1 INTRODUCTION:

Energy is the important driving force of world economy. Most of the developing countries and
industries are still totally relying on the conventional energy resources that are coal, oil and gas.
Conventional energy resources will vanish within few decades because of its limited availability.
Excessive uses of conventional energy resources are harmful for health of living things,
environment and ozone layer. Clean renewable energy is the best alternative to avoid further
deterioration of the earths environment. This can be possible only when there is solution of
many technical problems. All countries are individually or collectively taking efforts to solve
technical problems and developing new technologies in the field of renewable energy. Hydro
power, Wind, Solar and Ocean energy are some of the most common sources of clean renewable
energies. Hydro power energy is one of the first harnessed clean sources of energy and hence has
considerable development over the years. Hydro power is stored in the form of potential energy
by building dams across the river. This potential energy is converted in to kinetic energy by
turbines which are finally converted to electrical energy with the help of generators. To get land
fr bouilding dams and storage reservoir is big environmental and people rehabilitation issue in
most of the countries. Next comes the wind energy. Wind energy is an intermittent source of
small magnitudes. The research for improving the efficiency of wind turbine is still under
progress. Solar energy is the most abundant source of available energy. Solar cells are used to
convert solar energy to electricity. The main issue with solar energy is that the solar cells are very
costly and hence cannot be used for mass production. Latest addition in the field of renewable
energy is the energy from oceans. Ocean possess many forms of energy namely Thermal
energy, tidal energy, and energy from waves and circulating currents. The main focus of this
paper is tidal current energy. Research is still underway for developing devices to harness the
vast potential of tidal current energy possessed by ocean. In this paper an attempt has been made
to discuss the current scenario for tidal energy.

2 GENERATION OF TIDAL ENERGY

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tides. Other sources of energy originate directly or indirectly from the Sun, including fossil fuels,
conventional hydroelectric, wind, biofuels, wave power and solar. Nuclear energy makes use of
Earth's mineral deposits of fissile elements, while geothermal power uses the Earth's internal heat
which comes from a combination of residual heat from planetary accretion (about 20%) and heat
produced through radioactive decay (80%). Tidal energy is extracted from the relative motion of
large bodies of water. Periodic changes of water levels, and associated tidal currents, are due to
the gravitational attraction of the Sun and Moon. Magnitude of the tide at a location is the result
of the changing positions of the Moon and Sun relative to the Earth, the effects of Earth rotation,
and the local geography of the sea floor and coastlines. Because the Earth's tides are ultimately
due to gravitational interaction with the Moon and Sun and the Earth's rotation, tidal power is
practically inexhaustible and classified as a renewable energy resource. A tidal generator uses
this phenomenon to generate electricity. Greater tidal variation or tidal current velocities can
dramatically increase the potential for tidal electricity generation. The movement of the tides
causes a continual loss ofnmechanical energy in the EarthMoon system due to pumping of
water through the natural restrictions around coastlines, and consequent viscous dissipation at the
seabed and in turbulence. This loss of energy has caused the rotation of the Earth to slow in the
4.5 billion years since formation. During the last 620 million years the period of rotation has
increased from 21.9 hours to the 24 hours we see now; in this period the Earth has lost 17% of its
rotational energy. While tidal power may take additional energy from the system, increasing the
rate of slowdown, the effect would be noticeable over millions of years only, thus being
negligible.

GENERATING METHODS

Tidal power can be classified into three generating methods: Tidal stream generator, Tidal
barrage, Dynamic tidal power.

TIDAL STREAM GENERATOR

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 A tidal stream generator is a machine that extracts energy from moving masses of water,
or tides. These machines function very much like underwater wind turbines, and are sometimes
referred to as tidal turbines. Tidal stream generators are the cheapest and the least ecologically
damaging among the three main forms of tidal power generation.

Fig :steam generator

3 Types of tidal stream generators

Since tidal stream generators are an immature technology, no standard technology has yet
emerged as the clear winner, but large varieties of designs are being experimented with, some
very close to large scale deployment. Several prototypes have shown promise with many
companies making bold claims, some of which are yet to be independently verified, but they
have not operated commercially for extended periods to establish performances and rates of
return on investments.

