3. Tidal Stream
3.3. Technology Types
Tidal stream technologies are designed to harness the kinetic energy of the fast flowing water in tidal areas. Research and development in this emerging field have led to the design of several types of device to capture this energy:
Horizontal axis turbines – Horizontal axis turbines work much the same as a conventional wind turbine and some look very similar in design. A turbine is placed in a tidal stream which causes the turbine to rotate and produce power. Some turbines may also be housed in ducting/cowling to create secondary flow effects by concentrating the flow and producing a pressure difference.
Vertical axis turbines – Vertical axis turbines use the same principle as the horizontal axis turbines only with a different direction of rotation. A turbine is placed in a tidal stream which causes the turbine to rotate and produce power.
Reciprocating devices (oscillating hydrofoils) – These have hydrofoils which move back and forth in a plane normal to the tidal stream, instead of rotating blades. The oscillation motion used to produce power is due to the lift created by the tidal stream flowing in either side of the wing. One design uses pistons to feed a hydraulic circuit, which turns a hydraulic motor and generator to produce power.
Venturi effect tidal stream devices – The tidal flow is directed through a duct, which concentrates the flow and produces a pressure difference. This causes a secondary fluid flow through a turbine. The resultant flow can drive a turbine directly or the induced pressure differential in the system can drive an air-turbine.
