Tidal motion

The motion of the tides is caused primarily by the gravitational pull of the moon and the sun. This motion varies according to a number of cycles.

The main cycle is the twice daily rise and fall of the tide as the earth rotates within the gravitational field of the moon. A second, 14-day cycle which results in spring and neap tides is caused by the moon and the sun being alternately in conjunction or opposition. There are other cycles that add 6 monthly, 19- and 1600-year components but these are much smaller.

Tidal amplitude in the open ocean is around 1 m. This increases nearer land. Amplitude can be substantially enhanced by the coastal land mass and by the shape of river estuaries. Under particularly propitious conditions, such as are found in the Severn estuary in southwest England, or the Bay of Fundy in Canada, the tidal amplitude will increase substantially. For example, the Severn has an exploitable amplitude of 11 m.

The energy that can be extracted from tidal motion waxes and wanes with the tide itself. Power output is generally not continuous. It is, however, extremely predictable. Unlike most other forms of renewable energy, it is not subject to vagaries of weather. This means that the future output of a tidal power station can be determined with great accuracy.

The tidal resource

The World Energy Council has estimated the global annual energy dissipation as a result of tidal motion to be 22,000 TWh. Of this, 200 TWh is considered economically recoverable. Less than 0.6 TWh is actually converted into electricity.

There has been considerable interest in tidal power since the 1960s and a number of countries have identified sites where tidal power production would be possible. However in most cases proposed schemes have been judged too expensive to build.

One of the most thorough research projects into tidal potential was carried out in the UK between 1983 and 1994. This project looked at a range of possible schemes in England and Wales. It concluded that if every practicable tidal estuary with a spring tidal range of more than 3.5 m was exploited, around 50 TWh of power could be generated each year. This represented around 20% of the electricity consumption in England and Wales in the mid-1990s. The UK's best site is the Severn estuary.

In Canada, the Bay of Fundy has the highest tides in the world. This region, on Canada's east coast, has been the subject of intense examination. A comprehensive study of the region, carried out in the mid-1960s, focussed on sites with a total generating capacity of nearly 5000 MW. However tentative schemes to build projects were abandoned during the changing economic climate at the end of the 1970s.

Russia has significant potential for tidal generation, particularly in the White Sea on the Arctic coast and in the Sea of Okhotsk. A site at Tugur bay with the potential to generate 6800 MW has been identified as promising but the future of this project is uncertain.

Korea has potential tidal sites on the country's east coast. Consultants from various countries have carried out studies at several of these sites. Tidal span on this coast is not great but the region benefits from reflection from the South China Sea. The most promising project is at Garorim where a scheme with a projected capacity of 400 MW has been studied.

India also has substantial tidal potential. The Gulf of Kutch on the northwest coast has been studied and a 600 MW project proposed. The Indian government has estimated the country's tidal potential to be 10,000 MW.

China has studied various potential sites. Its southeast coastline is thought to offer particularly good opportunities. Mexico has looked at a site on the Colorado estuary, Brazil and Argentina have studied projects and the USA has examined a site in Alaska.

Australia's northwestern coast has some of the highest tidal ranges in the world and there are a number of inlets which could be harnessed to generate electricity. A novel two-basin project was proposed near the town of Derby, but the scheme was rejected by the Western Australian government in 2000 in favour of a fossil fuel plant. However other projects are being discussed.

Tidal power need not be tied to estuaries. In the 1960s France developed plans for an offshore project in Mont St Michel bay. The scheme was shelved when the country decided to invest heavily in nuclear power.

The Mont St Michel project involved a tidal plant that did not make use of an estuary. Instead, a circular barrage was to be constructed which would completely enclose an area of open sea. This type of plant would operate in exactly the same way as an estuary plant, with water flowing into the enclosed reservoir when the tide rises, and flowing out through turbines during the ebb tide. While this approach would involve enormous construction costs, it does have the merit of allowing a large tidal plant to be built where no suitable estuary exists.

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