Ellen Macarthur Foundation. Rethink the future

Tidal Power Generation

• by Editorial Team
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• 20 Aug 2010
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As an island, the UK is once again ideally placed to harness the energy of the tides. However the correct geographical locations with the right environment to generate from tidal flow currents are hard to come by. Eight sites have been identified around the UK with only 20 in the world classed as suitable for tidal barrages.

It is often thought that there is only one way to harness the power of the tides, the tidal barrage. However, there has been a great deal of research into this area and now there are several different options available for tidal schemes.

The Severn Barrage consultation has been the major push for engineers to take a new look at tidal technology. A tidal barrage is a large dam like structure with a hole to the centre. As the tidal flow moves through the hole, the motion rotates turbines to generate electricity. Another method involves water passing through an air chamber, as the water level rises, the air is forced upwards causing rotation of the turbine. There are drawbacks to this type of scheme mainly the destruction of natural habitats, as the tide never fully retracts. There is also disruption to fish migration and shipping.

Marine Turbine

A marine turbine operates in the same manner as a wind turbine, except that, rather than the wind, it is a current/tidal flow that rotates the blades. One of the leading manufacturers is a company called Marine Current Turbines who operate a technology branded SeaGen (image below). The blades can be rotated 180º allowing them to operate in both the ebb and flow. The rotors can also be lifted out of the water for easy maintenance. The two blades rotate in opposite directions resulting in a balanced torque. At the end of July 2008, the SeaGen turbine installed in Strandford Lough in Northern Ireland by Marine Current Turbine with the support of local authorities, has started to produce electricity which was fed directly to the grid. The company who developed the system had already installed one of the first ever commercial tidal turbines in Devon in 2003, and is now working on a large scale turbine farm project for Western Wales. In France, an experimental immersed turbine is currently being tested at the entrance of the Odet river (Finistère), and the leaders of the project estimate that in the long term this technology could provide enough electricity to meet 40% of the Brittany region’s needs.

Tidal Reef

The tidal reef is a new generation of tidal barrage designed specifically for the Severn Estuary. It looks like a barrage but in fact is ‘low impact’ meaning that tidal levels are hardly disturbed. This would allow for the safe passage of shipping, fish and the preservation of mud flats. As the reef would operate with lower water levels, the designers claim they could harness more energy over the whole length of the tide, rather than a barrage which works best at high and low tide. Another positive feature is the reef’s ability to adjust to tide height and protect low-level land from storm surges.

SwanTurbine

SwanTurbine is a company that grew out of research at Swansea University, with the concept to create a submerged turbine that is both efficient and robust. They stripped the turbines down to the raw essentials; they do not have gearboxes or other non-essential parts that could break. The turbines are able to rotate freely to face into the strongest part of the current.

Vertical Axis Turbine

As the name suggests, these are turbines that are mounted with a vertical axis. Rather than large scale barrages, these turbine units are aimed at fast flowing tidal/river inlets. The gearbox and generator are situated above water level meaning safe and easy access for maintenance.

Wave power

Another major axis of development is the use of wave power rather than tidal currents: a floating device is put in motion by the movement of waves, and the energy created by that motion is then converted into electricity. The most famous example of this technology is probably the “sea snake”, conceived and produced by Scottish company Pelamis Wave Power, and which Portugal is currently acquiring “en masse”, having increased its objectives in terms of renewable energy. PWP claims that “Wave energy could play a major part in the world’s efforts to combat climate change, potentially displacing 1 – 2 billion tonnes of CO2 per annum from conventional fossil fuel generating sources”. Each Pelamis unit is made of a 3,5 m diameter cylinder, 150 m long, with three points of articulation where hydraulic rams activate a generator. Waves cause the system to rise and fall in a snake-like motion.

OTEC

Looking at non-mechanical sources, Ocean Thermal Energy Conversion (OTEC), a principle known since the 19th century, is being considered by some countries: the idea is to use the temperature differential between deep and shallow waters to activate a heat engine – a turbo generator produces the electricity. In order to be efficient, this system requires at least 20°C of temperature difference, is particularly suited for inter-tropical regions, and some OTEC plants are in use in Japan and Hawaii. A first prototype was presented in the early 1930s by French engineer Georges Claude, founder of the Air Liquide company.

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