Tidal Device Principles

Tidal devices exploit kinetic energy in the tidal flow of water caused by the interaction of the gravitational fields of the earth, moon and sun. Tidal currents are magnified by topographical features, such as headlands, inlets and straits, or sea bed channels. Many of the devices that use these currents are broadly similar to submerged wind turbines. Sea water is around 800 times denser than air but average flow speeds for viable projects are of the order of one fifth of corresponding wind speeds, giving a power density of around 5-10 times. This means that for a tidal device of the same rating as a wind turbine, the rotor will be smaller. Individual tidal devices are unlikely to reach the same rated power as offshore wind turbines; however, as to have sufficient space to deploy such large devices requires development of deeper water sites that generally have less significant tidal flows. EMEC has identified four main types of tidal energy convertors that are outline below.

Tidal energy device concepts (graphics courtesy of EMEC).

Example 1	Example 2
The flow of water turns the rotor by generating lift due to the flow around the blades.  This rotational movement is used to generate electricity.  The device can be housed within a duct to accelerate the flow through the rotor, thus increasing energy capture.	This device extracts energy in a similar way to a horizontal axis turbine, but the axis of rotation of the rotor is vertical and perpendicular to the flow of water.  Some vertical axis turbine concepts work by the principle of drag rather than lift.

Example 3	Example 4
A hydrofoil is attached to an arm.  The flow of water around the hydrofoil causes it to oscillate by generating lift.  This oscillating movement is used to generate electricity.	A duct has the effect of accelerating the flow through the narrowest section.  This flow causes a pressure differential that drives an air turbine.  The rotational movement of the turbine is used to generate electricity.