**4. Manufacturing of wind turbine blades**

The wind turbine blade structure usually consists of upper skin, lower skin, and spar (**Figure 9**). These structural elements help in resisting the direct and shear stresses applied to the blade.

Spar is the main structural element in the wind turbine blade. It transforms all the blade loads to the wind turbine hub. Thus, the selection of appropriate spar shape is a corner stone in structural design. In the following figures, a comparison between the most common spar cross-sectional shapes is provided. **Figure 10** shows a comparison between the rectangular shape, circular shape, I section, double I shape, and C section spar elements. **Table 2** provides the mathematical equations for a detailed comparison. Two performance parameters are defined to measure the stiffness of the spars ð Þ *m* and *md* . In terms of the performance parameters, the I and C sections are found to have the best bending and torsional stiffnesses in comparison to the other candidates.

**Figure 9.** *Wind turbine construction [8].*


**Figure 10.** *Comparison between different spar shapes [9].*

**Figure 10** and **Table 2** can help in selecting the appropriate spar cross section for bending and torsional applications.

In **Table 2**, *bw* is the web height (the section height), *b <sup>f</sup>* is the flange width (the section width), *Di* is the inlet diameter, *t* is the thickness, *tw* is the web thickness, *EI*^ is the equivalent bending stiffness, *GI*^ is the equivalent torsional rigidity, *d*, *a*66, *d*<sup>66</sup> are composite stiffness coefficients, *ytip* is the tip displacement, *ψtip* is the tip rotation, *p* is the applied load, *Tmax* is the maximum torque, and *L* is the beam length.

In small wind turbines, it is difficult to add a spar inside the wind turbine blade because the blade thickness is small. In this case, a lateral and longitudinal stiffener can be bonded inside the wind turbine skin to stiffen the skin. In composite manufacturing, those stiffeners can be inherent inside the skin during the manufacturing process. We found this technique efficient in increasing the wind turbine blade stiffness.


*On the Design and Manufacture of Wind Turbine Blades DOI: http://dx.doi.org/10.5772/intechopen.104490*

> **Table 2.**

*Detailed comparison between different spar cross sections [9].*
