Module No 5c.tidal Energy
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Transcript of Module No 5c.tidal Energy
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Non Conventional Energy Sources
TIDAL ENERGY 1 Vaibhav Vithoba Naik (BE. Mech)
5.1 Introduction to Tidal Power Plants
The development of a nation is estimated from the total amount of energy it
produces and consumes in relation to its size and Population. Human-
Progress has been judged from the ways in whi ch man has been able to develop
and harness energy.
The present shortage of the energy production sources due to
deplitable nature of fossil flues has awakened the world human community to
search for new unconventional and replenish able energy so urces. According
to the energy experts, exhaustible resources extracted from the ground (coal,
oil, gas, and uranium), in the face increasing demand, results into a cycle.
This cycle starts at zero production increases exponentially than decreases
exponentially and finally comes to zero. It is estimated that Petroleum
reserves of the world will be nearly exhausted by 2020. Under such
circumstances, man has to find out some source of energy for his survival. In this
context, three sources of energy, tidal, wind and solar have been brought to be
most promising.
What is tidal power? Tide is periodic rise and fall of the water level of
the sea. Tides occur due to the attraction of seawater by the moon. These
tides can be used to produce electr ical power which is known as tidal power.
When the water is above the mean sea level, it is called flood tide and when the
level is below the mean level, it is called ebb tide. A dam is constructed in
such a away that a basin gets separated from the sea and a difference in thewater level is obtained between the basin and sea. The constructed basin is
filled during high tide and emptied during low tide passing through sluices and
turbine respectively. The Potential energy of the water stored in the basin is
used to drive the turbine which in turn generates electricity as it is directly
coupled to an alternator.
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Though the idea of utilizing tides for human service relates to eleventh
century when tidal mills were used in England but the use of tidal power for
electric power generation is hardly a decade old as the worlds first Rance
tidal power plant of 240 MW capacities in France was commissioned by
President be Gaulle in 1965 who described it a magnificent achievement in the
human life.
Tidal power has been a dream for engineers for many years and it
remained dream because of large capital cost involved in its development.
But after the inauguration of Rance Tidal Project, a new chapter in the history
is now opened.
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How tides are formed
1. Tides are produced mainly by the gravitational attraction of the moon and the
sun on the water of solid earth and the oceans.
2. About 70% of the tide producing force id due to the moon and 30% to the sun.3. The moon is thus major factor in tide formations.
4. Surface water is pulled away from the earth on the sides facing the moon,
and at the same time the earth is pulled away from the water on the opposite
sides. Thus high tides occur at these two areas with low tides at intermediate
points.
5. As the earth rotates, the position of a given area relative to the moon
changes, and so also do the tides.
6. Thus there are periodic succession of high and low tides
7. Although there are exceptions, two tidal cycles (two high tides and two low
tides) occur during a lunar day of 24 hour and 50 minutes. That is to says the
time between the high tide and low tide at any given location is a little over a 6
hours.
8. A high tide will be experiences at a point which is directly under moon. At the
same time the diametrically opposite sides also experiences a high tide due
to dynamic balancing. Thus a full moon and no moon produce a high tide.
9. In a period of 24 hrs and 50 minutes, there are therefore two high tides and
two low tides. These are called semi -diurnal tides.
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Tidal Energy
1. The difference in water level between the high and low tide is used for
production.
2. The hydraulic turbines are installed as shown in the figure.
Figure: Tidal Power Plant
3. During high tides, the gates are opened and water flows from sea to tidal basin.
The water is used to drive the turbine. The turbine is coupled to the generator,
which produces the electrical energy. When the level of water in the sea
decreases, the gates are closed.
4. During the low tides, when the water head is a sufficient, gates are opened and
water flows from tidal basin to the sea
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5.2 Factors affecting the suitability of the site for tidal power plant
The feasibility and economic vulnerability of a tidal power depends upon the
following factors.
1. The power produced by a tidal plant depends mainly on the range of tide and
the cubature of the tidal flow occurring in the estuary during a tidal cycle which
can be stored and utilized for power generation. The cubature of the tidal flow
not only depends on the tidal range but on the width of estuary mouth.
2. The minimum average tide range required for economical power production is
more.
3. The site should be such that with a minimum cost of barrage it should be
possible to create maximum storage volume. In addition to this, the site
selected should be well protected from waves action.
4. The site should not create interruption to the shipping traffic running through the
estuary other wise the cost of the plant will increase as locks are to be
provided.
