Page 78 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

70 Fundamentals of Ocean Renewable Energy

FIG. 3.22 Tidal Energy Limited DeltaStream device prior to installation in Ramsey Sound, Wales.
(From Tidal Energy Limited.)

into the possibility of floating tidal devices, potentially reducing capital and
installation costs. The hydrodynamic theory of horizontal axis turbines is
covered in Section 3.13.

3.12.2 Vertical Axis Turbines

Vertical axis turbines are similar in principal to horizontal axis turbines, but
with a different axis of rotation (Fig. 3.23). These devices are more suited to
regions where there is a large variation in the direction of tidal streams (e.g.
rotary currents) in contrast to the rectilinear flows exploited by horizontal axis
turbines. An example vertical axis turbine is the Gorlov Helical Turbine, which
utilizes three twisted blades in the shape of a helix [e.g. 11].

3.12.3 Oscillating Hydrofoils
Oscillating hydrofoils have one or more hydrofoils attached to an oscillating
arm (Fig. 3.24). The oscillating motion used to produce power is due to the lift
generated by the tidal stream flowing either side of the wing. The motion can be
used to drive a hydraulic system to produce useful power. Oscillating hydrofoils
are generally suited to shallower water environments such as estuaries, because
competing technologies such as horizontal axis turbines require significant
clearance for the rotor to operate (in addition to navigational constraints).