①The formation of wind
Wind is a natural phenomenon. The horizontal movement of air on the earth’s surface is called wind. Wind is the air movement caused by the sun’s thermal radiation in the atmosphere on the earth’s outer surface. The uneven heating of the earth’s surface by solar radiation is the main cause of the formation of wind. The solar radiation on the earth reaches the surface of the earth after passing through the thick atmosphere, everywhere on the earth’s surface (ocean and land: alpine rock and plain soil; desert, wasteland The heat absorption is different from vegetation and forest areas; due to the influence of the earth’s rotation, revolution, seasons, climate changes and the temperature difference between day and night, the heat dissipation conditions of the ground are also different. In areas with a lot of heat dissipation, the air close to the ground is heated and expanded. The pressure decreases to form a low-pressure zone. At this time, the air flows from the high-pressure zone to the low-pressure zone, and wind is generated, which means that the wind energy comes from solar energy.
The differences in topography and landforms, and the influence of the earth’s rotation and revolution, exacerbate the variability of the force and direction of air flow, and make the changes in wind speed and direction more complicated. Simply put, the sun’s radiation causes uneven heating of the earth’s surface, causing uneven pressure distribution in the atmosphere. The air moves in the horizontal direction to form wind, and the formation of wind is the result of air flow.
The difference in atmospheric pressure is the root cause of wind. Due to the uneven pressure distribution in the atmosphere, the air moves in a horizontal direction. The kinetic energy formed by air flow is called wind energy. It is estimated that although only about 2% of the solar energy reaching the earth is converted into wind energy, the total amount is still considerable. The total wind energy on the earth is estimated to be about 2x1017kW, of which the available is about 2x1010kW, which is quite large. It is 10 times the earth’s hydropower, so it can be said that wind energy is an inexhaustible and inexhaustible renewable energy.
②The change of wind
1) Wind changes over time
In a day, the strength of the wind changes randomly. On the ground, the wind is strong during the day, but the wind is small at night: On the contrary, in the sky, the wind is strong at night, and the wind is small during the day. In coastal areas, due to the difference in heat capacity between land and sea, sea breeze (blows from the sea to the land) during the day and land breeze (blows from the land to the sea) at night. In different seasons, the relative positions of the sun and the earth also change, so that there is a seasonal temperature difference on the earth. Therefore, the wind direction and the intensity of the wind will also change seasonally.
2) Wind changes with height
Due to the influence of air viscosity and ground friction, wind speed varies with height, ground flatness, surface roughness accuracy, and temperature changes on the wind channel, especially the rough surface accuracy. From the surface of the earth to an altitude of 1000, the flow of air is affected by factors such as vortex, viscosity and ground friction, and the wind speed increases with the increase in altitude. Through experiments, there are two commonly used formulas for calculating the change of wind speed with height.
In the formula, v1 is the wind speed at height h1: h1 is the height (usually 10m); v is the speed at the measured height h: h is the height of the measured point from the ground: h0 is the height where the wind speed is zero: n is the index , Depending on the flatness (roughness) of the ground and the stability of the atmosphere, the value range is 1/8~1/2. In an open, flat, and stable area, the value of n is 1/7. The roughness is large In large cities, it is often taken as 1/3. Generally, the wind speed difference between the upper and lower sides is small, and the value of n is small. On the contrary, the value of n is large.
3) The randomness of wind changes
Natural wind is a wind in which the average wind speed coincides with the instantaneous turbulent airflow that changes drastically. The turbulence of the airflow is mainly related to the friction of the ground. In addition, when the wind speed and the stable layer are vertically distributed, gravity waves are generated. Mountain waves will also be generated on the lower side. This kind of turbulent airflow not only affects the wind speed, but also obviously affects the wind direction. If distinguished by time, the change of wind direction can be divided into:
⑴ The trend of wind direction in one year or one month.
⑵ Turbulent air flow that fluctuates in a short period of time.
⑶ Average wind direction between the two.
For the first type of wind direction change, if you make a wind direction rose diagram (wind direction frequency), you can clearly see the general trend of the wind direction, as shown in Figure 1. Generally, the annual average wind speed time curve is used (representing the number of hours of various wind speeds in a place in a year)
To record the wind speed, as shown in Figure 2.
③The characteristics of wind
As a natural phenomenon, wind has its own characteristics and is usually expressed by basic indicators such as wind speed and wind frequency.
1) Wind speed
The magnitude of the wind is usually measured by the speed of the wind, which is the distance the air moves in the horizontal direction per unit time. Instruments that specifically measure wind speed include rotary anemometers, heat-dissipating anemometers, and acoustic anemometers. The unit of wind speed is often m/s, km/h,
mile/h and so on. For example, if the air moves 3m in 1s, the wind speed is 3m/s. Since the wind is constantly changing, the wind speed usually refers to the arithmetic average of the instantaneous wind speeds over a period of time, that is, the average wind speed.
2) Wind frequency
Wind frequency is divided into wind speed frequency and wind direction frequency.
⑴Frequency of wind speed: refers to the frequency of wind of various speeds. For the wind energy utilization of wind power generation, in order to facilitate the smooth operation of the wind generator and facilitate the control, it is desirable that the average wind speed is high and the wind speed change is small.
