# A Feature-Based Cost Estimation Model for Wind Turbine Blade Spar Caps
## 2 Literature Review
### Blade Design and Operational Performance  
In a turbine blade, the spar is the principal blade component and carries 85-90% of the bending load acting on the blade. Spars and internal webs in the blades are used to restrict the effect of flapwise bending loads, while the blade profile trailing edges carry edgewise bending loads. [PERSON], [PERSON], [PERSON], and [PERSON] provide comprehensive reviews of wind turbine blade structural design [31, 48, 49, 50]. Where GFRP blades replace GFRP blades having the same design and mechanical properties, there should be no additional design issues for the retrofit blades. The hub, pitch mechanism, bearings, spinner, nose, shaft, and gearbox of the turbine are designed to carry a given blade load for a rated maximum wind speed. Where lighter, longer, thinner, and higher modulus S glass, R glass, or CFRP blades are being considered as replacements for lower modulus shorter E-glass blades, the modifications required for the hub, pitch mechanism, bearings, spinner, nose, shaft, and gearbox when subjected to a lower load regime should be assessed in addition to operational, safety, and performance issues. A retrofitted blade must be specified for a given wind loading regime and have a maximum allowable tip deflection to avoid tower strikes so that power generation performance targets are met. The following engineering parameters are key for optimised turbine blade performance: natural frequency, wind loading, blade tip deflection, aerodynamic efficiency, and edgewise fatigue loading. Large blade design can be driven by the requirement to maintain a high natural frequency so that resonance caused by the pressure drop as the blade passes the tower is avoided. However, above approximately 45 m to 50 m blade length, the design becomes dominated by tip deflection and maintaining tower clearance. A solution is to increase stiffness by adding more unidirectional GFRP in the spar cap as the prinicpal load bearing component, but this is not ideal as the glass has to be added further and further away from the outer surface resulting in a thicker, less aerodynamically efficient design. Furthermore, additional glass increases blade weight, thereby increasing fatigue loads in the edgewise direction. The edgewise fatigue loads are caused by the weight of the blade creating bending firstly in one direction as it climbs to the highest point above the nacelle, and then reversed loading as the blade heads back towards the ground. Higher modulus glass fiber materials, such as E-glass prepreg, R-glass, and S-Glass permit thinner sections, thereby increasing aerodynamic efficiency. A high modulus glass (92 GPa) prepreg will enable a 5-10% increase in blade length for the same blade mass. However, longer blades with glass spar caps require more trailing edge and/or shell laminate to deal with increasing edgewise fatigue loads, resulting in increased weight, loss of aerofoil efficiency, and increased edgewise loading. As CFRP spar caps have higher stiffness and strength-to-weight ratios compared with E-glass GFRP, a CFRP solution results in longer, thinner, more efficient blades leading to more energy capture and a lower overall LCOE compared to an equivalent GFRP blade of the same weight [35]. However, compared with GFRP, CFRP has relatively lower damage tolerance and lower compressive strength than CFRP. [PERSON] [51] observed that although CFRP is becoming cost competitive with GFRP, CFRP remains much more expensive than GFRP, so the economic advantages of a CFRP spar cap do not generally transfer to other parts of the blade, as strength rather than stiffness is the key criterion and the weight penalty of glass becomes less significant. Therefore, the optimal compromise in terms of performance, loading, and cost is to use CFRP in the spar cap for maximum stiffness and E-glass GFRP in the outer periphery for maximum strength.