FTMC Turbine: A Marine Turbine for the Gulf Stream Current Off the Coast of Florida

This paper describes a marine turbine concept specifically designed to harness the energy from the Gulf Stream Current. This current achieves, at some points off the East coast of Florida, a speed that ranges between 2 and 4 knots in the first 100m of water depth, for 85% of the time. A device in this region shall be at least 40m deep to avoid navigation and wave interference. Since in these fast regions the seabed is 200 to 500m deep, moored solutions are the most feasible option. Moored turbines require buoyancy to allow the device to be located at the optimal depth quite easily. In our design the buoyancy is given by a nozzle, which does not obstruct the flow but accelerates it. Consequently the blade size is reduced and protected from shock. The mooring consists of a tension-tether arrangement that significantly reduces the vertical displacements, minimizing the pressure variation in the air tanks. This tension-tethered mooring also reduces the footprint and can be arranged to allow the turbine to passively turn into the current direction, increasing the capacity factor. The device is installed by controlling the nozzle ballast water and air pressure, and by attaching the mooring line to a concrete block on the seabed. If maintenance is needed the operation can be reversed, helping to reduce OPEX. The combination of a TLP (tension leg platform)-like mooring and a robust floater arranged as a nozzle makes the concept feasible. A magnetically-geared Pseudo Direct Drive (PDD®) generator ensures a robust, direct-drive, high torque, low speed and highly efficient solution that can operate in the inherently rough immersed conditions. The design has been validated in 2018 through prototype testing at INTA-CEHIPAR model basin in Spain. Thus, the concept as an offshore alternative energy converter has been confirmed. The small-scale demonstrator is designed for 600 Nm and 6 kW, with rotational speeds of up to 100 rpm. During the tests, the generated power has been measured at different generator and current speeds with the device fixed. The levels of generated power and efficiency agree with the CFD calculations. Furthermore, the stability of the marine turbine has also been confirmed. Several configurations of mooring lines and stabilizers have been tested to ensure that not only the unit is self-steerable, but that it also minimizes Vortex-Induced Motions. Regarding the business case, competitive values of the levelized cost of energy (LCOE) can be achieved because of the high power density available in the selected coastal locations combined with high capacity factors. This overcomes the higher capital expenditure (CAPEX) per MW of each unit, because it is fully immersed, compared to offshore wind solutions. The project has been co-financed by the Eurostars Program.