In this work we develop a numerical methodology for the structural analysis and optimization of composite blades for wind and hydrokinetic turbines. While the methodology presented here is equally applicable to the design of wind turbines, this paper focuses on its application to hydrokinetic turbines.
First, we derive a structural mechanics model which is based upon a combination of classical lamination theory with an Euler-Bernoulli and shear flow theory applied to composite beams. The development of this simplified structural model was motivated by the need for an accurate and computationally efficient method that is suitable for parametric design and optimization studies of composite blades. An important characteristic of this structural model is its ability to handle complex geometric shapes and isotropic or anisotropic composite layups.
After validating our simplified structural model, we formulate a structural optimization problem which determines an optimal layup of composite materials within the blade. For a specified design load, the objective of the structural optimization is to minimize the blade’s mass while satisfying constraints on maximum allowable stress, blade tip deflection, buckling, and placement of blade natural frequencies. We demonstrate this optimization methodology to produce a hypothetical design for a composite blade of a utility-scale horizontal-axis hydrokinetic turbine operating in the Admiralty Inlet of Puget Sound, Washington, USA. This particular blade design uses a combination of E-glass, carbon fiber, and foam composite materials. In solving this structural optimization problem, we compare the efficiency of two deterministic optimization algorithms (gradient search and pattern search) and a stochastic particle swarm algorithm.
Finally, we quantify the effects that uncertain material properties can have on the structural performance of composite blades and provide an estimate of the probability of structural failure for a given design. Studying the relationships between material properties and structural performance provides further insights into creating higher-performance, more reliable, and cheaper turbine blades.