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Tidal Energy Devices

Tidal energy exploits the natural ebb and flow of coastal tidal waters caused principally by the interaction of the gravitational fields of the earth, moon and sun. The fast sea currents are often magnified by topographical features, such as headlands, inlets and straits, or by the shape of the seabed when water is forced through narrow channels. The tidal stream devices which utilise these currents are broadly similar to submerged wind turbines and are used to exploit the kinetic energy in tidal currents. Due to the higher density of water, this means that the blades can be smaller and turn more slowly, but they still deliver a significant amount of power. To increase the flow and power output from the turbine, concentrators (or shrouds) may be used around the blades to streamline and concentrate the flow towards the rotors.

We have identified four main types of Tidal Energy Convertors (TEC):

Emec - horizontal axis turbineA) Horizontal axis turbine
This device extracts energy from moving water in much the same way as wind turbines extract energy from moving air. Devices can be housed within ducts to create secondary flow effects by concentrating the flow and producing a pressure difference

Emec - vertical axis turbineB) Vertical axis turbine
This device extracts energy from moving in a similar fashion to that above, however the turbine is mounted on a vertical axis.

Emec - oscillating hydrofoilC) Oscillating Hydrofoil
A hydrofoil attached to an oscillating arm and the motion is caused by the tidal current flowing either side of a wing, which results in lift. This motion can then drive fluid in a hydraulic system to be converted into electricity.

Emec - venturiD) Venturi Effect
By housing the device in a duct, this has the effect of concentrating the flow past the turbine. The funnel-like collecting device sits submerged in the tidal current. The flow of water can drive a turbine directly or the induced pressure differential in the system can drive an air-turbine.

E) Other Designs

This covers those devices with a unique and very different design to the more well-established types of technology or if information on the device’s characteristics could not be determined.

 

Methods to fix the TEC to the seabed:

Further to the categories of devices identified above, there is also a range of methods to fix the converter to the seabed.

i) Seabed Mounted / Gravity Base
This is physically attached to the seabed or is fixed by virtue of its massive weight. In some cases there may be additional fixing to the seabed.

ii) Pile Mounted
This principle is analogous to that used to mount most large wind turbines, whereby the device is attached to a pole penetrating the ocean floor. Horizontal axis devices will often be able to yaw about this structure. This may also allow the turbine to be raised above the water level for maintenance.

iii) Floating (with three sub-divisions)
Flexible mooring: The device is tethered via a cable/chain to the seabed, allowing considerable freedom of movement. This allows a device to swing as the tidal current direction changes with the tide.
Rigid mooring: The device is secured into position using a fixed mooring system, allowing minimal leeway.
Floating structure: This allows several turbines to be mounted to a single platform, which can move in relation to changes in sea level.

iv) Hydrofoil Inducing Downforce
This device uses a number of hydrofoils mounted on a frame to induce a downforce from the tidal current flow. Provided that the ratio of surface areas is such that the downforce generated exceeds the overturning moment, then the device will remain in position.