Experimental investigation of velocity-augmented barriers for tidal stream turbines

In recent years, research on environmentally sustainable energy production has endeavoured to exploit new renewable resources, as a consequence of the technological level increase. Particularly, the harvesting of tidal energy has found an emerging interest. The utilization of tidal turbines could be very appealing in numerous sea locations, where local conditions are favourable to energy harvesting, as rip tides at natural or artificial coastal inlets. Nonetheless, often tidal streams may be too slow for achieving an acceptable efficiency of kinetic energy conversion to electricity by means conventional turbine technologies. Indeed, low velocities correspond to low power extraction. Therefore, a device able to augment the flow velocity could be a proper solution to enhance the exploitation of tidal stream energy. The aim of this work is to investigate the possibility of using a barrier, embodying multiple assemblies of turbine plus velocity-augmented device, to enhance the performance of each turbine. An experimental campaign was carried out on a downscaled model of a semi-permeable barrier aiming to mimic this system by means of orifices of equivalent pressure loss. Tests were conducted in a water flume, in subcritical flow conditions at various Froude numbers. Free surface elevation, velocity profiles and pressure drop were measured. The decrease of the free surface level across the barrier model was also studied, as it results in a favourable feature for enhancing the turbine performance. Furthermore, an analytical actuator-disk model was developed for the barrier plus turbine. Finally, a comparison of the model prediction with experimental results is presented in this article.