift coefficient and drag coefficient and airfoil angle of attack
As shown in Figure 5, suppose that the x-axis is the direction of airflow, and the y-axis rotates around the origin of the coordinates as the rotating surface of the wind wheel. The angle θ between the alignment of the blade and the rotating surface of the wind wheel is called the blade installation angle, that is, the pitch or pitch angle. The y-axis direction indicates the direction of movement of a certain cross-section of the blade when the wind wheel rotates. If the rotating blade is used as the reference system, there is a relative movement between the airflow and the blade opposite to the y-axis direction. Considering the actual movement of the airflow along the x-axis direction, the action direction of the airflow relative to the moving blade is represented by Wr. Therefore, for the same horizontal wind, the angle of attack when the blade is rotating is different from the angle of attack when the blade is stationary.