Adaptive Composite Blades for Horizontal Axis Tidal Turbines

Abstract

The oceans are a huge resource of untapped energy. There are many marine renewable energy sources however tidal energy has the advantage of being highly predictable. The range of devices for tidal energy extraction is extensive with power being generated either by extracting potential or kinetic energy. Due to environmental concerns regarding potential devices it is thought that a breakthrough will occur in the area of kinetic energy devices, most likely horizontal axis tidal turbines (HATTs). No HATT technology is yet commercial and further research into the field is required to advance the concepts, improve the feasibility of maintenance and make devices more efficient and economic.
The blades of a HATT are the source of energy extraction for the device and are required to operate in a harsh subsea environment for a long (20 year) life cycle with minimal need for maintenance. Choice of an appropriate foil section for the blade is integral. A novel bi-directional section has been developed for use on a HATT and compared to a variable pitch device. The concept of using adaptive composite blades in order to improve energy capture but also decrease design complexity has been considered. Preliminary analysis was undertaken and it suggested that a 2.5% increase in annual energy capture and a 10% decrease in thrust loading could be expected through the use of a bend-twist coupled adaptive HATT blade. An experimental method was developed for the assessment of bend and induced twist in a coupled spar, and the results compared well to numerical analysis with the conclusion that the numerical model is very sensitive to manufacturing accuracy. A design tool for the development of adaptive composite bend-twist coupled HATT blades has been developed. The tool couples finite element analysis, computational fluid dynamics and blade element theory to create optimal blades.
Coupling the passively adaptive bend-twist coupled spar with actively adaptive snap-through asymmetric laminates to create an efficient blade that works optimally in both directions of tidal flow has been considered. A comparison between four arrays of 4 turbines in a staggered grid, one each with standard fixed pitch, bi-directional, passively adaptive and actively adaptive blades illustrated that annual energy capture could be increased by up to 120% with the use of actively adaptive blades when compared to a base fixed bladed rotor

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

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