Performance Prediction of a Free Stream Tidal Turbine with Composite Bend-Twist Coupled Blades

Free stream tidal turbines are a source of growing interest in the marine renewable energy field. Some designs that use an actuated mechanism to control individual blade pitch, as the tidal current varies and even during a revolutionary cycle, face the challenge of ensuring high reliability under the loads imposed on the device in the harsh sub sea environment. Adaptive materials have been used in the wind industry to create bend-twist coupled blades in an effort to increase turbine efficiency, thus increasing annual energy capture and reducing blade fatigue loads. A computationally efficient, yet realistic, empirical model has been developed in order to estimate the amount of induced twist present on a bend-twist coupled blade in a tidal stream. The method has been incorporated into a Blade Element Momentum code, modified to predict the performance of free stream tidal devices. Results show that the use of bend-tweist coupled blades could improve the energy capture of a 20m free stream horizontal axis tidal turbine by around 12.5% without a significant increase in production costs.