Debasis- Tidal Energy

8/8/2019 Debasis- Tidal Energy

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Background

Tides, the daily rise and fall of ocean levels relative to

coastlines, are a result of the gravitational forces of

the moon and sun as well as the revolution of the

earth. The principle of harnessing the energy of tides

dates back to as early as the Middle Ages, when the

phenomenon of tides was not yet explained and were

used to turn waterwheels, producing mechanical

power. A large amount of energy is stored in tides

and it is possible to tap into this renewable resource

with tidal power plants. Tidal electricity generation

involves the construction of a barrage across a delta,

estuaries, beaches, or other places that are affected

by the tides .

Tidal power is a means of electricity generation achieved

by capturing the energy contained in moving water mass

due to tides. Two types of tidal energy can be extracted:

kinetic energy of currents between ebbing and surging tides

and potential energy from the difference in height (or head)

between high and low tides. The former method - generating

energy from tidal currents - is considered much more


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feasible today than building ocean-based dams or barrages,

and many coastal sites worldwide are being examined for

their suitability to produce tidal (current) energy.

One method of extracting tidal energy involves building a

barrage and creating a tidal lagoon. The barrage traps a

water level inside a basin. Head is created when the water

level outside of the basin or lagoon changes relative to the

water level inside. The head is used to drive turbines. In any

design this leads to a decrease of tidal range inside the basin

or lagoon, implying a reduced transfer of water between the

basin and the sea. This reduced transfer of water accounts

for the energy produced by the scheme.

Tidal power is classified as a renewable energy source,

because tides are caused by the orbital mechanics of the

solar system and are considered inexhaustible within a

human timeframe. The root source of the energy comes from

the slow deceleration of the Earth's rotation. The Moon gains

energy from this interaction and is slowly receding from the

Earth. Tidal power has great potential for future power and

electricity generation because of the total amount of energy

contained in this rotation. Tidal power is reliably predictable

(unlike wind energy and solar power). In Europe, Tide Mills

have been used for nearly 1,000 years, mainly for grinding

corn.


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The efficiency of tidal power generation in ocean dams

largely depends on the amplitude of the tidal swell, which

can be up to 10 m (33 ft) where the periodic tidal waves

funnel into rivers and fjords. Amplitudes of up to 17 m (56 ft)

occur for example in the Bay of Fundy, where tidal

resonance amplifies the tidal waves.

As with wind power, selection of location is critical for a tidal

power generator. The potential energy contained in a

volume of water is

E = xMg

where x is the height of the tide, M is the mass of water and

g is the acceleration due to gravity. Therefore, a tidal energy

generator must be placed in a location with very high-

amplitude tides. Suitable locations are found in the former

USSR, USA, Canada, Australia, Korea, the UK and other

countries (see below).

Several smaller tidal power plants have recently started

generating electricity in Norway. They all exploit the strong

periodic tidal currents in narrow fjords using sub-surface

water turbines.


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Basically there are thre methods of operation that can be

performed in order to harness the tidal energy, which are as

follows :-

KINETIC ENERGY

POTENTIAL ENERGY

OTEC

The details of the above methods are given below

Another form of ocean energy is called tidal energy. When

tides come into the shore, they can be trapped in reservoirs

behind dams. Then when the tide drops, the water behind

the dam can be let out just like in a regular hydroelectric

power plant.

In order for this to work well, you need large increases in

tides. An increase of at least 16 feet between low tide to

high tide is needed. There are only a few places where this

tide change occurs around the earth. Some power plants are


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already operating using this idea. One plant in France makes

enough energy from tides to power 240,000 homes.

The further categorization of the potential energy are :-

Ebb generation

Flood generation

Pumping

Two basin model

Ebb method of generation of power from tides

The basin is filled through the sluices and freewheeling

turbines until high tide. Then the sluice gates and turbine

gates are closed. They are kept closed until the sea level

falls to create sufficient head across the barrage and 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 ebbs

Flood method of generation of power from tides


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The basin is emptied through the sluices and turbines

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 (the

domain of flood generation). This is compounded by the fact

that there is usually a river flowing into the basin, filling the

basin as the tide rises and making the difference in levels

between the basin side and the sea side of the barrage (and

therefore the available potential energy) less than it wouldotherwise be. This is not a problem with the lagoon model:

the reason being that there is no current from a river to slow

the flooding current from the sea.

