Marine Energy - Tidal Extraction Devices

Continuing our exploration of Marine Energy we now look at tidal stream energy, how tidal energy devices operate, and some of the devices used to capture this energy.

Tidal stream energy is a relatively new technology enabling energy in underwater currents to be extracted. There are three categories:

• Tidal Stream Turbines – these are like underwater wind turbines.
• Oscillating Tidal Stream Devices - operate in the flow with a hydrofoil action.
• Venturi or Ducted Tidal Stream Energy Devices – the stream is directed through a venturi ducting causing a pressure difference.

The most modern of these, the Seagen tidal stream turbine, is producing power from the tidal streams under the waters of Strangford Lough in Northern Ireland. The device consists of a mono-pile (this is a single pile developed by the offshore oil and gas industry for unmanned oil/gas production facilities) that is driven by a pile driver about 20m into the seabed, depending on the seabed bedrock.

This mono-pile supports the two drive units made up of a twin-bladed turbine propeller that rotates through 180 degrees to facilitate the changing directions of the tidal stream due to the ebb and flood tides. This design negates the requirement for another set of turbines to rotate in a different direction with the turn of the tide.

Each propeller drives a generator through a gearbox and produces power which is cabled down to the seabed thence shore-side to supply the grid via a subsea cable.

The drive units themselves are mounted on horizontal arms that are attached to a collar on the mono-pile. This collar is connected to the maintenance platform with cables and this allows the arms to be raised above sea-level for maintenance of the power units.

The complete structure weighs 300Tonnes and the turbine blades are 16m diameter, with the whole array producing 1.2MW of power for Northern Ireland’s national grid. The devices were vigorously tested and closely monitored for any effect on marine environment as Strangford Lough is a wild bird sanctuary. These tests proved satisfactory producing no adverse affects.

An example of this device is shown below.

One of these oscillating tidal stream devices that uses the power from the sea as a renewable energy resource is the Stingray. This was conceived in 1993 and following government grants, further research, and development since then has led to several prototypes being built and successfully tested in tidal streams.

These devices use the principle of a hydroplane, oscillating in the current much like the wing of a stingray, from which this device got its name.

The Stingray is a steel fabricated underwater structure to which a pair of pivoting arms is attached. Connected to the arms is a hydroplane known as the wing. The action of the tidal stream over the hydroplane causes the arms to lift and fall. When the arms reach the top of their stroke the hydroplane incline angle is reversed causing the arm to be lowered again, the cycle then being repeated. These arms are linked hydraulic rams that due to the oscillating motion, pump the hydraulic oil to accumulators which supply high pressure fluid to the hydraulic motors and run the power generators

The structure is secured to the seabed using steel piling or adjustable steel anchor wires.

A drawing showing the working principles of a Stingray oscillating device follows.

This is actually a device in its own right as well as an add-on component to existing devices in the form of a venturi hood. This has been estimated to increase the efficiency and power output by a factor of between two and four.

It uses the principle of a venturi to create an eddy of low pressure behind the turbine thus encouraging the flow to be drawn across the turbine entrance.

It can be fitted to most tidal stream turbines retrospectively and has also been tested on wind turbines.

It is especially suited to those tidal stream devices which are exposed to a rocky seabed, the venturi hood smoothing out the undulant currents caused by the terrain.

In one example; the Lunar Energy Ducted Turbine the angle of the venturi to the stream direction of flow can be adjusted to ensure maximum efficiency of the turbine.

In this device the venturi is encased in a structural steel frame that is anchored securely to the seabed. The venturi incorporates angle adjusters ensuring that it is maintained facing the direction of stream flow.

My interpretation of this device is shown below.