4 Energy calculations

Various turbine designs have varying efficiencies and therefore varying power output. If the
efficiency of the turbine is known the equation below can be used to determine the power output
of a turbine. The energy available from these kinetic systems can be expressed as:

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 Where:
= the turbine efficiency
P = the power generated (in watts)
= the density of the water (seawater is 1025 kg/m)
A = the sweep area of the turbine (in m)
V = the velocity of the flow

Relative to an open turbine in free stream, depending on the geometry of the shroud shrouded
turbines are capable of as much as 3 to 4 times the power of the sameturbine rotor in open flow.
5 Resource assessment

While initial assessments of the available energy in a channel have focus on calculations
using the kinetic energy flux model, the limitations of tidal power generation are significantly
more complicated. For example, the maximum physical possible energy extraction from a strait
connecting two large basins is given to within 10% by:

Where
= the density of the water (seawater is 1025 kg/m),
g = gravitational acceleration (9.81 m/s2),
Hmax = maximum differential water surface elevation across the channel,
Qmax= maximum volumetric flow rate though the channel.

TIDAL BARRAGE

A Tidal barrage is a dam-like structure used to capture the energy from masses of water
moving in and out of a bay or river due to tidal forces.

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 Instead of damming water on one side like a conventional dam, a tidal barrage first
allows water to flow into the bay or river during high tide, and releasing the water back during
low tide. This is done by measuring the tidal flow and controlling the sluice gates at key times of
the tidal cycle. Turbines are then placed at these sluices to capture the energy as the water flows
in and out.

6 +Generating methods

The barrage method of extracting tidal energy involves building a barrage across a bay or
river that is subject to tidal flow. Turbines installed in the barrage wall generate power as water
flows in and out of the estuary basin, bay, or river. These systems are similar to a hydro dam that
produces Static Head or pressure head (a height of water pressure). When the water level outside
of the basin or lagoon changes relative to the water level inside, the turbines
are able to produce power.

The basic elements of a barrage are caissons, embankments, sluices, turbines, and ship
locks.

An artistic impression of a tidal barrage, including embankments, a ship lock and caissons
housing a sluice and two turbines
7 Ebb generation

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 The basin is filled through the sluices until high tide. Then the sluice gates are closed. (At
this stage there may be "Pumping" to raise the level further). The turbine gates are kept closed
until the sea level falls to create sufficient head across the barrage, and then are opened so that
the turbines generate until the head is again low. Then the sluices are opened, turbines
disconnected and the basin is filled again. The cycle repeats itself. Ebb generation (also known as
outflow generation) takes its name because generation occurs as the tide changes tidal direction.

8 Flood generation

The basin is filled through the turbines, which generate at tide flood. This is generally much
less efficient than ebb generation, because the volume contained in the upper half of the basin
(which is where ebb generation operates) is greater than the volume of the lower half (filled first
during flood generation). Therefore the available level difference important for the turbine
power produced between the basin side and the sea side of the barrage, reduces more quickly
than it would in ebb generation. Rivers flowing into the basin may further reduce the energy
potential, instead of enhancing it as in ebb generation. Of course this is not a problem with the
"lagoon" model, without river inflow.

9 Pumping

Turbines are able to be powered in reverse by excess energy in the grid to increase the water
level in the basin at high tide (for ebb generation). This energy is more than returned during
generation, because power output is strongly related to the head. If water is raised 2 ft (61 cm) by
pumping on a high tide of 10 ft (3 m), this will have been raised by 12 ft (3.7 m) at low tide. The
cost of a 2 ft rise is returned by the benefits of a 12 ft rise. This is since the correlation between
the potential energy is not a linear relationship, rather, is related by the square of the tidal height
variation.

10 Two-basin schemes

Another form of energy barrage configuration is that of the dual basin type. With two basins,
one is filled at high tide and the other is emptied at low tide. Turbines are placed between the
basins. Two-basin schemes offer advantages over normal schemes in that generation time can be

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adjusted with high flexibility and it is also possible to generate almost continuously. In normal
estuarine situations, however, twobasin schemes are very expensive to construct due to the cost
of the extra length of barrage. There are some favorable geography, however, which are well
suited to this type of scheme.

11 Tidal lagoon power

Tidal pools are independent enclosing barrages built on high level tidal estuary land that trap
the high water and release it to generate power, single pool, around 3.3W/m2. Two lagoons
operating at different time intervals can guarantee continuous power output, around
4.5W/m2. Enhanced pumped storage tidal series of lagoons raises the water level higher than the
high tide, and uses intermittent renewable for pumping, around 7.5W/m2 i.e. 10 x 10 km delivers
750MW constant output 24/7. These independent barrages do not block the flow of the river and
are a viable alternative to the Severn Barrage.

