**2. Wind Power**

The wind power captured by a turbine is commonly expressed as a function of the turbine's swept area and a coefficient of performance, the air density and the wind speed [8]

© 2012 Gitano-Briggs; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Gitano-Briggs; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

$$\mathbf{P\_{urb}} = \text{\textquotedblleft C\_p} \rho \text{ A V}^3 \tag{1}$$

Where:

Pturb is the mechanical power of the turbine in Watts

Cp is the dimensionless coefficient of performance

ρ is the air density in kg/m<sup>3</sup>

A is the swept area of the turbine in m2

V is the speed of the wind in m/s

For wind sites near sea level the atmospheric pressure is approximately 1.18 kg/m<sup>3</sup> and de‐ creases with altitude. The coefficient of performance is related to the turbine design, and has a theoretical upper limit of 0.593, referred to as the Betz limit [5]. Most sub 10kW wind tur‐ bines are rated for speeds from 8 to 12m/s. The coefficient of performance of commercial small turbines generally falls in the range of 0.25 to 0.45 based on manufacturers rated pow‐ ers, speeds and diameters. The power of a turbine is directly proportional to the swept area, thus it is proportional to the blade length squared. The factor with the largest influence on turbine power, however, is the wind speed. From the turbine cut-in speed to the rated speed a turbine's power is proportional to the cube of the wind speed. That means that a 10m/s wind will deliver eight times the power of a 5m/s wind. This is why most turbines have a fairly high rates wind speed: it is the easiest way to achieve a high power output.
