Wind power generation

In 1891, the Danish Poul LaCour designed and built the world’s first wind power test station. After more than a century of development, wind power has now become the most important form of wind energy utilization. The process of wind power generation is the process of converting the kinetic energy of the wind into mechanical energy and then into electrical energy. As shown in Figure 1, a simplified wind generator is mainly composed of blades and generators. The rotating shaft of the impeller where the blades are located and the rotating shaft of the generator can be connected by a connecting device. When the wind speed reaches a certain requirement, it acts on the blades to drive the impeller to rotate, converting kinetic energy into mechanical energy. If the impeller shaft is connected to the generator shaft at this time, the rotation of the impeller will drive the generator to rotate, thereby generating electrical energy.

Wind power generation
Figure 1 – Schematic diagram of wind power generation principle

The natural characteristics of wind energy resources and the technical characteristics of wind power generation itself bring many significant characteristics that distinguish wind power generation from other power generation technologies.

1. Volatility of wind power

The volatility of wind power is caused by the volatility of wind energy resources. Wind energy is energy generated by the flow of air. Wind power generators use wind energy close to the ground to generate electricity. Due to the randomness and instability of natural wind, it also brings volatility to wind power generation. For example, wind gusts in the sub-second to minute period can cause turbulence peaks, which can cause short-term power generation fluctuations; the daily peak value brought about by the change of daily wind speed, as well as the weather peak value brought about by the weather and seasonal cycle, will have an impact on the long-term power balance of the power system.

2. Incomplete utilization of wind energy resources

After the wind energy is converted into the kinetic energy of the rotating machinery of the wind wheel, the moving speed of the air mass is reduced, but it is impossible to completely stop in the rotating surface, and the wind energy cannot be fully utilized by the wind turbine. According to the Betz theory proposed by the German physicist Betz, the maximum efficiency of converting wind energy into mechanical energy is 59.3%. If the efficiency of converting mechanical energy into electrical energy is considered, the actual utilization rate of wind energy will be reduced.

3. Hysteresis and tangential phenomena

Under normal circumstances, when the wind speed is 12~16m/s, the wind turbine reaches the rated output power (this value is different according to different wind turbine designs). If the wind speed reaches 20~25m/s, in order to ensure safety, the wind turbine will be out of operation. The wind speed that makes the wind turbine out of operation is called the cut-out wind speed. When there is a big storm, the wind turbine will often appear unexpectedly. When the wind speed drops below the cut-out wind speed, the wind turbines that exit operation often require a period of delay before restarting operation. Restarting the wind turbine after it is cut out is called hysteresis start.

For the power system, the hysteresis and cut phenomenon of wind turbines may suddenly cause a large amount of wind power loss and bring unreliability to the power system. At present, some manufacturers have made improvements to wind turbines so that the output power is gradually reduced with the increase of wind speed, rather than suddenly cut off, to a certain extent, to reduce the negative impact of excessive wind speed on the power system.

4. Concentration effect

With the increase in the number of wind turbines in wind farms and the expansion of the distribution area of wind farms, the quality of the output power of wind farms will be greatly improved, and it will also have a positive impact on the operation of the power system, showing a certain concentration effect. Increasing the number of wind turbines can reduce the impact of turbulence peaks. Short-term strong winds will not impact all wind turbines at the same time, and the output power variation of the entire wind farm will decrease as the number of wind turbines increases. Expanding the distribution area of wind farms, weather changes cannot affect all wind turbines at the same time, which can greatly reduce the impact of daily peaks and weather peaks.

5. Utilization time is less than that of traditional power plants

According to the types of wind resources and wind turbines, the annual utilization of wind farms is approximately 2,200 to 3,500 hours. If the capacity of the power grid is considered, the annual utilization of wind farms will be reduced, and the utilization time of wind farms will be much shorter than that of traditional power plants such as thermal power plants and hydropower plants. For example, the average annual utilization of power generation resources in my country in 2011 was 5294h for thermal power, 3028h for hydropower, 7772h for nuclear power, and 1920h for wind power. The average annual utilization of power generation resources in 2012 was 4,965h for thermal power, 3555h for hydropower, 7838h for nuclear power, and 1890h for wind power. Wind farms use less time, which means that if they want to obtain the same electric energy as thermal power, nuclear power and other power plants, the installed capacity of wind farms is much larger than that of traditional power plants.

6. Other

Wind power also has some other characteristics, such as clean energy, does not consume fossil resources, and does not pollute the environment; the construction period is short, and the construction period of a 10MW wind farm is less than one year; the stand-alone capacity is small, the installed capacity is flexible, and it can be flexibly configured according to funds and actual needs; operation and maintenance are simple, and it can work normally without being on duty, without major repairs, and only need to perform necessary maintenance on a regular basis; there are diversified power generation methods, which can be connected to the grid, or form a complementary system with diesel power generation, photovoltaic power generation, and hydropower generation, and operate independently.