Page 79 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

Tidal Energy Chapter | 3 71

FIG. 3.23 Conceptual diagram of vertical axis turbines. (From www.Aquaret.com.)

3.12.4 Venturi Effect Devices
These are funnel-like devices, which direct the water flow through a constricted
section of a duct (Fig. 3.25). This increases the water velocity and decreases
the pressure. The resultant flow can be used to directly drive a turbine, or the
induced pressure differential in the system can drive an air turbine. Venturi
effect devices are useful to exploit lower tidal streams and could potentially
be deployed in much shallower water than horizontal axis turbines.

3.12.5 Tidal Kites

A tidal kite turbine is an underwater kite system that converts tidal energy
into electricity by moving through the tidal stream. The water current creates
a hydrodynamic lift force on the wing of a ‘kite’ (Fig. 3.26), which pushes the
kite forward. The kite is steered in a figure-of-eight trajectory by a rudder, and
reaches a speed many times the water current speed. As the kite moves, water
flows through the turbine to generate electricity. The electricity is transmitted
through a cable in the tether attached to the wing. The Minesto 1/4 scale Deep
Green tidal kite has been successfully trialled in Northern Ireland, and plans are
well underway for Minesto to install a small array of full-scale devices in the
Irish Sea.