# Effects of Blade Extension on Power Production and Ultimate Loads of Wind Turbines
## 6 Conclusions  
As a potential technical transformation method, blade extension can directly increase the swept area of wind rotors and efficiently enhance the energy production. However, there is still a lack of systematic load evaluation considering all the design load cases for wind turbines. Therefore, we carried out the load evaluation based on a IIA 1.5 MW wind turbine prototype with three different blade extension lengths (1 m, 1.5 m and 2 m) according to the IEC 61400-1 standard. The main conclusions are as follows:  
1. Blade extension can significantly increase the power of wind turbines and decrease the rated wind speed to 11 m/s, 10.8 m/s and 10.7 m/s. With the increase in blade length, the optimum tip-speed ratio \(\lambda_{opt}\) and the corresponding power coefficient \(C_{pmax}\) slightly increase, while the thrust coefficient \(C_{T}\) decreases at the same \(\lambda\). In addition, a significant increase in the steady loads of trust of wind rotor and bending moment at blade root can be observed.
2. By assessing the ultimate loads in different wind conditions, including NTM, ETM, EOG, ECD and EWS, and analyzing their load time series, we reveal that the load fluctuation trend is similar in the same wind condition. Wind turbine fault, shutdown and gust can lead to large load fluctuations and the amplitude of fluctuations shows a monotonic increase with the extension length of the blades. The trend of ultimate load variation with the blade extension is similar for different wind conditions.
3. By evaluating the ultimate loads for all design load cases, we find that the ultimate loads of \(M_{y}\) and \(F_{x}\) at blade root and tower bottom, and the \(M_{x}\), \(M_{y}\) and \(M_{yz}\) at the hub all increase significantly after the blade extension. In particular, the percentage increase of these quantities is over 10% when the blade extension achieves 2 m. Compared with the steady operational loads, the dynamic ultimate loads have the same growth trend, but often increase by a larger percentage.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{2}{c}{**Blade Extension Length**} & **1 m** & **1.5 m** & **2 m** \\ \hline Cost of material (\(\Psi\), thousand yuan) & 15.30 & 22.95 & 30.60 \\ Cost of mold (\(\Psi\), thousand yuan) & 130 & 140 & 150 \\ Cost of tip manufacture (\(\Psi\), thousand yuan) & 40 & 50 & 60 \\ Cost of transport and section (\(\Psi\), thousand yuan) & 150 & 165 & 180 \\ Revenue increase of power generation(\(\Psi\), thousand yuan) & 78.97 & 114.09 & 148.89 \\ Payback period (year) & 4.25 & 3.32 & 2.83 \\ \hline \hline \end{tabular}
\end{table}
Table 18: The cost of wind turbines after blade extension.  
Considering the cost and power generation revenue, the economy improves with the increase in blade length within the safety margin.  
In the paper, we assessed the ultimate loads of wind turbines with different blade extension lengths, which can provide an important reference for this type of technical transformation.  
Conceptualization, Y.L. and M.G.; methodology, M.G.; software, X.L.; writing--review and editing, Y.L., X.L., A.C., L.Z. and W.L. All authors have read and agreed to the published version of the manuscript.  
This work was supported by Huaneng Group Headquarters 'Science and Technology Project' HNKJ20-H12 Research on \"Wind Farm Performance Improvement Technical Improvement Safety Requirements and Evaluation\".  
Not applicable.  
Data Availability Statement: Data is contained within the article.  
The authors declare no conflict of interest.