On the Design and Manufacture of Wind Turbine Blades

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.

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 $\left(C_p\right)$with the tip speed ratio $\left(\lambda \right)$ for two different blade designs. Design 1 has the maximum $C_p$ but with large drop with small and high $\lambda$. Design 2 has smaller $C_p$, but performs better over the range of $\lambda$. Therefore, design 2 seems to be better than design 1; however, it has smaller maximum power coefficient $C_p$.

Figure 1.

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

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.

Figure 2.

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

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

Figure 3.

Wind turbine blade variables [3].

Figure 4.

Relation between 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; 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).

Figure 5.

The control point motion [4].

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 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 $\frac{C_l}{C_d}$ ratio. The maximum is $\frac{C_l}{C_d}$; 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 $3\times {10}^4$. The performance of each airfoil is different in relative to the angle of attack.

Figure 6.

Comparison between different airfoils at Re = 3 × 10⁴.

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.

2.2 Unsteady aerodynamic

Before starting the design optimization process, steady and unsteady aerodynamic analyses should be conducted. These analyses can be accomplished either using the blade element momentum (BEM) or by applying computational fluid dynamics (CFD). A detailed comparison between the two methods can be found in [6]. The two methods are used to solve the blade mathematical model, which is usually has a form of differential equation. The mathematical model should provide the relation between the different variables and parameters of a typical wind turbine blade. One of the most popular and widely used mathematical models in wind turbine analysis and design is the blade element momentum (BEM) method. A general procedure for applying the BEM method can be summarized as follows [1]:

1. Define the geometry.

2. Discretize the blade into elements.

3. Initialize the induced and relative blade velocities.

4. Determine the airfoil data including the lift.

5. Compute new values of wind velocities.

6. Compare between the old and new values till convergence is achieved.

More details about the BEM method can be found in [6].

2.3 Design optimization

The design of wind turbine blades is twofold: first, the correct selection of the optimization method, and, second, the proper definition of design variables and other optimization parameters. A genetic algorithm (GA) is one of the popular methods that are widely used in design optimization [7]. The GA is based on the process of natural selection, in which the new generation is selected based on the fitness of the parents. Thus, the parents with high fitness supposed to produce offspring better than those with low fitness score in the optimization process. The process is keeping in iteration until the best design variables are selected through mutation, crossover, and selection steps. Figures 7 and 8 show details of the GA optimization process.

Figure 7.

GA process [4].

Figure 8.

Single-point example for the crossover process [3].

One of the advantages of using the GA in optimization is that it can be applied to both discrete and continuous optimization. In the GA, the population is generated randomly, and a candidate solution is defined for the design variables. The best solutions are selected based on their fitness defined from the objective function. Those solutions define the parents. Their children are produced by crossover operation. Then, to ensure global optimization, a mutation operation is applied [3].

Wind turbine optimization requires the definition of an objective function. The objective function differs based on the purpose of the optimization. If the purpose is to improve the wind turbine aerodynamic performance, the objective function may be to increase the wind turbine lift and/or decrease the drag. If the optimization purpose is to improve the wind turbine structure performance, then the design objective could be to maximize the wind turbine stiffness and/or minimize its weight. If one objective function is defined, we called it single-objective optimization. In case of more than one objective function, the optimization is called multiobjective optimization.

3. Structural design

After determining the wind turbine geometry, a structure design should be accomplished to create the wind turbine. The structure design includes several analyses, such as static analysis, modal analysis, dynamic analysis, and aeroelastic analysis. There are two schools in wind turbine structure analysis and design. The first school suggests the design of the wind turbine structure by approximating the wind turbine blade into a beam model [2]. This method is very efficient in computational time and cost, but it cannot provide a detailed solution for deformation, strain, and stress distribution. The second school prefers to analyze the wind turbine blade as a full 3D model using numerical software. The most popular method to conduct these analyses is the finite-element method. However, the 3D simulation can provide a detailed solution to the wind turbine blade structure; it costs lots of time and money in comparison to the 1D beam analysis. Table 1 provides a detailed comparison between the beam and numerical solutions.

	Beam analysis	3D simulation
Time cost	Low	High
Money cost	Low	High
Accuracy	Applies 1D approximation to blades using beam theory	Applies numerical approximation to the governing equation using variational methods
Solution	Based on analytical or numerical methods	Based on numerical methods
Results	Provides 1D solution	Provides detailed 3D solution

Table 1.

Comparison between 1D beam and 3D numerical solutions.

Usually, four solutions should be studied to make sure that the wind turbine structure is safe and stiff enough:

1. Static analysis—by which steady loads are applied to the structure and the static displacement and stresses are determined.

2. Modal analysis—by which the blades’ natural frequencies and mode shapes are calculated.

3. Dynamic analysis—by which the dynamic displacement and stresses are determined in response to unsteady aerodynamic loads.

4. Aeroelastic analysis—by which the divergence and flutter speeds are calculated to make sure that the blade is safe from any aeroelastic instability.

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.

Figure 9.

Wind turbine construction [8].

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 $\left(m\text{and}{m}_d\right)$. 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 10 and Table 2 can help in selecting the appropriate spar cross section for bending and torsional applications.

Figure 10.

Comparison between different spar shapes [9].

Table 2.

Detailed comparison between different spar cross sections [9].

In Table 2, $b_w$ is the web height (the section height), $b_f$ is the flange width (the section width), $D_i$ is the inlet diameter, $t$ is the thickness, $t_w$ is the web thickness, $\widehat{\mathit{EI}}$ is the equivalent bending stiffness, $\widehat{\mathit{GI}}$ is the equivalent torsional rigidity, $d,a_{66},d_{66}$ are composite stiffness coefficients, $y_{\mathit{tip}}$ is the tip displacement, ${\psi }_{\mathit{tip}}$ is the tip rotation, $p$ is the applied load, $T_{\mathit{max}}$ 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.

5. Conclusion

This chapter summarizes the methods and techniques usually used in the design and manufacturing of wind turbine blades.