12 Energy calculations

The energy available from a barrage is dependent on the volume of water. The potential
energy contained in a volume of water is:

Where:

h is the vertical tidal range,

A is the horizontal area of the barrage basin,

is the density of water = 1025 kg per cubic meter (seawater varies between 1021 and 1030 kg
per cubic meter)

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g is the acceleration due to the Earth's gravity = 9.81 meters per second squared.

The factor is half due to the fact that the basin flows empty through the turbines; the
hydraulic head over the dam reduces. The maximum head is only available at the moment of low
water, assuming the high water level is still present in the basin.

DYNAMICAL TIDAL POWER:

Dynamic tidal power or DTP is a new and untested method of tidal power
generation. It would involve creating large dam like structure extending from the coast
straight to the ocean, with a perpendicular barrier at the far end, forming a large 'T' shape.
This long T-dam would interfere with coast-parallel oscillating tidal waves which run
along the coasts of continental shelves, containing powerful hydraulic currents.
Description

Fig. 3. Top-down view of a DTP dam. Blue and dark red colors indicate
low and high tides, respectively.
A DTP dam is a long dam of 30 to 60 km which is built perpendicular to the coast, running
straight out into the ocean, without enclosing an area. The horizontal acceleration of the tides is
blocked by the dam. In many coastal areas the main tidal movement runs parallel to the coast.

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The entire mass of the ocean water accelerates in one direction, and later in the day back the
other way. A DTP dam is long enough to exert an influence on the horizontal tidal
movement,which generates a water level differential(head) over both sides of the dam. The head
can be converted into power using a long series of conventional low-head turbines installed in
the dam.
Benefits:
A single dam can accommodate over 8 GW (8000 MW) of installed capacity, with a capacity
factor of about 30%, for an estimated annual power production of each dam of about 23 billion
kWh (83 PJ/yr). To put this number in perspective, an average European person consumes about
6800 kWh per year, so one DTP dam could supply energy for about 3.4 million Europeans. If
two dams are installed at the right distance from one another (about 200 km apart), they can
complement one another to level the output (one dam is at full output when the other is not
generating power). Dynamic tidal power doesn't require a very high natural tidal range, so more
sites are available and the total availability of power is very high in countries with suitable
conditions, such as Korea, China, and the UK (the total amount of available power in China is
estimated at 80 - 150 GW).

Challenge
A major challenge is that a demonstration project would yield almost no power, even at a dam
length of 1 km or so, because the power generation capacity increasesas the square of the dam
length (both head and volume increase in a more or less linear manner for increased dam length,
resulting in a quadratic increase in power generation). Economic viability is estimated to be
reached for dam lengths of about 30 km. Other concerns include shipping routes,marine ecology,
sediments, and stormsurges. Amidst the great number of challenges and few environmental
impacts the method of utilizing tidal power to generate electricity has great potential and is
certainly a technology most of the countries will try to harness in near future.

1. ADVANTAGES :
It is a renewable source of energy.
Tidal energy is environmental friendly energy and doesnt produces green house gases
Because 71% of earth surface is covered by earth, it is possible to generate the energy on
a large scale.
Efficiency of tidal power is far greater as compared to coal , solar or wind energy. Its
efficiency is around 80%

2. DISADVANTAGES:
Cost of construction of tidal power plant is high.
Intensity of sea waves is unpredictable and there can be damage to power generation
units.
Influences aquatic life adversely and can disrupt migration of fish

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 6.CONCLUSION:

With the machine demand of energy and fast depletion of fossil fuels there is
necessity to explore the renewable source of energy. Tidal energy is promising,
predictable and very clean source of energy. There is abundance of potential sites which
need to be explored world over for setting up of tidal current turbines. The basic design
of HATCT is similar to that of wind turbines while that of vertical axis turbines is similar
to cross flow hydraulic turbines. However due to their location on seabed their
construction becomes
different. Due to the difference in the working environment as well as flowing fluid there
is a need for much research for improving the design of tidal current turbine

7. REFERENCES:

[1] Desmukh. T, Gawas.A. Tidal Current Energy An Overview. International Journal Of

[2] Shaikh Md.Rubayiaat Tousif, Shaiyek Md. Buland Taslim. Tidal Power: An Effective
Method Of Generating Power. International Journal Of Scientific And Engineering Research,
Volume: 2, Issue: 5, May 2011, 1-5.

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