The Wind.

Earth is surrounded by a fluid that we call air. When air is heated creates High pressures in the atmosphere, and wind flows from High (H) to Low (L) pressure points. But, why does our planet have different temperatures? And even the most important, how can we take advantage of this natural power? In order to explain these questions, on this post we will see how wind acts and the latest researches on the aerodynamic field to convert the wind power in a useful and ecological energy.

There are some reasons to explain why the atmosphere has temperature gradients. Since Christopher Columbus discovered that Earth is a sphere (or almost) five hundred years ago, now we can understand easily that Sun’s radiations impinge on the surface with different power depending on the latitudes. Also, we have the rotation. When the Earth’s face is in front of the Sun, it means that the another face is in the dark, and as we know, days are usually warmer than nights. Finally, our planet’s axis is inclined and when it is summer in the North means it is winter in the South. Furthermore, our planet has other peculiarities (clouds, oceans which keep better the heat than land, white glaciers, so on) that provoke High and Low pressures in the atmosphere.

However, wind does not move straight from the High to Low pressures, and it is influenced by other natural forces such us gravity, friction against Earth’s surface, Coriolis, centrifugals, turbulences.  Furthermore, wind must cope with craggy terrains and obstacles. Knowing this, another question arises, where is the most suitable place to use the wind power?

Wind is air in motion and therefore, it has Kinetic Power. Pkin = ½ m v2 = ½ ρ A v3. As we can see, the most important factor is the wind’s velocity. However, wind is not constant and it depends on time and other factors. In order to study the annual average of wind in a place, we must determine its frequency distribution, the Weibull distribution and the prognosis for the long-term (by using, for example, the Wind Atlas Method).

Example of Frequency Distribution of the Wind.

Wind Atlas Method.

After studying where most profitable places are to exploit the wind power, we might determine which the best wind turbine is to achieve good performances. The first principle we must assume it isEnergy can neither be created nor destroyed, it can only be converted.

As we can see on the image below, the air flow has mass m and comes with velocity v1. Then, it impinges on the rotor disc and goes with velocity v2. Knowing this, we can calculate the Power Electric that we can utilize. Pel=Pbefore-Pafter, it means: Pel = ½ m (v12– v22).

Albert Betz demonstrated that the maximum Pel is reached when v2=⅓ v1 and the power that the wind turbine can attain can be expressed as P = ½ ρ A v3 CP (where CP is the Power Coefficient. CPMax=16/27 (0,593)).

On the other hand, we must know the rotational speed of the rotor in relation to the wind speed. The relation, λ = Vtip / Vwind, we call it Tip Speed Ratio which shows us the optimal speed of the rotor blades related to the wind speed. As we can guess, each wind turbine will have different Tip Speed Ratio as we can see on the next diagram.

Finally of this basic aerodynamic lesson, we must be familiar that there are two forces which blades rotors use: Drag (D) and Lift (L) forces. The Lift force makes the rotor of modern wind turbine rotate as we can see on the picture below. A deeper explanation of these principles will come in coming posts. Are you still interested?

Source: “Developing Wind Power Projects” by Tore Wizelius.

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