# A Feature-Based Cost Estimation Model for Wind Turbine Blade Spar Caps
## 3 Cost Model Methodology, Selection of Cost Modelling Approach
### Technical Cost Modelling: Tooling, Capital, Labor, Production Rate
#### 3.3.1 Resource Costs: Material, Tooling, Capital, Direct Labor, Indirect Labor, Utilities  
Material Cost  
For a carbon fiber composite, the fiber weight fraction is:  
\[\text{ Fiber Weight Fraction}=\ \rho_{f}\text{FVF}\big{[}\rho_{m}+(\rho_{f}-\ \rho_{m}\big{]}/\text{FVP} \tag{10}\]  
where \(\rho_{f}\) carbon fiber density = 1550 kg/m\({}^{3}\); \(\rho_{m}\), epoxy matrix density = 1100 kg/m\({}^{3}\); FVF = carbon fiber volume fraction = 0.56  
From Equation (9), the fiber weight fraction = 0.64. Hence, the matrix weight fraction = 0.36. Based on 500 kg for a 35 m blade spar cap (Figure 3):  
\[0.64\times 500=321\text{ kg of carbon fiber, }0.36\times 500=179\text{ kg of resin}\]  
Fiber cost: 321 kg \(\times\)\(\epsilon\)33/kg = \(\epsilon\)10,593. Resin cost = 179 kg \(\times\)\(\epsilon\)2/kg = \(\epsilon\)358 so total cost = \(\epsilon\)10,951. The scrap rate for unidirectional fiberglass and carbon fiber material was 5% [16]. The resin scrap rate was 15% [16], therefore, the material cost of a 35 m carbon fiber spar cap is \(\ell\)11,979 and the material cost of a 35 m E-glass fiber spar is \(\ell\)3352. Figure 4 in Section 4.1 shows material cost results for 35 to 75 m.  
Dedicated Tooling and Capital Cost  
The dedicated tooling cost of a 35 m spar cap, defined as a Calibration Standard, will be more expensive than the tooling cost of a simple rectangular shape analogue of a 10 m long spar cap, defined as a Reference Standard, by the increased complexity of the Calibration Standard features compared with the complexity of the Reference Standard. These features are mass, complexity, and volume fraction. For the Reference Standard, volume fraction, complexity, and part mass are assumed to be much smaller than for the Calibration Standard. Therefore, dedicated tooling cost of the Calibration Standard is related to the tooling cost of the Reference Standard and Cost Scaling Feature Factor by:  
Calibration Standard Dedicated Tooling Cost = Reference Standard Tooling Cost \(\times\) Feature Factor  
\[\text{ Tooling Cost, }35\text{mm Standard}=\ C_{t}\text{ Reference Standard}/\left(1-V_{f}\right)^{x}\left(1-\overline{C}\right)^{y}\left(1- \overline{M}\right)^{z} \tag{11}\]  
where \(C_{t}\) = Tooling Cost of Reference Standard  
In general, spar cap dedicated tooling cost will vary with the Cost Scaling Feature Factor as:  
\[\text{Cost, }35\text{mm Standard}=\ C_{t}\text{ Reference Standard}/\left(1-V_{f}\right)^{x}\left(1-\overline{C}\right)^{y}\left(1- \overline{M}\right)^{z} \tag{12}\]  
Using the same reasoning as for tooling cost, the capital cost for making a 35 m Calibration Standard spar cap of given complexity, size, and fiber volume fraction compared with the capital cost of the Reference Standard will vary with the Cost Scaling Feature Factor as:  
\[\text{Capital Cost}=\ C_{c}\text{ Reference Standard}/\left(1-V_{f}\right)^{x} \left(1-\overline{C}\right)^{y}\left(1-\overline{M}\right)^{z} \tag{13}\]  
where \(C_{c}\) = Capital Cost of Reference Standard. Figure 4: Resource cost variation with spar cap length. Data for the Reference Standard (Table 2) and a 35 m spar cap Calibration Standard (Table 3) was used to infer Cost Factor Indices \(x\), \(y\) and \(z\) for predicting GFRP and CFRP spar cap cost. The following assumptions were made. Volume fraction \(Vf\) was taken as 0.56, so \((1-Vf)=0.44\). For 35 to 75 m, volume fraction is assumed constant. Complexity Factor \(\overline{C}=0.50\), so \((1-\overline{C})=0.50\), where complexity is assumed equal to 0.5 and maximum complexity equals 1. Mass Factor \(\overline{M}\) increases as \(m\) increases in line with spar cap length. For one 35 m blade spar cap, mass \(m\) is 500 kg and Dedicated Tooling Cost is \(\ell 265\),000 for vacuum infusion (Table 3). The cost of the Reference Standard is \(\ell 20\),000 (Table 2).