## 4 Impact of Yaw Operation on Energy Capture Performance
### Effects of Blade Deflection  
Due to the aerodynamic loads, the wind turbine blades will deflect inevitably, which will affect the energy capture performance of the turbine rotor during yaw operation. To further understand the coupling effects of yaw operation and blade deflection, the energy captured by the wind turbine rotor during yaw operation under different blade deflection conditions are calculated in this section. Considering blade deflection happens mainly from the middle to the tip sections of the blade (see Figure 7), the 3D models of the turbine rotor with deflected blades that are characterized by different deflection angles are established. They are shown in Figure 8. Where Figure 8a is the rotor model when the edgewise deflection of the blade is 30\({}^{\circ}\); Figure 8b is the model when the flapwise deflection of the blade is 30\({}^{\circ}\); Figure 8c shows the model when the blade has 30\({}^{\circ}\) deflections in both edgewise and flapwise directions.  
Then the energies captured by the wind turbine rotor during yaw operation under different blade deflection conditions are calculated. The results are shown in Figure 9, where Figure 9a,b shows the results obtained when the rotor speed is 17 r/min, blade deflects in flapwise direction, and wind speed is respectively 11.5 m/s and 9 m/s. Figure 9c,d shows the corresponding results obtained when blade deflects in edgewise direction, while Figure 9e,f shows the corresponding results when the blade deflects in both edgewise and flapwise directions.  
Figure 8: Rotor models with deflected blades, (**a**) Bending deflection 30\({}^{\circ}\) in edgewise direction. (**b**) Bending deflection 30\({}^{\circ}\) in flapwise direction. (**c**) Bending deflection 30\({}^{\circ}\) in both edgewise and flapwise directions.  
Figure 7: Schematic diagram of blade deflections in edgewise and flapwise directions.  
From Figure 9, it is found that when wind speed is 9 m/s, the blade's flapwise deflection has more influence than its edgewise deflection on the energy capture performance of the turbine rotor during yaw operation. For example, when the blade's flapwise deflection angle is \(2^{\circ}\), \(4^{\circ}\), \(6^{\circ}\), \(8^{\circ}\), \(20^{\circ}\), and \(30^{\circ}\), the corresponding \(\Lambda\) is 23.5%, 23.5%, 23.9%, 24.2%, 25.4%, and 26.0%, respectively, while when the blade's edgewise deflection angle is \(2^{\circ}\), \(4^{\circ}\), \(6^{\circ}\), \(8^{\circ}\), \(20^{\circ}\), and \(30^{\circ}\), the corresponding \(\Lambda\) is 22.8%, 22.8%, 22.6%, 23.0%, 22.4%, and 22.1%, respectively. From Figure 9, it is interestingly found that the corresponding influence is mitigated when the blade deflects simultaneously in both flapwise and  
Figure 9: Effect of yaw on energy capture under different blade deformations.  
edgewise directions. For example, when wind speed is 9 m/s and 20\({}^{\circ}\) deflections occur in both flapwise and edgewise deflections of the blade, the power fluctuation factor \(\Lambda\) is only 20.3%, as opposed to 25.4% when 20\({}^{\circ}\) deflection happens only in flapwise direction and 22.4% when 20\({}^{\circ}\) deflection happens only in edgewise direction. Moreover, the power fluctuation during yaw operation will be further reduced at higher wind speeds. For example, the corresponding power fluctuation factor \(\Lambda\) is only 7.4% when wind speed is 11.5 m/s and 20\({}^{\circ}\) deflections occur in both flapwise and edgewise directions of the blade.