This type of energy will change kinetic energy (0.5 mv ^ 2) of wind energy into other forms (electricity, windmill, and pumps). So for power generation, a common formula used is P [W] = 0.5 x rho [kg / m ^ 3] x A [m ^ 2] x (v [m / s]) ^ 3 x efficiency. So factor that plays an important role in wind energy generation is speed, followed by expansion turbine (blade), and efficiency. Let us first talk about the wind factor.
Wind speed.
The interesting thing is the wind speed. Generally (for reasons of convenience and price), the anemometer to measure air velocity. If the rate of 3% accuracy of the tool, which means that power output will be in the range of + / – 9%.
Another thing is the problem of stability of the wind speed. As known, the wind speed will fluctuate with time and place. In Indonesia, for example wind speed during the day can be much faster than the night. In some locations even at night there is no significant air movement. Whether for a situation like this, the average velocity to represent (?), Whereas at night the wind turbine is not moving at all.
Moving air near the soil surface will have zero velocity and then increased to a height (see Figure below). This phenomenon occurs naturally in the flow near the surface that does not move (when the earth rotates? Khan).
What is interesting?
First, it is too close to the soil surface, wind speeds obtained will be small so that power generated very little. The higher the better. To obtain wind speeds in the range of 5-7 m / s is generally required height of 5-12 m. Second, for a large propeller (say a diameter of 20 m), wind speed at the tip of the propeller section of roughly 1.2 times the speed of the wind vane tip bottom. That is, the propeller at the top will be exposed to a greater thrust than the propeller at the bottom. These factors need to be considered when designing the power of the propeller and mast (tower), especially on large wind turbines.
If the wind speed at the blades top and bottom are significantly different, then at what wind speeds are reasonable and fair to design power output of a wind turbine?.
Wind speed is also influenced by the contour of the surface. In urban areas with many houses, terraced apartments and offices, wind speed will be low. Compare this with wind speeds in the field area. Object density (porosity,?) On the surface of the earth will cause the wind is easy to move or not. Porosity are also important factors to consider when designing a wind turbine.