Client

Partner consortium created for the project CENIT-E Ocean Lider (Líderes en Energías Renovables Oceánicas)

Objective

Developing the technology for the efficient and comprehensive ocean renewable energy

Seaplace-501: A TLP Turbine for the Gulf Stream or the Kuroshio Current?

The mass transport of the Gulf Stream current is more than 30 times the total flow of the fresh water rivers of the entire world. This huge current achieves at some points in the East coast of Florida an annual power density of about 2,0 kW/m2 (corresponding to a mean current speed of about 3 knots). In these locations the current speed ranges 85% of the time between 1 and 2 m/s at depths of up to 100 m. This way placing a device 40 to 70 m deep should be optimal because for less than 40 m it can interfere with navigation. Since in these fast regions the seabed is 200 to 500 m deep a moored solution seems the best option

Moored turbines require buoyancy, which increases the structure cost but allows the device to be located at the optimal depth without significant impact, making the fabrication of a series of turbines easier. A ballast system is frequently used to control the emersion / immersion of the turbine for installation and maintenance

Stability in most moored designs is achieved using a kite concept. However in these cases a change of thrust (e.g. if the turbine is not working) could result in considerable displacements of the turbine. A significant vertical displacement would modify substantially the pressure on the watertight tanks and probably would require active control of these tanks. This problem might be solved using passive floaters, but then the installation process could turn out to be much harder. On the other hand a significant excursion (axial and lateral displacement) would require more space between turbines, probably leading to a smaller exploitation of a specific field. If only a single forward mooring line is required a big space shall be left between adjacent turbines. Other moored designs use forward and aft mooring lines, providing a higher footprint on the seabed, and not allowing the device to turn 180º if the stream changes its direction

By using vertical tethers a TLP mooring significantly reduces the vertical displacements, minimizing the pressure variation in the air tanks. A TLP (tension leg platform) mooring also allows a better use of a specific field since the footprint is much smaller. Besides, this mooring can be arranged to allow the turbine to turn into the current direction, easily increasing the capacity factor of the turbine. This is possible as long as the center of gravity, buoyancy center and the hydrodynamic center are appropriately located

Considering all the points mentioned above we chose a TLP arrangement and designed a turbine (Patent pending, PCT/ES2013/070328) to harness currents such as those of the Strait of Florida. In our design, the Seaplace-501 Marine Turbine, the required buoyancy is given by a nozzle and/or diffuser. This structure also helps us to significantly reduce the size of the screw, a 6-blade in-line solution (shaftless turbine). In the preferred embodiment, the diffuser is first installed by controlling the air pressure. Then the nozzle, a much smaller structure that includes the electric equipment and the screw, is connected to the diffuser. If maintenance of the turbine is needed we do not need to uninstall the TLP mooring, just uncouple the nozzle from the diffuser

We put a big effort in optimizing the nozzle, diffuser and turbine, and we confirmed with experimental tests the good behavior of the concept: a CFRP model was built and tested at the CEHIPAR in 2012-2013. During the tests we checked the stability for several loading conditions and buoyancy distributions. The turbine is capable of smoothly turning into the current direction. There are several available configurations for the tendons depending on the specific needs for each case. Some are shown in the attached figures

The device can be fully submerged, allowing navigation and some fishing activities. Furthermore, several devices can be fitted to the same anchoring system one above the other

From an environmental standpoint, the nozzle protects the tips of the blades from damaging the fauna, and the footprint is rather small due to the short distance between the tendons at the seabed, affecting very little the bottom fauna & flora. Dismantling the device is also possible after fulfilling its operating life

Note: The following video shows a summary of the project Ocean Lider




And the following video shows a summary of the model tests that have been carried out the CEHIPAR model basin