Preliminary Design of Composite Wind Turbine Towers

Abstract

Over the past decade, wind turbine towers have grown taller and wider to support high capacity turbines. It may, therefore, be prudent to investigate materials alternative to steel to mitigate an increase in tower mass, cost, and complexities in transportation & manufacturing associated with the steel towers. The current research focuses on the preliminary design of economically feasible composite wind turbine towers.Some of the project objectives involve setting up the design tool for tubular and lattice tower made up of Glass Fiber Reinforced Plastic (GFRP) and Carbon Fiber Reinforced Plastic (CFRP) material, preliminary design of possible joining techniques, setting up the cost model for various manufacturing approaches and assembly techniques and comparison of composite towers to steel tower design regarding mass and cost. The design tool will incorporate all the essential load cases, structural and geometric constraints and will help to analyze the composite towers with various hub heights and for 2.1 MW and 5 MW turbinecapacities.Loads and constraints are estimated through the literature study. Preliminary design of the tubular tower has been carried out using an analytical approach. For the lattice towers, a Finite Element Method (FEM) approach using Matlab was the suitable method to perform the analysis. A minimum first natural frequency constraint of 0.27Hz and 0.22Hz have been incorporated during the design for 2.1MW and 5MW turbine capacity towers respectively. For the joint design, bolted and adhesive joints are considered and the failure modes associatedwith these joints have been incorporated into the design to get an estimate of the joint mass and cost. The cost modeling of composite structures was done using the parametricequations that fit the Process Cost Analysis Database (PCAD) cost model in the region of interest. These parametric equations are usually functions of the surface area of the structure, perimeter, the number of plies and the complexity of the part. The final part involves comparison of the GFRP and CFRP towers with steel towers and comments on the feasibility of the composite towers.CFRP tubular and lattice towers show a mass reduction of up to 60% for lower hub height towers (<=100m), but this advantage decreases with height. Due to the high material cost, the CFRP towers are at least 3-4 times costlier than steel towers. For the GFRP material system, up to 35% mass reduction was found in the tubular design with lower hub height (<=100m). 5MW-100m hub height GFRP tubular tower showed the highest mass advantage (35%) and was the closest in cost to the steel design with the GFRP tower being 4% costlier. The GFRP lattice towers showed a similar trend with up to 35% but were at least 27% costlier than steel tower due to the high material and joint costs.The only region where composite currently shows any promises of mass and cost feasibility is for smaller hub heights (<=100m) and on tubular towers. The major existing difficulties with steel towers are for heights in the range of 125-150m, and in this region, the composite tower design does not show mass or cost advantages. Based on the results of the various trade studies and optimized designs, it was concluded that the composites do not hold a definitive promise as an alternative material for wind turbine towers over steel. With the current technology and understanding of the tower designs, the mass advantage promised by composite towers is not enough for composites to be deemed a viable option that can thrive in a competitive market for renewable energy. Thus a few scopes for future research are provided that can help in strengthening the understanding of the composite tower design