## 5 Results and Discussion
### Tower Optimization Results  
The distributions of outer diameters and wall thicknesses along the tower height are shown in Figure 18. Both of the optimal towers have larger outer diameter and thickness distributions to meet the structural constraints. Table 6 shows the comparison of the optimization results between the optimal towers and the original one. Both of the optimal towers are taller than the original one. With the increase of tower height, the annual average wind speed also increases. Hence, the rotor can capture more wind power to improve the AEP. However, with the increase of wind speed, the aerodynamic loads acting on the rotor and hence the thrust will also increase. The tower's structural design needs to consider that an increase in thrust on the tower top will cause a larger deformation. The tower mass increases when the tower height increases. Compared to the original tower, the two optimal towers have an increase in mass of about 43.65% and 42.49% (CRP). The main reason is that the tower height with 118m and 120m has higher annual average wind speed, and the greater rotor thrust requires a better structural performance with thicker walls. The height of the optimal tower considering rated power is higher but its mass is less than the one without considering rated power. The main reason for this is that the wind turbine with longer blades has larger thrust acting on the tower top.  
Table 7 shows the costs of rotor, tower, and foundation in the cases of the original design, the optimal design (without CRP) and the optimal design (with CRP). The rotor costs of the optimal wind turbines are reduced compared with that of the original one. The main reason is that the optimal blades have a shorter length and lighter blade mass. The tower costs of the optimal wind turbines  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Initial Value** & **Optimal Value** & **Optimal Value (Rated)** \\ \hline Height (m) /Mass (kg) & 95/186,780 & 117.2/268,320 & 119.8/266,140 \\ Top deformation (m) & 0.65 & 0.828 & 0.816 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Optimization design results of the tower.  
Figure 18: Tower outer diameter and wall thickness distributions of the baseline and the optimized towers.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Initial Value** & **Optimal Value** & **Optimal Value (Rated)** \\ \hline Length (m) /Mass (kg) & 59.8/13,240 & 57.84/12,540 & 56.2/11,620 \\ Tip deflection (m) & 7.576 & 6.92 & 6.31 \\ AEP (GW) & 5.7830 & 6.0921 & 5.8324 \\ Cost of energy (\$/kWh) & 0.057 & 0.0478 & 0.047 \\ Tip speed ratio & 10.6 & 9.8 & 9.6 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Optimization design results of the blade.  
are increased compared with the original one due to the higher height and thicker wall distributions. Because of the shorter blade length and significant decrease in chord length distributions, the rotor thrust of the optimal wind turbine is less than that of the original one. Thus, the foundation costs do not change much, although the tower of the optimal wind turbine is higher than that of the original one.