# A Feature-Based Cost Estimation Model for Wind Turbine Blade Spar Caps
## 3 Cost Model Methodology, Selection of Cost Modelling Approach  
An Excel-based cost model was developed to derive a cost curve of spar cap cost versus spar cap blade length from 35 to 75 m in order to estimate the cost of GFRP and CFRP retrofit spar cap blades. The model is intended as a tool for assessing the economic feasibility of retrofitting wind turbine blades (Figure 1). The life extension for this model is five years. If the cost of the CFRP and GFRP retrofits are known in addition to turbine investment, operational, and maintenance costs, the Levelized Cost of Energy (LCOE) for these retrofits over a 25-year extended life can be compared with each other and with LCOE for 20-year baseline GFRP turbine blades. CFRP retrofits were 3 m longer than the replaced blade. For example, a 38 m blade replaces a 35 m blade, a 48 m blade replaces a 45 m blade, and so on. Longer length CFRP retrofits were not considered due to likely turbulence effects and planning restrictions. GFRP retrofits will have the same length as replaced GFRP turbine blades.  
A resource-based technical cost modelling approach, in which historical cost data is either not available or does not exist, was used to derive a cost curve for lengths 40 to 75 m. Spar cap manufacturing resources are material, tooling, capital (plant infrastructure), direct and indirect labor, and utilities. Material for each spar cap length was already known. Indirect labor costs are for management and administration. Utilities are electricity costs. Both indirect labor and utilities costs are assumed to vary only with the production rate over the 35 to 75 m range. Tooling, capital, and direct labor costs for 40 to 75 m were unavailable. Employing available tooling, capital, and direct labor cost data, technical cost modelling was used to develop a Cost Scaling Feature Factor in which tooling, capital, and direct labor costs scale with spar cap features defined as mass, volume fraction, and complexity. The production rate will determine cost per part: the higher the rate, the lower the cost per part. Based on the known production rate for the 35 m spar cap, and assuming the production rate decreases with spar cap mass, volume fraction, and complexity, the Cost Scaling Feature Factor can estimate production rates for the 40 to 75 m range. As more data becomes available, detail can be added to the model to improve predictive power.  
From estimated cost data, a cost curve was plotted for spar cap cost against blade length. For validation, the curve was compared with one independently derived by a 35 m wind turbine blade manufacturer for a box spar turbine blade design in 35 to 75 m blade lengths. For each blade length, CFRP 3 m extended length retrofit costs and GFRP retrofit costs were derived from the cost curve. Retrofit costs, model-generated turbine investment costs, operation and maintenance costs, and power generation data were used to find the Levelized Cost of Energy (LCOE) at 25 years for retrofitted CFRP and GFRP blades, which was then compared with each other and with LCOE for a GFRP 20-yearbaseline. To account for the effect of higher GFRP mass on Direct Labor cost, embedded learning as a measure of manufacturing complexity was investigated as a cost factor.