Pumping

Turbines can be powered in reverse by excess energy in the

grid to increase the water level in the basin at high tide (for

ebb generation and two-way generation). This energy is

returned during generation.

Two basin method

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 adjusted with high flexibility

and it is also possible to generate almost continuously. In


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normal estuarine situations, however, two-basin schemes

are very expensive to construct due to the cost of the extra

length of barrage. There are some favourable geographies,

however, which are well suited to this type of scheme.

The major draw back of the potential energy method is that it requires large investments

and the environmental impact of this method is large .

What are the current developments in the Tidal

Energy technology?

For many hundreds of years tiny amounts of this energy source have

been harnessed for agricultural purposes. In more recent years variousriver and estuary schemes have been proposed where the opportunity

of generating electrical power in close proximity to the energyconsumer is recognised. However with a couple of notable exceptions,

these tidal barrage schemes have stalled due to concerns related tothere environmental impact. Within the UK the Severn Estuary has

been extensively considered as a potential site for a tidal barrage

scheme, the grandest proposal for a 6.8 GW development scheme wasabandoned in 1987.

Within the last decade interest has concentrated upon the offshore

tidal resource with several development teams achieving large scaleprototype deployment in real marine


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Kinetic energy (movement) exists in the moving waves ofthe ocean. That energy can be used to power a turbine. In

this simple example, to the right, the wave rises into a

chamber. The rising water forces the air out of the chamber.

The moving air spins a turbine which can turn a generator.

When the wave goes down, air flows through the turbine and

back into the chamber through doors that are normally

closed.

This is only one type of wave-energy system. Others actually

use the up and down motion of the wave to power a piston


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Intermittent nature of power output

Tidal power schemes do not produce energy 24 hours a day.

A conventional design, in any mode of operation, would

produce power for 6 to 12 hours in every 24 and will not

produce power at other times. As the tidal cycle is based on

the period of revolution of the Moon (24.8 hours) and the

demand for electricity is based on the period of revolution of

the Sun (24 hours), the energy production cycle will not

always be in phase with the demand cycle. This causes

problems for the electric power transmission grid, as

capacity with short starting and stopping times (such as

hydropower or gas fired power plants) will have to be

available to alternate power production with the tidal power

scheme

The energy from the sun heats the surface water of the

ocean. In tropical regions, the surface water can be 40 or

more degrees warmer than the deep water. This

temperature difference can be used to produce electricity.

The OTEC system must have a temperature difference of at

least 25 degrees Celsius to operate, limiting use to tropical


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regions. Hawaii has experimented with OTEC since the

1970s. There is no large-scale operation of OTEC today.

There are many challenges. First, the OTEC systems are not

very energy efficient. Pumping water is a giant engineering

challenge. Electricity must also be transported to land. It will

probably be 10 to 20 years before the technology is

available to produce and transmit electricity economically

from OTEC systems.

Advantages

Once you've built it, tidal power is free.

It produces no greenhouse gases or other waste.

It needs no fuel.

It produces electricity reliably.

Not expensive to maintain.

Tides are totally predictable.


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Offshore turbines and vertical-axis turbines are not

ruinously expensive to build and do not have a large

environmental impact.

Local environmental impact

The placement of a barrage into an estuary has aconsiderable effect on the water inside the basin and on thefish. Lagoons, on the other hand, could be used for fish orlobster farming, adding to their economic viability.

Turbidity

Turbidity (the amount of matter in suspension in the water)

decreases as a result of smaller volume of water being

exchanged between the basin and the sea. This lets light

from the Sun to penetrate the water further, improving

conditions for the phytoplankton. The changes propagate up

the food chain, causing a general change in the ecosystem.

Pollutants


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Once again, as a result of reduced volume, the pollutants

accumulating in the basin will be less efficiently dispersed.

Their concentrations will increase. For biodegradable

pollutants, such as sewage, an increase in concentration is

likely to lead to increased bacteria growth in the basin,

having impacts on the health of the human community and

the ecosystem.

The concentrations of conservative pollutants will also

increase.

Tidal energy is the energy of the future and lots of research

are needed to be done on it and bench marks are to be set

so that the generation of the power form this source is

environmental friendly as well as its economical.


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