On the Design and Manufacture of Wind Turbine Blades

*Mohamed Mahran Kasem*

### **Abstract**

Wind turbines become extremely important worldwide along with the need for clear energy sources. The concept of wind turbines is based on using the wind energy to produce lift that turns into toque, which rotates the wind turbine blades and subsequently produces electric power using a proper generator. However, the wide use of wind turbines and their design and manufacturing process are a challenge. Therefore, much research has been conducted to improve and develop new methods for the design and manufacturing of wind turbines. In this chapter, the author discusses some techniques for wind turbine design and manufacturing, including airfoil appropriate selection, design optimization methods, and manufacturing techniques. One of the manufacturing techniques that are found to be superior is the use of chordwise and spanwise stiffeners to increase the stiffness of the skin of carbon fiber wind turbine blades. Those stiffeners are not bonded externally to the skin; otherwise, they are layers of carbon fibers that are buried inside the skin of the wind turbine blades.

**Keywords:** wind turbine blades, blade manufacturing, blade design methods

## **1. Introduction**

The design of wind turbine blades has two objectives: (1) to determine the blade geometry that can produce an optimum power and (2) to determine the optimum structure required to create the wind turbine blade. The objective of the former is to obtain the wind turbine blade geometry that maximizes the power generated at different tip speed ratios. **Figure 1** illustrates the variation of the power coefficient *Cp* with the tip speed ratio ð Þ*<sup>λ</sup>* for two different blade designs. Design 1 has the maximum *Cp* but with large drop with small and high *λ*. Design 2 has smaller *Cp*, but performs better over the range of *λ*. Therefore, design 2 seems to be better than design 1; however, it has smaller maximum power coefficient *Cp*.

The aerodynamic design also includes the selection of optimum chord and twist distribution for the wind turbine blade. The objective of the latter is to create a wind turbine blade structure that satisfies the aerodynamic requirements. A typical blade cross section is shown in **Figure 2**. A blade structure is usually constructed from external skin and internal spar.

This chapter summarizes the key steps required to perform an appropriate aerodynamic and structural designs for wind turbine blades. This includes the design

**Figure 1.** *Variation of power coefficient with tip speed ratio for two different blade designs [1].*

#### **Figure 2.**

*Typical cross section of a wind turbine blade [2].*

process, unsteady aerodynamic analysis, design optimization, and structural design of the blade.

## **2. Design of wind turbine blades**

A major objective of wind turbine design is to maximize the output power and improve its performance. This objective can be accomplished by maximizing the aerodynamic lift and minimizing the drag. The process of designing a wind turbine blade starts by the airfoil selection in addition to selecting the appropriate wind turbine geometries according to the required performance. **Figure 3** shows the main variables in a typical wind turbine blade. **Figure 4** shows the relation between the wind turbine power and diameter.

#### **2.1 Airfoil selection**

Wind turbine blades are usually constructed with high taper ratio and twisting angle. Small wind turbines usually have one airfoil type, whereas large-scale wind turbines need different airfoils along the blade radius. An airfoil should be selected with maximum lift-to-drag ratio and minimum pitching moment coefficient. Most optimization models concentrate on improving wind turbine blade performance by enhancing the taper ratio, aerodynamic twist, and geometric twist of the blade;

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

**Figure 3.** *Wind turbine blade variables [3].*

**Figure 4.**

*Relation between wind turbine power and diameter.*

however, some optimization models improve the wind turbine performance by changing the airfoil shape. The latter can be conducted either by considering different airfoil shapes in the optimization problem or by defining control points over the airfoil and change its shape during the optimization process (**Figure 5**).

Sometimes, special types of airfoils are required for the wind turbine based on its characteristics. For instance, low-speed wind turbines require special types of airfoils to generate the torque required to rotate the blades [5]. In most cases, it is required to

**Figure 5.** *The control point motion [4].*

**Figure 6.** *Comparison between different airfoils at Re = 3 104 .*

compare between different airfoil types and select the best airfoil to be integrated with a certain wind turbine. The airfoils are evaluated based on their *Cl Cd* ratio. The maximum is *Cl Cd* ; the airfoil can produce more lift and smaller drag.

There are several airfoils' families suitable for wind turbine blades, such as the NACA family and the S series. **Figure 6** shows a comparison between nine airfoils from different series at Reynold's number 3 <sup>10</sup>4. The performance of each airfoil is different in relative to the angle of attack.

Large wind turbines are usually constructed from more than one airfoil. It could have two or three different airfoils along its radial position. In this case, a linear or higher order chord variation can be assumed between the airfoils.
