Oceans and seas have enormous amounts of energy lying in them. They are mainly tidal energy, wave energy, ocean thermal energy. The water in the sea or ocean has kinetic energy which can be utilized to generate electricity.
Tidal energy is due to tides generated by the gravitational forces of the sun and the moon on ocean water. All coastal areas experience two high tides and two low tides over a period of about 24 hours. The tides are the periodic rise and fall of ocean water.
The tidal movement is different from the wave movement. Waves have a period of about 6 seconds whereas tides have a period of about 12 hours. Waves are caused by the interaction of wind with the water surface. Whereas tides are caused by interaction of gravitational forces of the moon and sun on ocean water.
For the tidal energy to be able to harness, the difference between high and low tides must be more than 5 meters. These are only about 40 sites on earth with the difference of high and low tides of this magnitude suitable for harnessing tidal energy.
The tidal power projects are very expensive as compared to river dams, because large structures must be built in difficult locations of coastal areas. There are very few coastal locations in the world where tidal range is large enough to justify harnessing the tidal energy.
PHENOMENON OF TIDE
Tide is a periodic rise and fall of the water level of the sea or ocean. Tide occurs due to the attraction of seawater by the moon and sun. The effect of the moon is about 2.6 times more than the sun. Tides occur twice in a period of 24 hours 50 minutes called lunar day when the sea water rises and falls. The rotation of earth causes two high tides and two low tides everyday at any place.
When the sun and moon align in a line then it gives rise to spring tide on the occasion of the new moon and full moon. During these periods the sun and the moon act in combination to produce tides of maximum range. This occurs twice in a lunar month (about 28 days i.e., time taken by the moon for a complete revolution around the earth).
The sun and moon are at right angles with respect to earth on the first and third quarter of the moon. The tides produced are neap tides, when the tidal range is minimum.
During the remaining period tidal range gradually decreases from spring tide to neap tide and gradually increases from neap tide to spring tide.
When water is above mean sea level, it is called flood tide and when the water is below the mean sea level, it is called ebb tide.
POTENTIAL OF TIDAL POWER
There are limited sites for tidal power plants having tidal power potential all over the world. They include Canada, Argentina, England, France, China, Russia, USA, India.
Highest potential is available in Canada and as much as 29000 MW can be generated in the bay of Fundy.
A very few tidal power plants have been constructed so far. The largest tidal power plant is located in La Rance in France, which has completed 50 years of successful operation. Russia and China have very low potential.
In India tidal power is available at three main locations mainly Gulf of Cambay, Gulf of Kutch in Gujarat and Sunderbans, Durgaduani in West Bengal. India has tidal power potential of 8000 MW to 9000 MW having 7400 MW in Gulf of Cambay and 1200 MW in Gulf of Kutch and 100 MW at Sunderbans.
The world's maximum potential for electric power generation by tidal energy is estimated at about 550 billion kilowatt hour per year. Even though tidal energy has a huge potential, there is very limited utilization at present because technology is yet to develop.
TIDAL ENERGY AND POWER
The tidal energy and average power that can be produced depends on the tidal basin of an area (A) which is filled at high tides and emptied at low tides. When water is released to sea through a turbine it generates electrical energy E and power P.
R = tidal range, i.e., the difference of water level at high tide and low tide.
r = head of pool water below which the turbine will stop to run or minimum head for turbine operation.
ρ= density of seawater, i.e., 1025 kg/m³
g= acceleration due to gravity = 9.81 m/s²
The energy E generated in one filling or emptying process is given by the following expression.
E=1/2ρgA(R²-r²),Joules
Power,P=Energy,E/Time,t
=ρ.g.A.(R²-r²)/2t, watt
Where, t=duration of tide=6hours 12min 30 secs.
= 22350
COMPONENTS OF TIDAL POWER PLANT
The main components of tidal power plant are:
1. Dam or barrage
2. Sluice-ways from basin to the sea and vice versa
3. Power house
(1) Dam or Barrage
Dam or barrage is used for storage of water. Channels for turbines are provided in the barrage. Generally the barrage is constructed where there is sufficiently a high tide range to obtain a good water head.
The best sites are bays and estuaries. The nearer the barrage built to the mouth of the bay the larger the basin but smaller the tidal range.
(2) Sluice ways
Tidal power basins have to be filled and emptied. Gates are opened and closed regularly and frequently. lift gates are generally used in existing plants. However, flap gates technology has been under consideration as they are operated by water pressure and require no mechanical means of operation. However, flap gates allow only in the direction from sea to the basin.
(3) Power House
Because small heads are available in the tidal power plants, large turbines are needed as a huge amount of water is required at low head to produce sufficient power. Thus the power house is of large structure.
For low heads, three types of turbine are used:
1. Bulb turbine
2. Tube turbine
3. Straight flow rim type turbine.
The selection of the turbine is made according to the suitability of the tidal plant.
TYPES OF TIDAL POWER GENERATION SYSTEMS:
There are four main types of tidal power generation systems:
1. Single basin single effect tidal power generation system
2. Single basin double effect tidal power generation system
3. Double basin with linked-basin tidal power generation system
4. Double basin with paired-basin tidal power generation system
CHALLENGES AND LIMITATIONS
1. There are limited sites suitable for tidal power generation as it requires a large tidal range. At least 5 m of tidal range should be available.
2. Construction in the sea or in estuaries is difficult.
3. Sea water is saline, so it corrodes the machinery.
4. The head available for tidal power plants is very low which requires a large turbine size of proper design, and developed technology.
5. The water head available with the turbine varies over a wide range. This adversely affects turbine efficiency and overall efficiency of the power plant.
6. Long gestation period and low plant load factor.
7. Design and manufacturing of machinery poses a challenge such as turbulence and cavitation effects, etc.
8. The tidal power plant must take into account the damaging effects of erratic weather conditions, cyclones, and tsunamis.
SELECTION OF SITE
Criterion for Selection of site:
1. The site must have a large tidal range.
2. The site should have capacity to store large quantities of water with minimum dyke construction.
3. It should be located near an estuary or creek for achieving a large storage capacity.
4. The site should be as near as possible to the load center to reduce transmission costs.
ADVANTAGES AND DISADVANTAGES OF TIDAL ENERGY
Advantages:
1. It is a free, renewable and inexhaustible energy source.
2. It is pollution free as it does not use any fuel.
3. It does not depend on rain so it is better than river based hydro power plants.
4. Tidal power plant does not require valuable land as it is located on the sea shore.
5. Tidal power is predictable and reliable as the tides are available twice in a day.
6. Running cost is low.
Disadvantages:
1. Tidal power plants are located far away from the load centers so transmission cost is high.
2. Capital cost of the plant is high..
3. Sedimentation and siltation of basins are major problems of tidal power plants.
4. The size of turbine and generators required is large as compared with river-based hydro-power plants.
5. The navigation, i.e., water transport is obstructed.
6. Tidal Power Plant output is not constant. It varies with tidal range.
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