Wind power generation
Wind energy is a form of energy formed by atmospheric movement, and its energy comes from solar energy absorbed by the atmosphere. About 20% of the energy radiated by the sun to the earth is converted into wind energy. It has been preliminarily proven that the onshore wind energy resources in China’s 10m low-altitude range are about 253 million kW, and the offshore wind energy is about 750 million kW, totaling about 1 billion kW. If it is extended to a height above 50-60m, wind energy resources will at least double. Wind energy resources are mainly distributed in the southeast coast and nearby islands, Xinjiang, Gansu Corridor, Inner Mongolia, Northeast, Northwest, North China and Qinghai-Tibet Plateau. The annual wind speed is above 3m/s (according to the current wind turbine technology, the wind speed reaches 3m/s, and it can generate electricity) for nearly 4000h, and the average annual wind speed in some areas can reach 6-7m/s or more, which has great development and utilization value.
A wind turbine is an energy conversion device that converts wind energy into electrical energy, and it mainly includes two components: a wind turbine and a generator. The flowing air acts on the wind wheel of the wind turbine, pushing the wind wheel to rotate, and converting the kinetic energy of the air into the rotating mechanical energy of the wind wheel. The hub of the wind wheel is fixed to the wind turbine shaft, and the generator shaft and the rotor are driven to rotate through a transmission mechanism. The generator converts mechanical energy into electrical energy and transmits it to the power system or load. Figure 1 shows the working process of wind power generation.
Wind turbine classification
According to the different structures and technical solutions adopted by the two major parts of wind turbines and generators, wind turbines can be classified from multiple angles.
1. Wind power operation mode
From the classification of the operation mode of wind power generation, it can be divided into independent operation mode and grid-connected operation mode. Independently operating wind turbines are not connected to the power grid and have a relatively simple structure. They supply power to households or villages separately. It is mostly used in remote rural areas, pastoral areas and islands and other areas that are inconvenient to connect to the power supply. It can also be operated in conjunction with diesel generators and photovoltaic power generation to provide residents with electricity for life and production. Early small-capacity wind power generation equipment generally used small DC generators to supply power to resistive loads (such as lights) through battery energy storage devices. At present, AC generators are mostly used. After the output power is passed through the rectifier, the controller charges the battery and drives the DC load at the same time, as shown in Figure 2. If supplying power to AC load, an inverter needs to be added after the controller, as shown in Figure 3.
The grid-connected operation mode is that the wind generator set is connected to the power grid and transmits electric energy to the grid, so that users on the grid can enjoy green energy, which is mostly used in large and medium-sized wind power generation systems.
2. The relative position of the main axis and the ground
According to the relative position of the rotating main shaft of the wind turbine and the ground, it can be divided into horizontal axis wind turbine and vertical axis wind turbine. The horizontal axis wind turbine is a type widely used at home and abroad, and the rotation axis of the wind wheel is parallel to the ground, as shown in Figure 4. The main advantage is that the wind wheel can be installed higher than the ground, reducing the impact of ground disturbance on the dynamic characteristics of the wind wheel, and as the height increases, the power generation increases. The disadvantage is that the airfoil design and wind wheel manufacturing are more complicated, and the main mechanical parts are installed high in the sky, which is inconvenient to disassemble large parts.
According to the relative position of the wind wheel and the tower, the horizontal axis wind turbine can be divided into two types: upwind and downwind. The wind wheel of the upwind unit rotates in front of the tower. This type of model requires an active direction adjustment mechanism to ensure that the wind wheel is aligned with the wind direction. The wind wheel of the downwind unit is installed behind the tower, and the wind first passes through the tower and then reaches the wind wheel. The downwind unit can automatically align the wind direction, eliminating the need for direction adjustment devices. However, because part of the air passes through the tower and then blows to the wind wheel, the tower interferes with the airflow to the blades, forming the so-called tower shadow effect, which affects the output of the wind turbine and reduces its performance. Therefore, most of the current wind turbines are of the wind direction type on the horizontal axis.
The rotation axis of the wind wheel of the vertical axis wind turbine is perpendicular to the ground or the direction of the airflow, as shown in Figure 5. Its main advantage is that it can absorb wind from any direction. When the wind direction changes, there is no need to face the wind, so no direction adjustment mechanism is needed, which simplifies the structural design. The gearbox and generator can be installed on the ground, which is easy to maintain.