How much energy is available from the wind
People think that as long as they can feel the wind, ample electricity should be generated from the wind turbine. They have often dreamed of a small wind turbine that can be installed on the roof of a house can provide all the energy they need. However, this is not the case. We should learn more on the limits of wind. Like anything in nature, there are limits to what is possible.
The power that is available in the wind depends on the wind speed, the density of the wind (which varies with altitude and temperature), and the amount of turbulence (swirling) in the wind. Wind speed is high up in the sky and low at the ground level. You may notice that you are difficult to fly a kite at the ground. However, when the kite is flied up in the sky, you will feel the high wind force.
The power available at a given wind speed passing through an area (e.g. a window) can be calculated by using the following formula:
P = ½·ρ· A·v³
Where:
- P is power in Watt (W).
- ρ is the density of air in Kilograms per cubic meter (Kg/m3)..
- A is the cross section area of the wind passing through in sq. meter (m2).
- v is the velocity of the wind in meters per second (m/s).
The density of air at sea level and room temperature is approximately 1.3 Kg/m3.
The available power from the wind increases dramatically with the swept area but so do the stresses on the blades. The swept area is the area the windmill's blades cover during a rotation.
The power depends on the cube of the velocity. This means that when the wind speed doubles, the available power is increased by a factor of 8. In other words, when the wind speed reduces in half, the available power reduced to 1/8 of the original value. There is very little available power below 11 Km/h, but as wind speed increases the power becomes very significant.
Turbulence and Drag are always associated with wind. They cause rapid variation of pressure and velocity in space and time. They detract our ability to extract mechanical energy from the wind. In general, wind turbines are typically installed as far above ground level obstacles as possible, since trees and buildings add to turbulence.
In order to have an idea of how much energy is available in the wind, we can refer to the Beaufort scale. At wind speed of 11 Km/h or less, we can only felt the wind on our exposed skin. Thus, there is only limited wind energy under 11 Km/h.