Tidal Turbine Blades: Design and Dynamic Loads Estimation Using CFD and Blade Element Momentum Theory

Abstract

The interest in tidal power is constantly increasing thanks to its high predictability, the huge potential of tides and the actual need for renewable energy. It explains the emergence of many tidal turbine designs, especially in Europe, often inspired from wind turbines. All of them are at a more or less early stage of development. But because of the high density of water, environmental drag forces are very large compared with wind turbines of the same capacity. Therefore the knowledge acquired by the wind industry is certainly qualitatively useful, but it has to be reconsidered to be applicable to tidal turbines. The aim of the project presented in this paper is to create a 1 MW reference tidal turbine, whose small-scaled model has been tested in the towing tank of Marintek laboratory (Trondheim, Norway). The tests focused on dynamic loads, which are an important reason of failure, and thus will help tidal turbine designers in their work by gaining valuable experience in turbine performance in various operating conditions. The chosen turbine has a horizontal axis and two blades, which have been designed using the blade element momentum theory for a diameter of 20m. This paper states the project issues and the method used to design the blades, from the hydrodynamic properties of the hydrofoils to the computational fluid dynamic analysis. The tests on the small scaled model makes it possible to validate the concept and a comparison between efficiencies obtained analytically, experimentally and with CFD computation has been performed in this paper. The maximum power coefficient experimentally obtained is 0.427, i.e. 1.4% higher than the power coefficient obtained numerically. The blade element momentum theory is then used to estimate the loads on each blade when the rotor is subjected to regular waves of many heights and periods, with the intention of ranking the parameters of importance and introducing a fatigue analysis.