The Resource

Tidal energy occurs due to large movements of water in the sea. As tides come in and out (flow and ebb), water near the coast is raised and lowered and the potential energy of this tidal range can be exploited. In areas that constrict flow – eg between islands, water flow accellerates and can provide high velocity underwater currents.

It is also possible to harness the kinetic energy of the moving water in the tidal stream itself. The amount of energy possible to extract depends on the speed of the flowing stream and the area intercepted. This is similar to wind power extraction, but because water is much denser than air, an equivalent amount of power can be extracted over smaller areas and at slower velocities.

Tidal power has the huge advantage of being entirely predictable.

Technology

There are two main methods of capturing tidal energy, by a barrage installed across an estuary with a high tidal range or through underwater devices placed in areas of high sea currents around headlands (tidal stream). The tidal barrage is a long-established technique which essentially involves a structure with gated sluices and low-head hydro turbines. Bridging two sides of an estuary, the principle of operation is to allow water to flow into the area behind the barrage with the flood tide and out during the ebb tide. As water flows out, the collected head of water turns the turbines to generate power.Tidal barrage schemes have previously been proposed in the UK, notably at the River Severn.

Several types of device have been designed to capture tidal stream energy, these include (courtesy of the British Wind Energy Association www.bwea.co.uk):

• Tidal stream turbines – These work on a similar principle to wind turbines and indeed may look quite similar. The turbine may be coupled directly to a standard generator via a gearbox, or use an alternative power train design.
• Reciprocating tidal stream devices – These have hydrofoils which move back and forth in a plane normal to the tidal stream, instead of rotating blades. One design uses hydraulic pistons to feed a hydraulic circuit, which turns a hydraulic motor and generator to produce power.
• Venturi effect tidal stream devices – In these, the tidal flow is directed through a duct, which concentrates the flow and produces a pressure difference. This causes a secondary fluid flow through a turbine.

New findings from the Marine Energy Challenge (www.carbontrust.co.uk) suggest that the technical UK tidal stream resource is 18 TWh/year, which is about 10-15% of the known worldwide tidal stream resource.

Cost

The cost of tidal energy is very site specific, and influenced by geography, distance to grid, and speed and volume of the current.

Initial tidal stream farms are estimated to have costs of energy between 9p/kWh and 18p/kWh, with central estimates in the sub-range 12p/kWh to 15p/kWh.

Tidal stream energy could become competitive with current base costs of electricity within the economic installed capacity estimated for the UK, 2.8 GW. It is accepted that the larger the tidal stream device (in terms of Mega Watts) the cheaper the energy produced will be. For example it is predicted that the for the Pelamis project energy prices could fall as low as 3p/kw hour by 2010.

Examples of current uses

Although the technology required to harness tidal energy is well established, tidal power is expensive, and there is only one major tidal generating station in operation. This is a 240 megawatt station at the mouth of the La Rance (seen in picture) river estuary on the northern coast of France (a large coal or nuclear power plant generates about 1,000 MW of electricity). The La Rance generating station has been in operation since 1966 and has been a very reliable source of electricity for France. La Rance was supposed to be one of many tidal power plants in France, until their nuclear program was greatly expanded in the late 1960's. Elsewhere there is a 20 MW experimental facility at Annapolis Royal in Nova Scotia, and a 0.4 MW tidal power plant near Murmansk in Russia.

The world’s first offshore tidal energy turbine was launched off the coast of Devon in 2003. The single 11 metre-long rotor blade is capable of producing 300 kilowatts of electricity and will be a test-bed for further tidal turbines.

Future development and potential

The UK has some of the best locations for tidal energy - enough to make a significant contribution to the UK’s energy needs. The UK is fast developing the industry and hopes to be a world leader in tidal technology.

The Department of Trade and Industry has stated that almost 10% of the UK’s electricity needs could be met by tidal power. It has been estimated that a barrage across the Severn River in western England could supply this 10%. However, tidal barrage schemes face major obstacles in dealing with the environmental issues of damming estuaries, which are often valued for their scenic beauty and wildlife.

The world’s first deep-sea tidal-energy farm will be built off the Welsh coast in 2008 to provide electricity for 5,000 homes. Eight underwater turbines, each 25 metres long and 15 metres high, are to be installed on the sea bottom off St David’s peninsula in Pembrokeshire, South Wales. Construction is due to start summer 2008 and the proposed tidal energy turbines, described as "a wind farm under the sea", should be operational by 2010.