Behaviour of a tidal turbine system using a diffuser in waters characterized by low speeds and the presence of surface waves typical of the Mediterranean Sea

Abstract

The exploitation of the kinetic energy of the Mediterranean Sea tidal currents is hindered by low speed values. Therefore, it is useful to investigate methods for accelerating the flow to increase both the power generated by the single machine and the specific power, i.e. related to a certain sea surface. The aim of the study is predicting the power gain achievable by means of a diffuser in very low speed conditions and also to analyse the behaviour of the diffuser in the presence of small amplitude and high frequency surface waves, typical of the Mediterranean. Using 3D CFD simulations we predicted the performance of a system of 2 counter-rotating turbines, coupled side-by-side, equipped with a symmetrical convergent-divergent diffuser. The fluid dynamic interaction with the rotating blades is just mimicked by adopting the momentum source low-order approach, that in our case is based on the Virtual Blade Model (VBM). The diffuser not only enables more than doubled power output, but also dampens the relative fluctuation of torque in case of very low wave amplitude; however, in case of significant amplitudes the turbine angular speed needs to be increased in order to avoid stall, and therefore to preserve the blade structural integrity.