Coastal areas with huge and flowing tidal waters carry vast potential energy. 11th Century England was the first to harness this energy, using water wheels to produce mechanical power. These days the rise and fall of tides have become the basis to produce electrical power similar to the principles of hydroelectric power generation.
The daily rise and fall in the level of ocean water relative to the coastline is referred to as tide. Tides originate from the motions of the earth, moon and sun. The gravitational pull of the Moon and Sun along with the revolution of the Earth result in tides.
(The magnitude of the gravitational attraction of an object is dependant upon the mass of an object and its distance.)
The moon exerts a larger gravitational force on the earth, though it is much smaller in mass, because it is a lot closer than the sun. This force of attraction causes the oceans, which make up 71 percent of the earth's surface, to bulge along an axis pointing towards the moon. Tides are produced by the rotation of the earth beneath this bulge in its watery coating, resulting in the rhythmic rise and fall of coastal ocean levels.
The gravitational attraction of the sun also affects the tides similarly, but to a lesser degree. As well as bulging towards the moon, the oceans also bulge slightly towards the sun. When the earth, moon and sun are positioned in a straight line i.e on the occasion of a full or new moon, the gravitational attractions are combined, resulting in very large spring tides. At half moon, the sun and moon are positioned at right angles, resulting in lower neap tides. Coastal areas experience two high and two low tides over a period of 24 hours and slightly above.
The presence of geographical features such as bays and inlets result in higher tides. To produce enough amounts of power (electricity) that can be put to practical use, a difference of at least five meters between high and low tides is a must. There are about 40 suitable sites around the world with this kind of tidal range. The higher the tides, the greater is the amount of electricity that can be generated from a given site. It is inversely proportional to the cost of electricity produced, making such sites also more economical. Approximately 3000 GW (1 Giga Watt = 1 GW = 1 billion watts) of energy are available from the tides, worldwide. However considering the limitations as mentioned above, only about 2% (= 60 GW) can potentially be exploited for electricity generation.
Generating tidal energy
The technology required to convert tidal energy into electricity is comparable to technology used in traditional hydroelectric power plants. The first requirement is a dam across a tidal bay or estuary. However building a dam is expensive and the best sites are those where a bay has a narrow opening, thus reducing the length of dam required. Gates and turbines are installed. When there is adequate difference in the levels of the water on the different sides of the dam, the gates are opened. This causes water to flow through the turbines, turning the generator to produce electricity.
Electricity is generated by water flowing both inwards and out of a bay. There are periods of maximum generation every twelve hours, with no electricity generation at the six-hour mark in between. The turbines may also be used as pumps to pump extra water into the basin behind the dam at times when the demand on electricity is low. This water can later be released when the demand on the system is very high, thus allowing the tidal plant to function like a "pumped storage" hydroelectric facility.
Uses and economy
The friction of the bulging oceans acting on the spinning earth results in a very gradual slowing down of the earth's rotation but this is not expected to impact us for billions of years. Therefore, for practical purposes, tidal energy can be considered a sustainable and renewable source of energy. It can prove to be a valuable source of renewable energy to an electrical system. The demand of electricity from a grid varies with the time of the day. Tidal power, although variable, is reliable and predictable and can make a valuable contribution to an electrical system, which has a variety of sources. Tidal electricity provides a good alternative to conventional methods of generating electricity, which would otherwise be generated by fossil fuel (coal, oil, natural gas) etc, thus reducing emissions of greenhouse and acid gases.
Dam across La Rance
Although the technology is well in place, tidal power is an expensive affair. Operating and maintenance costs of tidal power plants are very low because the fuel, being seawater, is free. However the overall cost of electricity generated is still very high. There is only one major tidal generating station in operation - the 240-MW tidal plant (1 megawatt = 1 MW = 1 million watts) at the mouth of the La Rance river estuary on the northern French coast. In operation since 1966, this plant has been a very reliable source of electricity for France.
Researchers are examining the potential of several other tidal power sites and some of the prospects include the Severn River Dam in western England, the Bay of Fundy in Canada, Cook Inlet in Alaska, and the White Sea in Russia.
Environmental studies so far indicate that tidal energy does not result in the emission of gases responsible for global warming or acid rain associated with fossil fuel generated electricity. Use of tidal energy could also decrease the need for nuclear power, with its associated radiation risks. One concern is that the tidal flows caused by damming a bay or estuary could, result in negative impacts on the immediate environment. This is still unclear as very little is understood about how altering the tides can affect incredibly complex aquatic and shoreline ecosystems. However each specific site is different and the impacts depend greatly upon local geography. Local tides changed only slightly due to the La Rance dam, and the environmental impact has been negligible. It has been estimated that in the Bay of Fundy, tidal power plants could decrease local tides by 15 cm.
The negative environmental impacts of tidal barrages are probably much smaller than those of other sources of electricity. Tidal energy has the power to generate significant amounts of electricity at suitable sites around the world. But the potential is yet to be fully explored.