A model for semi-permeable barriers with velocity-enhanced vertical-axis turbines for tidal streams

A model is presented for assemblies of ducted vertical-axis tidal turbines, arranged as a two-dimensional semi-permeable barrier. The choice of the correct contraction ratio of the convergent, together with the design features of the vertical-axis turbine, results from a compromise between blockage, by-pass flow and power output. The barrier/turbine assembly is designed to determine, in subcritical conditions (Fr<1), a measurable downstream local decrease of the sea level. In order to predict the overall pressure loss of the assembly, this last is modelled separately in its three components: convergent, turbine and diffuser. The most relevant pressure loss is due to the enhanced-velocity vertical axis turbine; the pressure loss was predicted using a Blade Element turbine Model (BEM), with results matched against published performance data by a manufacturer. Finally, a study for model validation by means of physical tests in reduced scale to be performed in a laboratory flume is presented.