Performance prediction of a free stream tidal turbine with composite bend-twist coupled blades

Abstract

Free stream tidal turbines are a source of growing interest in the marine renewable energy field. Some designs use variable pitch blade control devices in order to maximize the efficiency of the turbines; however these are complex to design, construct and maintain under the severe load conditions sub sea devices experience. There is an interest in the use of composite materials for potential improvements in hydrodynamic and structural performance of Horizontal Axis Tidal Turbines (HATT). In addition to the advantages of high strength-to-mass and high strength to stiffness ratios, anisotropy of the laminated fibre composites can be designed to allow 3D tailoring of the blade deformation. Anisotropic structures show different levels of elastic coupling, depending on the ply angle in the layers that comprise such a material. Passive control of a turbine blade can be achieved by taking advantage of the directionality of the anisotropic composite material. A structure that undergoes both bending and twisting due to a pure bending load is said to exhibit bend-twist coupling. This type of behaviour has been identified as a potential method for load reduction - particularly fatigue loads, and an increase in both efficiency and annual energy capture in wind turbines [1]. Preliminary studies have since shown that this may also be the case for HATTs [2]. A computationally efficient, yet realistic, model has been developed in order to estimate the amount of induced twist present on a bend-twist coupled blade in a tidal stream. This model takes into account the effect on the induced twist of fibre orientation, blade loading and cross section, material mechanical properties, and shell thickness. The method has been incorporated into a Blade Element Momentum code, modified to predict the performance of free stream tidal devices; such that the performance of a HATT with composite bend-twist coupled blades could be estimated. It has been shown that, when compared to a free stream tidal turbine with fixed blades of a similar configuration, a HATT that utilises composite bend-twist couple blades can reduced fatigue loading, bring the turbine efficiency closer to the Betz limit and increase the annual energy capture.

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ORCID for S.R. Turnock: orcid.org/0000-0001-6288-0400

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