TIDAL RESOURCE
The motion of the tides is caused by the gravitational pull of the moon and 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 is caused by the moon and sun being alternately in conjunction or opposition. This results in spring and neap tides. There are other cycles that add 6-month, 19-year, and 1600-year components but these are much smaller.
Therefore, tidal energy is energy generated through the motion of the planets. As such, it is one of the few renewable energy sources that do not depend either directly or indirectly on radiant solar energy.
The actual size of the tidal movement depends on geographical location. Tidal amplitude in the open ocean is around 1 m but this increases nearer to land. Amplitude can be substantially enhanced by the coastal land mass and by the shape of river estuaries. Under particularly propitious conditions, such as those found in the Severn estuary in southwest England or the Bay of Fundy in Canada, the tidal amplitude will increase substantially. The Bay of Fundy, for example, has a recorded maximum tidal reach of 16.2 m, while that of the Severn estuary is 14.5 m. Note, however, that mean tidal amplitudes in these regions are likely to be much smaller than this.
The energy that can be extracted from tidal motion waxes and wanes with the tide itself. Under most conditions power output is not possible continuously. Tidal movement is, however, extremely predictable and the timing of the tides can be calculated with great accuracy. This makes tidal power a valuable form of renewable generation because, although intermittent, it is entirely reliable in its behavior.
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 are considered economically recoverable based on the use of tidal barrages. Aggregated national estimates (see below) suggest that the total recoverable energy is much higher than this but not all of it will be economic. Current production from tidal energy is probably around 1 TWh.
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, although a number of pilot projects have been launched, large-scale schemes have generally been judged too expensive to build.
One of the most thorough research projects examining national tidal potential was carried out in the United Kingdom 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 United Kingdom’s best site is the Severn estuary. The country probably has the best tidal regime in Europe but the European Atlantic coast offers a variety of other potential sites.
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, focused 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 Mezenski Bay on the White Sea could provide 15 GW of generating capacity and an annual output of 40 TWh, while a second at Tugurki Bay has a potential generating capacity of 7800 MW and 20 TWh/year. The Russian state utility has estimated that total Russian tidal potential is 250 TWh/year.
Korea has a variety of tidal sites and is home to the world’s largest tidal power plant at Sihwa that began operating in 2011. The country has plans for further large tidal schemes. India also has substantial tidal potential. The Gulf of Kutch on the northwest coast has been studied and a 600 MW project proposed. Meanwhile, the Gulf of Khambhat has an estimated generating capacity of 7000 MW. The Indian government has put the country’s tidal potential at 10,000 MW but it could be much larger than this.
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 United States 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 that 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 favor of a fossil fuel plant.
Tidal power does not need to be tied to estuaries. In the 1960s, France devel- oped 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, or bund, was to be constructed that 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.