Wind arises as a result of a group of natural factors such as the difference in temperature between night and day, and the sun’s thermal energy is the main source of wind energy, and scholars estimate that 1% of the amount of sunlight reaching the Earth’s surface is consumed in the formation of wind, as the solar rays heat the contact air. And close to the land at a higher rate than it is over water bodies such as seas and oceans, and therefore the density of air on land becomes less than it is over water bodies; These thermal differences between the layers of the atmosphere during the day lead to the generation of energy capable of converting the heat capacity resulting from the fall of the sun’s rays into a kinetic ability possessed by the air molecules, and this kinetic ability is represented by the movement of air from high pressure areas (tropics) to low pressure areas (the poles). ).
1.Wind speed Meseaurment with Anemometers
The wind strength is measured by a device called the anemometer ,and its direction is determined by a special device also called the air vane, and for the success of the measurement, these devices are required to be placed in a free position away from any obstruction to the movement of the wind in order to obtain correct readings..
2.Categories of wind turbines
Wind turbines are generally classified according to several categories:
1- Direction of its axis
It is either with an axis parallel to the surface of the earth (HAWT), which is the most widespread and has the highest yield and capacity, or with axes perpendicular to the surface of the earth, the axis of the turbine is perpendicular to the direction of the wind and is characterized by ease of operation and maintenance compared to the horizontal axis turbine, but its yield and capacity are relatively low ( VAWT photo(2)
2-Direction of the wind
It is either facing the winds UP Wind, in which the rotor with the blades is facing the movement of the wind and makes up the vast majority of the turbines, and these turbines have a twisting mechanism to keep the rotor facing the wind.
Or consistent with the direction of the wind Down Wind, in which the rotor is located behind the tower. It is characterized by that it does not have a twisting mechanism, especially the small ones, but rather rotates automatically so that the axis of the rotor remains parallel to the direction of the wind, but its main problem is the disturbance of the wind movement behind the tower, which affects the blade as a result of the effect of the shadow of the tower. Thus, increased stress loads on the blades and turret. Picture(3)
3- It is also classified according to the number of its blades and it is with one blade, two blades, three blades or multi blades, bearing in mind that the most common and efficient types are three blades, and it is the most widespread due to its multiple features such as high dynamic stability, and the decrease in vibrations where the high blade load is distributed over the The axis of rotation is parallel during movement. picture(4)
How many type of anemometre exist in the market?
Wind speed measurement is most often done by using a cup anemometer like the one you see in the image to the right. Such an anemometer is provided with a rotor of three cups which, under the effect of the wind, begins to rotate around a vertical axis.
The speed of rotation is recorded electronically.
Usually, the anemometer also has a wind vane indicating the direction of the wind. Instead of cups, the anemometer can be fitted with a propeller, a solution which is however quite rare.
Ultrasonic anemometers were first introduced in the 1950s. They use ultrasonic sound waves to measure wind speed. They time wind speed as a function of the time of flight of sound pulses between pairs of transducers. The data provided by the pairs of sensors can be combined to generate a measurement of the velocity in a flow in one, two or three dimensions.
4.Ping-pong ball anemometers
This gadget is developed from a ping-pong ball appended to a rope. At the point when the breeze bearing is level, it pushes ready and makes it move. Since ping pong balls are light, they move easily even in quiet breezes. Figuring the point between the rope gadget and the upward gives an estimation of the breeze speed.
Before you buy an anemometre
If you are planning to build a wind farm, it will indeed be an economic disaster if you employ an anemometer that measures wind speeds with a 10% error. If so, you risk relying on an energy content of the wind that is 33% higher than the actual content.
Also, if you need to recalculate your measurements for a different hub height (ranging, from 10 to 50m in height), you can even multiply that error by a factor of 1.3. So you will end up having made a calculation error of 75%!
It is possible to buy for around 700-900 EUR / USD a professional and well-calibrated anemometer with a measurement error of only 1%. This amount is negligible compared to the potential risk of making an economically disastrous measurement error. It stands to reason that you cannot always rely on the price as an indicator of the quality of the anemometer. Before purchasing an anemometer, therefore, preferably consult a wind energy research institute.
Note: The anemometer placed at the top of the nacelle is normally only used to determine if the wind is blowing hard enough or not to make it worthwhile to start the wind turbine.