# A Feature-Based Cost Estimation Model for Wind Turbine Blade Spar Caps
## 4 Results and Discussion
### Commercial Feasibility, Material Efficiency. Levelized Cost of Electricity  
The global weighted average Levelized Cost of Electricity (LCOE) for offshore wind is predicted to fall to \(\ell\)0.045/kWh from \(\ell\)0.078/kWh (2014) by 2020, with UK offshore wind electricity priced at \(\ell\)0.057/kWh in 2019 [1]. In a context of decreasing prices, retrofitting blades for wind turbine life extension is emerging as an alternative strategy to very expensive system redesign and replacement options. The environmental effects of material production and processing are becoming critical [40]. GFRP spar caps over 50 m blade length show increased blade flexure and excessive weight on turbine hub and bearings [48; 49; 50; 31]. Specific stiffness for CFRP is higher than for GFRP, therefore CFRP blades will flex less. For the same weight, CFRP blades are longer than GFRP blades and will generate more power. Levelized Cost of Electricity was modelled for three scenarios: a 20-year baseline rated turbine life with GFRP blade lengths 35 to 75 m (Table 10); a 5-year life extension with a GFRP retrofit for each blade at the same length (Table 10); and 3 m longer CFRP retrofits for each blade length 35 to 75 m (Table 11). For both retrofit scenarios, CFRP pric was varied to assess CFRP retrofit feasibility. Tables 10 and 11, and Figure 8, compare LCOE for glass blades from 35 to 75 for a 20-year life with LCOE for same length GFRP blade retrofits for life extension to 25 years, and LCOE for 3 m longer length CFRP blade retrofits for life extension to 25 years. The CFRP price was \(\ell\)33/kg. LCOE decreases for GFRP retrofits as more power is generated compared to the 20-year baseline. A 35 m GFRP retrofit decreases LCOE by 6.4%, with the percentage decrease declining to 5.8% at 75 m (Table 10). A longer length CFRP retrofit for a 35 m blade (Table 11) decreases LCOE by 10.49% as the longer CFRP refit generates more power. The percentage decrease in LCOE declines to 8.39% at 75 m. In both cases, the decline is due to dominance of material and labor costs at annual production rates of 285 for 35 m and 272 for 75 m, with material and direct labor increasing as a proportion of overall cost and increasing absolute blade cost with length. Figure 10 compares LCOE for the three scenarios at a CFRP price of \(\ell\)5/kg. Tables 12 and 13 show how the decrease in LCOE percentage reduction for longer length CFRP retrofits compared with GFRP  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Spar Cap** & **GFRP, 20 Years** & **GFRP Retrofit, 25 Years** & **Reduction in** \\
**Length (m)** & **LCOE (\(\ell\)/kWh)** & **LCOE (\(\ell\)/kWh)** & **LCOE (\%)** \\ \hline
35 & 0.157 & 0.147 & 6.42 \\
40 & 0.154 & 0.144 & 6.40 \\
45 & 0.150 & 0.140 & 6.37 \\
50 & 0.146 & 0.137 & 6.31 \\
55 & 0.142 & 0.133 & 6.23 \\
60 & 0.138 & 0.130 & 6.13 \\
65 & 0.134 & 0.126 & 6.03 retrofits varies with CFRP price. At \(\epsilon\)33/kg, the LCOE percentage reduction for CFRP is lower for all blade lengths and more marked with increase in blade length than for \(\epsilon\)5/kg (Figures 9 and 10). As the percentage reduction in LCOE for CFRP retrofits decreases with increasing blade length, this implies that at some blade length greater than 75 m, a CFRP retrofit will give the same reduction in LCOE compared with a GFRP retrofit. Above this length, a CFRP retrofit will not be economically viable due to much higher material price compared with GFRP. Figure 11 shows that a lower CFRP price will increase the viability of CFRP retrofits at higher blade lengths (Tables 12 and 13). This is a simplified sensitivity analysis, as the only variable changed was CFRP price. If only material was considered, CFRP retrofits would not be financially viable compared with GFRP retrofits above a certain blade length. However, new CFRP retrofits will generate more power with reduced LCOE, since longer and thinner CFRP blades retrofits are aerodynamically more efficient than thicker GFRP retrofits.