Energy is an important input for any country’s economy and for any form of life. The standard of living of a given country can be directly related to per capita energy consumption. Energy crisis is due to two reasons:
1. Rapid increase in world’s population
2. Increase in the standard of living of the human beings
The conventional energy sources are depleting in nature and may get exhausted by the end of this century or at the beginning of the next. Though coal based and naphtha based power project are riding over other non-conventional energy sources, environmental problems associated with such mega-projects are many. Due to oil shocks of 1970s, oil-fired power generation has come down to 15%. This has increased the dependence on alternating energy resources such as solar energy, wind energy, Tidal energy, biomass etc.
2. INTRODUCTION
Tidal power is another way to generate electricity extracting energy from the sea (see also wave power). Twice a day enormous volumes of water rise and fall with the tides, and the UK UK France has an active Tidal power generator - the 240MW La Rance experimental tidal power plant in Brittany
The word "tides" is a generic term used to define the alternating rise and fall in sea level with respect to the land, produced by the gravitational attraction of the moon and the sun. They are caused by slight variations in gravitational attraction between the Earth and the moon and the sun in geometric relationship with locations on the Earth's surface. Tides are periodic primarily because of the cyclical influence of the Earth's rotation.
4. WHAT ARE TIDES?
Tides are periodic rises and falls of large bodies of water. Tides are caused by slight variations in gravitational attraction between the Earth and the moon and the sun in geometric relationship with locations on the Earth's surface During high tide, the water is deeper and comes further onto the beach. Another name for high tide is flood tide. During low tide, the water is shallower and does not come as far onto the beach. Another name for low tide is ebb tide. Tidal range may vary over a wide range (5-17m) from site to site
4.1. Spring tide:
Around new and full moon when the Sun, Moon
and Earth form a line (a condition known as syzygy), the tidal forces due to the Sun reinforce
those of the Moon. The tides' range is then at its maximum: this is called the "spring tide,"
4.2. Neap tide:
When the Moon is at first quarter or third quarter, the Sun and Moon are separated by 90° when viewed from the earth, and the forces due to the Sun partially cancel those of the Moon. At these points in the lunar cycle, the tide's range is at its minimum: this is called the "neap tide," or "neaps".
5. TYPES OF TIDES
Diurnal or daily tides - have a single high tide & a single low tide per tidal day
Semidiurnal or semidaily tides - have two high tides & two low tides of approximately equal height each tidal day. Semidiurnal tides may have daily inequity, where successive high tides have different heights. Semidiurnal tides often easy to predict because high (or low) tides occur a consistent length of time after the moon has passed overhead When heights of two successive high tides or two low tides are markedly different, we have a mixed tide. Identify higher high water tide (HHW), lower high water tide (LHW), higher low water tide (HLW), & lower low water tide (LLW). Times of high stands & low stands are not simply related to passage of moon overhead
5.1. Global distribution of the tide types
6. ENERGY IN TIDES
Types of tidal energy can be extracted:
Ø Kinetic energy of currents between ebbing and surging tides and
Ø Potential energy from the difference in height (or head) between high and low tides.
E=gρA∫hdh = 0.5gρAR2
Where,
E= the energy,
g = acceleration of gravity,
ρ= the sea water density, which equals its mass per unit volume,
A= the sea area under consideration.
h = the head and
R=the tide range.
The power is generated during emptying (or filling) and no power is generated during rest of the time. The average theoretical power delivered by the water is W divided by the total time it takes each period to repeat itself. (6 h, 12.5 minutes=22,350seconds). Assuming an average sea density=1025Kg/m3.
The average power per unit basin area is given by
Pav/A=0.225R2Watts/m2 (MW/Km2)
7. TIDAL ENERGY TECHNOLOGIES
Harnessing the energy in the tides
There are two fundamentally different approaches to the exploitation of tidal energy. The first is to exploit the cyclic rise and fall of the sea level through entrainment (tidal barrages or dams) and the second is to harness local tidal currents in a manner somewhat analogous to wind power (tidal turbine farms).
7.1 Tidal barrage methods
Essentially the approach is always the same. An estuary or bay with a large natural tidal range is identified and then artificially enclosed with a barrier. This would typically also provide a road or rail crossing of the gap in order to maximize the economic benefit. The electrical energy is produced by allowing water to flow from one side of the barrage, through low-head turbines, to generate electricity.
Essentially the approach is always the same. An estuary or bay with a large natural tidal range is identified and then artificially enclosed with a barrier. This would typically also provide a road or rail crossing of the gap in order to maximize the economic benefit. The electrical energy is produced by allowing water to flow from one side of the barrage, through low-head turbines, to generate electricity.
7.2 Tidal turbine farms
In this method the electricity is generated through windmill like structures which are immersed in the see water, as shown in figure bellow.
8. COMPONENTS OF TIDAL POWER PLANT
The power house:
The turbines, electric generators and other auxiliary equipments are the main equipments of a power house.
a) The dam or barrage to form the pool or basin:
It acts as a barrier between the sea and the basin or between one basin and the other in case of multiple basins.
b) Sluice ways from the basins to the sea and vice-versa: These are used either to fill the basin during the high tide or empty basin during the low tide, as per the operational requirement. Generally the power house and the sluice ways are aligned with the dam.
8.1 Recent turbine developments
Bulb turbines incorporated the generator-motor unit in the flow passage of the water. These turbines are used at the La Rance power station in France
.
