Windis the directed movement of air masses. Wind energy can be considered as one of the forms of manifestation of solar energy.
The wind changes over time. In most regions, significant seasonal changes in wind flows are observed. And in the winter months the wind speed is usually higher than in summer. Daily changes in wind speed are usually observed near seas and large lakes. In the morning, the sun heats the earth faster than water, so the wind blows towards the coast. In the evening, the earth cools faster than water, so the wind blows from the coast.
Wind speed depends on the height above ground level. Closer to the ground, the wind slows down due to friction on the earth's surface. Thus, winds are stronger at high altitudes relative to the earth. For agricultural fields and desert areas with an increase in height above the ground twice the wind speed is increased by about 12%.
Wind speed is significantly influenced by geographical conditions and the nature of the earth’s surface, including various natural and artificial obstacles, such as hills, etc., as well as trees and buildings. For this reason, wind turbines are located, where possible, on elevated and distant from tall trees, residential buildings and other facilities, because such obstacles reduce wind speed and lead to flow turbulence, making it difficult to convert wind energy.
The average annual wind speed (Va.s.) characterizes the wind potential of the territory. This is the wind speed, which is defined as the arithmetic average of all observed wind speeds during the year. Average wind speeds can also be calculated for other periods, for example: monthly, daily, hourly.
The kinetic energy A contained in the air stream depends on its mass m and velocity V, and can be found with using the formula:
If in this formula to substitute the value of the mass of air flowing through the wind wheel of the engine for 1 second, then we get the expression for the second energy of the flow, or, equivalently, for its power:
where – air mass density;
γ – Air weight;
g – Gravity Acceleration;
F – The area through which air flow;
V – Wind speed.
The specific gravity of air depends on the value of barometric pressure and temperature.:
where – mass density of air at temperature and atmospheric pressure ;
B and tº – respectively, atmospheric pressure, mm. Hg Art., and the air temperature in the new conditions;
and T – absolute air temperatures at 15ºC and in new conditions.
From the second formula it is clear that the second energy, or the power of the flow, is proportional to the cube of speed, that is, if the wind speed increased, for example, twice, the energy of the air flow increases times. Thus, the average wind speed of 5 m/s can give about 2 times more energy than wind with an average speed of 4 m/s. Also, the power developed by a wind turbine varies in proportion to the square of the diameter of the wind wheel, i.e. with an increase in diameter by 2 times, the power, at the same wind speed, increases by 4 times.
Wind characteristics are measured at weather stations. On the surface, wind speed is determined using an anemometer (anemiain Greek means wind). The height of the wind speed measurement is officially adopted - 10 meters from the surface of the earth The purpose of this is to eliminate the influence of surface turbulence and air friction on the surface.
There are various types of anemometers that use a propeller or rotating cups. They measure the actual wind speed because they move with the wind. Pitot tube and floating ball - anemometers of another type, whose readings vary according to air density (depending on temperature and altitude), they determine the wind speed by its dynamic pressure. Impressions depend little on altitude, temperature and humidity.
Data on the speed and direction of the wind come to the weather center every 3 hours. In this case, the averaging of readings in the direction and speed of the wind for 10 minutes preceding the observation period is automatically done. And gusts are considered the maximum wind speed for 2 minutes over a given observation period (3 hours). The observation period is strictly regulated on Greenwich time, it is 00:00 h, 03:00 h, 06:00 h, 09:00 h, 12:00 h, 15:00 h, 18:00 h, and 21:00 h.
Based on long-term observations of wind speeds in different parts of the world, specialized wind maps are composed.