Although there are many types of wind turbines, their basic principles and structures are similar. Given that horizontal axis wind turbines are currently the most widely used and the technology is the most mature, the structure of horizontal axis wind turbines is mainly introduced in detail. Horizontal-axis wind turbines mainly include wind wheels (including blades and hubs), yaw systems (including wind direction sensor, yaw drive device, yaw bearing, yaw hydraulic circuit, yaw counter, torsion cable protection device, yaw brake), transmission mechanisms (including low speed shaft, gear box, coupling, high speed shaft and brake mechanism), speed control devices, power generation systems, towers, etc., as shown in Figure 1.
1. Wind wheel
The wind wheel, also called the impeller, is the core component of the wind turbine. Its cost is about 20% to 30% of the total cost, and it has a design life of at least 20 years. The wind wheel is mainly composed of blades and a hub. The blades are mounted on the hub to convert wind energy into rotating mechanical energy. The geometric parameters of the wind wheel mainly include the number of blades, the diameter of the wind wheel, the height of the wind wheel center and the wind wheel elevation angle, as shown in Figure 2.
Generally speaking, the greater the number of blades, the greater the wind energy utilization coefficient, the greater the output torque of the wind turbine, and the lower the start-up wind speed of the wind turbine. However, the more complex the hub design, the greater the manufacturing cost. Because the three-blade wind wheel can provide the best efficiency, the force is more balanced, the hub design can be simpler and more beautiful, so modern wind turbines often use three-blade wind wheels.
The diameter of the wind wheel refers to the diameter of the projected circle of the wind wheel on the plane of rotation. The power of the wind wheel depends on the diameter of the wind wheel. Generally, the larger the diameter, the larger the sweeping area and the greater the power of the wind wheel. The height of the center of the wind wheel refers to the vertical distance from the center of rotation of the wind wheel to the base plane. In theory, the higher the height of the wind wheel center, the greater the wind speed, but the corresponding increase in the tower height, the greater the tower cost and installation difficulty. The wind wheel elevation angle refers to the angle between the rotation axis of the wind wheel and the horizontal plane, and its function is to avoid the collision of the blade tip with the tower.
(1) Blade. The blade is the main component that receives wind energy, and its material must be high in strength, light in weight, and stable in physical and chemical properties in harsh environments. In practice, the blades are made of wood, aluminum alloy, stainless steel, glass fiber resin-based composite materials (glass reinforced plastic), carbon fiber resin-based composite materials, etc. The blade is shown in Figure 3. For blades, lightning protection should also be fully considered in the design stage. Blades are the most susceptible parts of wind turbines to direct lightning strikes. Therefore, it is necessary to reliably guide the lightning from the hub to avoid damage to the blade due to lightning strikes.
(2) Wheel hub. The hub is the connection between the root of the blade and the main shaft, as shown in Figure 4. It bears the thrust, bending moment, torque and gyro moment exerted on the blade by the wind, and then transmits the force of the wind wheel to the rear-stage transmission mechanism or the tower. Usually the shape of the wheel hub is trigeminal or spherical.
2. Transmission mechanism
The mechanical energy generated by the wind wheel is transmitted to the generator by the transmission mechanism in the nacelle. Transmission mechanisms generally include low-speed shafts, gearboxes, couplings, high-speed shafts, and brake mechanisms (which can stop the wind turbine in an emergency), etc., as shown in Figure 5. Since the speed of the wind wheel is very low, it is necessary to increase the speed of the wind wheel from 20~50r/min to 1000~1500r/min through a gear box to reach the speed required to drive most generators, so the gear box is also called the speed increasing box. Figure 6 shows the structure of the gearbox. The brake mechanism is composed of a brake disc installed on a low-speed shaft or a high-speed shaft and hydraulic clamps arranged around. In order to monitor the internal state of the brake mechanism, a temperature sensor and a sensor indicating the thickness of the brake pad are installed in the brake caliper.