Rim turbines allow the generator to be mounted in the barrage, at right angles to the turbine blades. It is difficult to regulate the performance of these turbines and it is unsuitable for use in pumping.
Finally, in tubular turbines, the blades are connected to a shaft which is oriented at an angle that allows the generator to be located on top of the barrage.
9. GENERATION
There are a variety of suggested modes of operation. These can be broken down initially into single-basin schemes and multiple-basin schemes. The simplest of these are the single-basin schemes.
Single-BasinTidalBarrageSchemes.
These schemes, as the name implies, require a single barrage across the estuary, There are, however, three different methods of generating electricity with a single basin. All of the options involve a combination of sluices which, when open, can allow water to flow relatively freely through the barrage, and gated turbines, the gates of which can be opened to allow water to flow through the turbines to generate electricity.
These schemes, as the name implies, require a single barrage across the estuary, There are, however, three different methods of generating electricity with a single basin. All of the options involve a combination of sluices which, when open, can allow water to flow relatively freely through the barrage, and gated turbines, the gates of which can be opened to allow water to flow through the turbines to generate electricity.
EbbGenerationMode.
During the flood tide, incoming water is allowed to flow freely through sluices in the barrage. At high tide, the sluices are closed and water retained behind the barrage. When the water outside the barrage has fallen sufficiently to establish a substantial head between the basin and the open water, the basin water is allowed to flow out though low-head turbines and to generate electricity.
During the flood tide, incoming water is allowed to flow freely through sluices in the barrage. At high tide, the sluices are closed and water retained behind the barrage. When the water outside the barrage has fallen sufficiently to establish a substantial head between the basin and the open water, the basin water is allowed to flow out though low-head turbines and to generate electricity.
The system can be considered as a series of phases. Typically the water will only be allowed to flow through the turbines once the head is approximately half the tidal range. This method will generate electricity for, at most, 40% of the tidal range.
FloodGenerationMode.
The sluices and turbine gates are kept closed during the flood tide to allow the water level to build up outside the barrage. As with ebb generation, once a sufficient head has been established the turbine gates are opened and water can flow into the basin, generating electricity.
The sluices and turbine gates are kept closed during the flood tide to allow the water level to build up outside the barrage. As with ebb generation, once a sufficient head has been established the turbine gates are opened and water can flow into the basin, generating electricity.
This approach is generally viewed as less favourable than the ebb method, as keeping a tidal basin at low tide for extended periods could have detrimental effects on the environment and on shipping. In addition, the energy produced would be less, as the surface area of a basin would be larger at high tide than at low tide, which would result in rapid reductions in the head during the early stages in the generating cycle.
Two-WayGeneration.
It is possible, in principle, to generate electricity in both ebb and flood. Unfortunately, computer models do not indicate that there would be a major increase in the energy production. In addition, there would be additional expenses associated in having a requirement for either two-way turbines or a double set to handle the two-way flow. Advantages include, however, a reduced period with no generation and the peak power would be lower, allowing a reduction in the cost of the generators.
It is possible, in principle, to generate electricity in both ebb and flood. Unfortunately, computer models do not indicate that there would be a major increase in the energy production. In addition, there would be additional expenses associated in having a requirement for either two-way turbines or a double set to handle the two-way flow. Advantages include, however, a reduced period with no generation and the peak power would be lower, allowing a reduction in the cost of the generators.
Double-BasinSystems.
All single-basin systems suffer from the disadvantage that they only deliver energy during part of the tidal cycle and cannot adjust their delivery period to match the requirements of consumers. Double-basin systems, have been proposed to allow an element of storage and to give time control over power output levels.
All single-basin systems suffer from the disadvantage that they only deliver energy during part of the tidal cycle and cannot adjust their delivery period to match the requirements of consumers. Double-basin systems, have been proposed to allow an element of storage and to give time control over power output levels.
The main basin would behave essentially like an ebb generation single-basin system. A proportion of the electricity generated during the ebb phase would be used to pump water to and from the second basin to ensure that there would always be a generation capability.
It is anticipated that multiple-basin systems are unlikely to become popular, as the efficiency of low-head turbines is likely to be too low to enable effective economic storage of energy. The overall efficiency of such low-head storage, in terms of energy out and energy in, is unlikely to exceed 30%. It is more likely that conventional pumped-storage systems will be utilised. The overall efficiency of these systems can exceed 70% which is, especially considering that this is a proven technology, likely to prove more financially attractive.
10. LA RANCE – A WORKING CASE STUDY
La Rance Tidal Power Plant is the only full scale power station of its type in the world, located in northern France
11. ADVANTAGES OF TIDAL POWER SYSTEM.
1. Permits the simultaneous use of the dam for a road and/or a railway line.
2. Protects vulnerable coastline from strong waves and floods.
3. Provides a non-polluting and inexhaustible supply of energy.
4. The technology does not rely on fuel to produce electricity.
5. Does not emit greenhouse gases.
6. The system is easy to operate & maintain.
7. It is non-polluting and almost silent when running.
8. Provides employment to a large number of people.
SOME OF THE MAIN DRAWBACKS ARE.
1. High capital cost
2. Construction time is several years for large projects
3. Limited number of potential sites (i.e. site-specific)
4. Power generation is intermittent
13. CONCLUSION:
Tides play a very important role in the formation of global climate as well as the ecosystems for ocean habitants. At the same time, tides are a substantial potential source of clean renewable energy for future human generations. Tidal power is a natural source of energy with many benefits. The planet's tidal capability greatly exceeds that of the world’s entire coal and oil supply.
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