⑵ Wind direction frequency: refers to the frequency of occurrence of various wind directions. For the utilization of wind energy in wind power generation, it is always desired that the frequency of a certain wind direction be as high as possible.
Wind energy is the kinetic energy of the air, which refers to the energy carried by the wind. The magnitude of wind energy depends on the wind speed and the density of the air. The energy of the wind is converted from solar radiant energy. The energy radiated by the sun per hour to the earth is 174 423 000 000 000 kW. In other words, the earth receives 1.74 x 1017 watts of energy per hour. Wind energy accounts for 1% to -2% of the total energy provided by the sun. Part of the solar radiation energy is converted into biomass energy by plants on the earth, and the total amount of wind energy converted is 50 to 100 times that of biomass energy.
⑤Wind power rating
According to theoretical calculations and practical results, the wind with a certain wind speed, usually 3~20m/s, is developed as an energy resource for work (such as power generation). The wind in this range is called effective wind energy. Or wind energy resources. Because when the wind speed is lower than 3m/s, its energy is too small and has no use value, and when the wind speed is greater than 20m/s, it is very destructive to the wind turbine and it is difficult to use. The World Meteorological Organization divides the wind into 17 levels, and when there is no anemometer, you can roughly estimate the wind speed based on it. The wind power ratings are shown in Table1 and Table 2.
|Wind level||name||Wind speed (m/s)||Wind speed (km/h)||Terrestrial objects||Sea waves||Wave height (m)||Highest (m)|
|0||No wind||0.0~0.2||＜1||Quiet, smoke straight up||calm||0.0||0.0|
|1||light air||0.3~1.5||1~5||Smoke shows wind direction||Microwave peak without droplets||0.1||0.1|
|2||Light breeze||1.6~3.3||6~11||Feel windy||Small wave crest is not broken||0.2||0.3|
|3||breeze||3.4~5.4||12~19||Flag unfolding||Wavelet peak rupture||0.6||1.0|
|4||Zephyr||5.5~7.9||20~28||Blow up the dust||Xiaolang white foam crest||1.0||1.5|
|5||Strong wind||8.0~10.7||29~38||Small tree swing||Zhonglang Zhemo Peak Group||2.0||2.5|
|6||Gale||10.8~13.8||39~49||The wire is sound||Big Waves Baimo Lifeng||3.0||4.0|
|7||blast||13.9~17.1||50~61||Difficulty walking||Breaking the peak of white foam into strips||4.0||5.5|
|8||Gale||17.2~20.7||62~74||Destroy the branches||Waves grow high and there are waves||5.5||7.5|
|9||Gale||20.8~24.4||75~88||Small damage house||Waves upside down||7.0||10.0|
|10||Strong wind||24.5~28.4||89~102||Uproot trees||The waves roll and roar||9.0||12.5|
|11||storm||28.5~32.6||103~117||Significant damage||The crests are all droplets||11.5||16.0|
|12||hurricane||＞32.6||＞117||Devastating||The waves are overwhelming||14.0||﹣|
|Wind level||Wind speed (m/s)||Wind speed (km/h)|
The energy of the wind is very large, with a wind speed of 9~10m/s, the force of blowing on the surface of the object is about 10kg/m2;
Level 9 wind with a wind speed of 20m/s, the force blowing on the surface of an object is about 50kg/m2; the energy contained in the wind is much greater than the energy that humans can control so far.
⑥ Advantages and limitations of wind energy
Wind energy is a very important energy with huge reserves. It is safe, clean and abundant. At present, the use of wind power has become the main form of wind energy utilization, which is highly valued by countries all over the world, and it is developing at the fastest speed. Compared with other energy sources, wind energy has obvious advantages, but also has its outstanding limitations.
1) Advantages of wind energy
(1) Large reserves. Wind energy is a conversion form of solar energy, an inexhaustible and inexhaustible renewable energy source. According to calculations, the sun can still illuminate the earth for at least 6 billion years as it does now.
(2) No pollution. In the process of converting wind energy into electrical energy, no harmful gas or waste is produced, and it does not pollute the environment.
(3) Renewable. Wind energy is produced by the flow of air, and this energy depends on the existence of the sun. As long as the sun exists, air currents can be formed continuously and regularly, and wind energy can be generated cyclically, which can be used forever.
(4) Widely distributed, with local materials, no transportation required. In remote areas such as plateaus, mountainous areas, islands, grasslands, etc., the lack of coal, oil, and natural gas has caused many inconveniences to the people living in this area. Moreover, due to the remoteness and inconvenience of transportation, even from the outside world Transporting fuel is also very difficult. Therefore, the use of wind energy to generate electricity can obtain local materials without transportation, which has great advantages.
(5) Strong adaptability and great development potential. The available wind resources in my country account for 76% of the country’s land area. The development of small-scale wind power generation in my country has huge potential and broad prospects.
2) The limitations of wind energy
(1) Low energy density. Since wind energy comes from the flow of air, and the density of air is very small, the energy density of wind is very small, only 1/816 of water power.
(2) Instability. Due to the rapid changes in air flow, the wind is sometimes big and small, and the changes in the day, month, season, and year are very obvious.
(3) Large regional differences. Due to changes in topography and different geographical latitudes, the areas of wind are very different. Due to the different topography of the two neighboring areas, the wind power may differ several times or even dozens of times.