What is Tidal Energy

Tidal energy is the utilization of the sun and moon's gravitational forces - as tides are formed by the gravitational pull of the sun and moon on the oceans of the rotating earth. Tides can be found with varying degrees of strength on any coastline, and sometimes even at sea, although these are better known as currents. A flood tide is one that is coming in or rising and an ebb tide is one that is going out. Tides are generated through a combination of forces exerted by the gravitational pull of the sun and the moon and the rotation of the earth. The relative motion of the three bodies produces different tidal cycles which affect the range of the tides. In addition, the tidal range is increased substantially by local effects such as shelving, funnelling, reflection and resonance.

Origin

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.

Since India is surrounded by sea on three sides, its potential to hamess tidal energy has been recognised by the Government of India. Potential sites for tidal power development have already been located. The most attractive locations are the Gulf of Cambay and the Culf of Kachchh on the west coast where the maximum tidal range is 11 m and 8 m with average tidal range of 6.77 m and 5.23 m respectively. The Ganges Delta in the Sunderbans in West Bengal also has good locations for small scale tidal power development. The maximum tidal range in Sunderbans is approximately 5 m with an average tidal range of 2.97 m. The identified economic tidal power potential in India is of the order of 8000-9000 MW with about 7000 MW in the Gulf of Cambay about 1200 MW in the Gulf of Kachchh and less than 100 MW in Sundarbans. The Kachchh Tidal Power Project with an installed capacity of about 900 MW is estimated to cost about Rs. 1460/- crore generating lectricity at about 90 paise per unit. The techno economic feasibility report is now being examined.

Advantages of Tidal Energ

The most important advantage of tidal energy is its economical benefits, as tidal energy does not require any fuel. Tides rise and fall every day in a very consistent pattern. The economic life of a tidal plant is very high. A plant is expected to be in production for 75 to 100 years, in comparison with the 35 years of a conventional fossil fuel plant. Besides the economical factors, tidal energy is clean and renewable, unlike fossil fuels. Tidal energy offers a lot of potential to be a substitute for hydrocarbon and fossil fuels. A very important feature of tidal energy is that it is non-polluting. A tidal barrage can prevent approximately one million tons of CO2 per TWH generated. A barrage can also safeguard coastlines from storms.

Disadvantages of Tidal Energy

The altering of the ecosystem at the bay is the biggest drawback of tidal power. Damages like reduced flushing, winter icing and erosion can change the vegetation of the area and disrupt the balance. The alteration of tidal currents affects the habitat of the seabirds and the fish. Similar to other ocean energies, tidal energy has several prerequisites that make it only available in a small number of regions. For a tidal power plant to produce electricity effectively (about 85% efficiency), it requires a basin or a gulf that has a mean tidal amplitude (the differences between spring and neap tide) of 7 meters or above. It is also desirable to have semi-diurnal tides where there are two high and low tides everyday. Tides out in the ocean have maximum amplitude of about one meter. As you move closer to shore, this can increase to as high as 12 or more. This can depend on local features such as shelving or funneling meaning the tidal range can vary considerably along any given coastline. This can mean that a lot of places just aren't suitable. When planning the location major consideration has to be given to see whether the tides ar high enough and if there is a suitable place for building the site.