Marine Renewable Energy - Tidal Stream

31 March 2013

Marine Professional Newsletter Issue 1

IMarEST TECHNICAL SECTION

By Martin Murphy MSc CEng, FIMAREST, Managing Director, Tidal Energy Ltd.

What is Tidal Energy?

Tidal energy is a form of hydropower that converts the energy of the tides into electricity or other useful forms of power. It is quite distinctive from wave energy which generates power from the motion of a device due to wave effect. Tides are well-understood phenomena caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth, and as such therefore offer a predictable form of renewable energy, which can be harnessed in two forms, as follows.

Tidal Range

Tidal Range is the vertical difference between the high tide and the subsequent low tide. The tidal range is not constant, but changes with cyclic dependency on the relative positions of the sun and the moon and the earth. Barrages and lagoons use this potential energy component of the tide to generate electricity in a similar way to dammed hydroelectric systems.

Tidal Stream

Tidal Stream is the flow of water as the tide ebbs and floods, and it manifests itself as tidal current. Tidal stream devices seek to extract energy from this kinetic movement of water, much as wind turbines extract energy from the movement of the air. The sea currents created by movement of the tides are often magnified where water is forced to flow through narrow channels or around headlands. There are a number of locations around the coastline of the UK where the tidal stream resource is high, and it is in these areas where early technology developments are taking place to explore the prospect of realising the energy.

Recent analysis indicates that the total availability of tidal stream power can contribute significantly to meet the UK’s electricity demand – researched estimates suggest in the region of 10%. Given that tidal power is more predictable than wind and solar power and that economic and environmental costs can be brought down to competitive levels, the untapped tidal stream resources of the UK coast have made it a natural home for innovative research. The industry is at a crucial stage in its development, progressing from a number of prototype installations to demonstration of commercial arrays, i.e., multiple devices connected subsea. Demonstration arrays in the coming 3 to 4 years will create the opportunity for commercial-scale viability and growth in a 2020 timeframe.

Over the last decade, research companies have striven to identify the most efficient designs that can be installed and maintained cost-effectively. In the tidal stream sector, the predominant adopted technology is a horizontal axis turbine which may yaw, or has controllable pitch blades to realise energy from the opposing tidal flows. Cardiff-based Tidal Energy Ltd. (TEL) have developed a commercial-scale tidal energy device called DeltaStream, and are poised to tap into the enormous potential for renewable power generation from the tidal currents around the UK and beyond. The inherent predictability of tidal power makes it highly attractive for grid management. Tidal stream energy presents a significant economic opportunity through synergies with the offshore wind industry and its suitability for clusters of large-scale projects, allowing a business model favoured by utility companies.

DeltaStream Technology

DeltaStream is a tidal energy conversion unit that generates electrical power. It is primarily designed to be located on the seabed in areas with high tidal stream flows, but could also be installed in suitable rivers and estuaries. It is a free-standing device which remains located on the seabed due to its own weight. Fixed-pitch turbines are yawed into the flow by a hydraulic yaw mechanism, and the drive train incorporates an epicyclic gearhub with the high-speed output shaft driving an induction generator. A submarine cable connects the device to its onshore power conversion system for connection to the local electricity distribution network. Tidal Stream is one of the many areas being worked on by the Offshore Renewables SIG.

Martin Murphy
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