On the design of a scaled rotor for a tethered tidal converter to deploy at sea

Model scale testing is vital for developing technologies like tidal turbines. Reduced-scale tests follow numerical design and are cost-effective in controlled environments, allowing parameter adjustments. However, controlled tests face drawbacks, such as blockage effects and the inability to replicate real sea conditions. Sea trials, conducted in actual marine environments, offer realistic data and continuous collection, potentially being cost-effective despite environmental variability and complex data processing. This work aims to measure real sea effects, such as marine growth and stream turbulence, on fixed and moving parts, of a scaled model of GEMSTAR tidal stream energy converter, a twin rotors submerged tethered device, designed to tap tidal current energy. The planned model installation site is at the Renewable Marine Energy Laboratory, a natural facility located on the East coast of the Strait of Messina (southern Italy), characterized by tidal currents with a maximum speed of about 1.25 m/s. Due to the low stream speed, in order to get significant data, an up-scaling of the existing model rotor diameter has been necessary, and a specific new blade design has been performed, in order to lower the cut-in speed, increasing in this way the useful power production time of the model. The paper illustrates the different phases of model scale design.