# Aerodynamic Design and Performance Analysis of a Large-Scale Composite Blade for Wind Turbines
## 2 Aerodynamic Design and Analysis  
Design requirements should be established to design wind turbine blades. First, the specification of the wind turbine generation system is established and then detailed design requirements are determined. The most important element of a wind turbine generation system is the design of blades that convert wind energy into a mechanical form. Aerodynamic design is performed first and structural design follows. In this study, the blade design requirements were defined, as shown in Table 1.  
The airfoil configuration of the wind turbine blade's cross-section is an important factor that determines the various performance metrics of the wind turbine generator. To design blades, electrical power is determined considering the loss factor, which is lost by friction, etc., for the aerodynamic power determined first. Equation (1) is the one that calculates the power, where \(P\) means the aerodynamic power, \(P_{e}\) is the electric power, and \(\eta\) is the efficiency of an alternator and step-up gearing. Based on it, Equation (2) is used to determine the diameter, which is the rotational surface of the blade, where \(D\) is the rotor diameter and \(V\) is the rated wind speed. The equations for the design in this work are referenced in wind power plant theory [36]. \[P=\frac{P_{e}}{\eta} \tag{1}\]  
\[D=\sqrt{\frac{P}{0.2V^{3}}} \tag{2}\]  
The diameter of the designed blade is 260 m. P is the aerodynamic power. V means the rated wind speed. \(\eta\) is the efficiency of the alternator and step-up gearing. After determining the diameter of wind turbine generation blades, the blade configuration is designed in detail. To design blades, aerodynamic design, such as the blade size, airfoil configuration, twisting angle, and tip speed ratio, is first carried out based on the design requirements, whether or not they meet the required performance is examined through aerodynamic analysis and aerodynamic experiments, and then the design is improved or confirmed [36]. Figure 1 shows the force vector of the blade. In this study, aerodynamic design was performed in accordance with the method suggested below. Setting angle (\(\theta\)), rotational wind speed ratio (h), and axial wind speed ratio (k), which are the variables for aerodynamic design, were calculated using the following equations. The twisting angle was calculated using Equation (7). I means the inclination angle or flow angle, and i is the incidence angle. \(\lambda_{e}\) is cot \(I\). \(\lambda\) means the tip speed ratio. \(\alpha\) \begin{table}
\begin{tabular}{c c} \hline \hline
**Design Requirements** & **Value** \\ \hline Rated power & 25 MW \\ Cut-in wind speed & 4 m/s \\ Rated wind speed & 12.5 m/s \\ Cut-out wind speed & 25 m/s \\ Survival wind speed & 55 m/s \\ Rotor orientation & Upwind \\ No. of blades & 3 \\ Tip speed ratio & 6 or 7 \\ Type & HAWTS \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Design requirements. means the twisting angle. \(l\) is the chord length, and \(C_{l}\) is the lift coefficient. \(b\) is the number of blades. The design result of the maximum twisting angle was 15.78 degrees. The design result of blade chord length was calculated using Equation (8). The aerodynamic design result is shown in Table 2. Figure 2 shows the aerodynamic design configuration. The optimal design was performed for each cross-section by applying the DU-series airfoil. \[\theta =\frac{\left(\pi+\tan^{-1}\lambda\right)}{3} \tag{3}\] \[k =\sqrt{\lambda^{2}+1}\cos\theta\] (4) \[h =\sqrt{\frac{1+(1-k^{2})}{\lambda^{2}}}\] (5) \[\lambda_{e} =\lambda\frac{1+h}{1+k}=\cot I\] (6) \[\alpha=I-i\] (7) \[l =\frac{8\pi r(1-k)}{C_{l}b(1+k)\lambda_{e}\sqrt{\lambda_{e}^{2}+1}} \tag{8}\]