# A Feature-Based Cost Estimation Model for Wind Turbine Blade Spar Caps
## 6 Conclusions and Recommendations for Further Work  
The model is intended as a tool for assessing the economic feasibility, in terms of reduced LCOE, of retrofitting wind turbines with new spar cap blades. Estimated spar cap costs were within 6.8% and 8.3% (CFRP and GFRP, respectively) of a manufacturer's cost estimates of spar cap blade lengths from 35 to 75 m having the same box spar design. LCOE was calculated for 20-year baseline GFRP blade turbines ranging from 35 to 75 m. The reduction in LCOE for longer length CFRP retrofits compared with same length GFRP retrofits decreased as blade length increased, implying that CFRP retrofits may not be viable at some blade length above 75 m. A lower CFRP price will increase CFRP retrofit viability at longer blade lengths. The model has two disadvantages. Firstly, results are estimates, which are not as accurate compared with other modelling approaches. Secondly, the model does not contain an engineering assessment of spar cap retrofits. To meet design and operational performance constraints, any assessment of blade retrofits in terms of decreased LCOE must account for a variety of conflicting cost and engineering factors to arrive at an optimum retrofit solution compared with a system redesign and complete turbine replacement. Engineering studies performed concurrently with financial modelling may confirm if longer CFRP retrofitted blades can meet given wind loading, tip deflection, and power generation performance criteria, and whether increased turbulence resulting from longer retrofit blades outweighs increased power generation. Unless these are controlled as far as possible, any cost estimates will be highly uncertain. Any engineering study should include an assessment of how lighter blades affect nacelles and rotors designed for heavier glass blades. An assessment must also consider the environmental cost implications of complete turbine replacement compared with life extension via blade retrofits, and account for planning restrictions on the maximum allowable blade length that can be retrofitted. Further work should explore: incorporating engineering criteria, such as wind loading, tip deflection, and edgewise fatigue limits into the model; how retrofitted blades impact engineering performance; extending the model to spar caps less than 30 m; and investigating the applicability of the model to other turbine components. These approaches will require detailed engineering assessments and readily available cost data. Cost drivers such as inflation, variation in part complexity, and volume fraction should be investigated. As increased organizational learning reduces cost, identification and recording of human memory patterns during spar cap manufacture may enable the development of a virtual human algorithm that reinforces learning and reduces cost. Additionally, an algorithm for automatic calculation of Cost Factor Indices and Cost Scaling Feature Factors is a concept worth pursuing. In tandem with a concurrent engineering philosophy to model different scenarios that account for specified engineering criteria covering varying wind loading regimes and power generation performance specifications, these approaches can result in an improved model to enable more accurate prediction of Levelized Cost of Energy so that spar cap retrofit design can be optimized and cost reduced. Conceptualization, J.C.; Data curation, J.C.; Formal analysis, J.C.; Investigation, J.C., T.D. ; Methodology, J.C.; Project administration, A.M. and E.A. ; Resources, A.M. and E.A. ; Software, J.C.; Supervision, A.M. and E.A. ; Validation, T.F. and P.S. ; Visualization, J.C., T.D. ; Writing - original draft, J.C., T.D. ; Writing - review & editing, J.C., A.M., T.F. and P.S. All authors have read and agreed to the published version of the manuscript. This research received no external funding  
The author would like to thank [PERSON] for providing valuable advice and guidance. Thanks also to [PERSON] of Eire Composites for useful information on composite Spar Cap manufacturing.