With a growing demand for longer blades in the wind turbine industry for higher rated power per turbine, a structurally sound blade-root connection is of commercial importance. Bushing connections have been a commercially favoured design in the past few years, replacing the commonly used T-bolt connection as the joining method of choice. The new design replaces the barrel nut in the T-bolt with an axial bushing that the bolt connects to and can be assembled with the laminate during the lay-up stage of the blade skin. It has been theorised that it can result in a reliable connection due to the elimination of laminate stress concentrations. However, literature outlining the performance of a blade-root connection with bushings is lacking in the current body of knowledge. While several patents for bushing designs exist, they don’t provide verifiable results on their efficacy due to trademark laws. The objective of this project is to design and conduct a numerical study of a blade-root connection with bushings with an aim to replace the T-bolt connection, along with providing evidence of the effect of various parameters on the structural performance of the blade root. The design is to be based on a Suzlon Energy-make blade with a pitch circle diameter of 3m and a blade length of 63m. The project has been planned in three phases: (1) Design of the root; (2) Validation of the design; (3) Comparative analysis. Modelling and FE analysis has been carried out in the ANSYS environment. Parameters of the bolted connection have been determined according to industry standards provided by VDI and GL. Design validation was conducted based on structural constraints; the key design constraint relevant to the blade-root as a sub-component of the wind turbine is the accumulated fatigue damage. For the final phase of the study, various parameters associated with the assembled root were identified and tested in iterations and their effect on the structural performance, weight, and cost of the assembly were studied. The results confirm the hypothesis of reduction of stress concentrations within the laminate; this eliminates several failure modes associated with composite laminates at the blade root. As is, the bushing connection can be considered a viable alternative to the T-bolt joint. Within the connection, higher absolute stresses and stress gradients were developed in the bolt joining the blade to the hub. Hence, this was the area of focus for fatigue damage evaluations. Conservative estimates of the accumulated damage show values well within the acceptable range. The connection has been designed keeping several concurrent variables in mind. Given the commercial applicability of the design, a rigid optmized design is not feasible due to unpredictable parameters like certification costs, total assembly times, and procurement costs. Therefore, an effort has been made to understand the effect of varying component parameters. The design lends itself to flexibility of dimensioning and material choice within the sub-components; parameters can be optimised according to cost, manufacturability, and performance requirements. While the base design configuration for the bushing connection is heavier than the T-bolt design, improved fatigue performance can be seen as a favourable trade-off.

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