5. Silt index of the water of the estuary should be as small as possible to avoid the
siltation troubles. The siltation leads to reduction of the range of tides and
reduces the power potential of the plant.
6. The fresh water prism that falls into the rese rvoir of the tidal plant (due to the
surface flows in the streams having out fall in the estuary) eats away the
valuable storage created for storing the tidal prism. Therefore, the ratio of fresh
water prism to tidal water prism becomes an important index in determining the
economic feasibility of a tidal scheme. The effective and cheaper will be the
power production with decreasing the ratio mentioned above.
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5.3Classification of tidal Power Plants
The tidal power plants are generally classified on the basis of the
number of basins used for the power generation. They are further subdivided
as one-way or two-way system as per the cycle of operation for powergeneration.
The classification is represented with the help of a line diagram as given below
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5.4 Working of Different Tidal Power Plants
5.4.1 Single basin-One-way cycle
1) This is the simplest form of tidal power plant.
2) In this system, a basin is allowed to get filled during flood tide and during the
ebb tide.
3) The water flows from the basin to the sea passing through the turbine and
generates power.
4) The power is available for a short duration during ebb tide.
5) Fig 5.1 shows a single tide basin before the construction of dam and Fig5.2shows the diagrammatic representation of a dam at the mouth of the basin
and power generation during the falling tide.
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5.4.2 Single-basin two-way cycle
1) In this arrangement power is generated b oth during flood tide as well as ebb
tide also.
2) The power generation is also intermittent but generation period is
increased compared with one-way cycle.
3) However the peak power obtained is less than the one -way cycle.
The arrangement of the basin and the power cycle is shown in Fig 5.3.
Fig 5.3.2Single-basin two-way cycle
4) The main difficulty with this arrangement, the same turbine must be used
as Prime mover as ebb and tide flows pass through the turbine in
opposite directions.
5) Variable pitch turbine and dual rotation generator are used for such
schemes.
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5.4.3. Single-basin two-way cycle with pump storage
1) The Rance tidal power plant in France uses this type of arrangement.
2) In this system, power is generated bo th during flood and ebb tides.The cycle of operation is shown in Figure.
Single-basin two-way cycle with pump storage
3) Complex machines capable of generation power and pumping the water in
either direction are used.
4) A part of the energy produced is used for introducing the difference in the water
levels between the basin and the sea at any time of the tide and this is done by
pumping water into the basin up or down.
5) The period of power production with this system is much longer than the
other two described earlier.
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5.4.4 Double basin type
1) In this arrangement, the turbine is set up between the two basins as
shown in Fig 5.5. one basin is intermittently filled by the flood tide and other is
intermittently drained by the ebb tide.
Double basin type,one way tidal plant
2) Therefore a small capacity but continues power is made available with this
system as shown in Fig5.5.
3) The main disadvantage of this system is that 50% of the Potential
energy is sacrificed in introducing the variation in the water levels of the two
basins.
5.4.5 Double basin with Pumping
1) In this case, off peak power from the base load plant in a
interconnected transmission system is used either to pump the water up the
high basin.
2) Net energy gain is possible with such a system if the pumping head is
lower than the basin-to-basin turbine generating head.
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5.5 Advantages and Disadvantages of Tidal Power Plants
Advantages
1) Exploitation of tidal energy will in no case ma ke demand for large area of
valuable land because they are on bays.
2) It is free from pollution as it does not use any fuel.
3) It is much superior to hydro-power plant as it is totally independent of rain
which always fluctuates year to year. Therefore, t here is certainty of
power supply a the tide cycle is very definite.
4) As in every form of water power, this will also not produce any unhealthy waste
like gases, ash, atomic refuse which entails heavy removal costs.
5) Tidal Power is superior to conventional hydro power as the hydro plants are
know for their large seasonal and yearly fluctuations in the output of
energy because they are entirely dependent upon the natures cycle of
rainfall, which is not the case with tidal as monthly certain power is
assured. The tides are totally independent on natures cycle of rainfall.
6) Another notable advantage of tidal power is that it has a unique capacity to
meet the peak power demand effectively when it works in combination
with thermal or hydroelectric system.
7) It can provide better recreational facilities to visitors and holiday makers, in
addition to the possibility of fish forming in the tidal basins.
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TIDAL ENERGY 12 Vaibhav Vithoba Naik (BE. Mech)
Disadvantages
1) These Power plants can be developed only if natural sites are available.
2) As the sites are available on the bay which will be always far away from the
load centers. The power generated must be transported to long distances.
This increases the transportation cost.
3) The supply of power is not continuous as it depends upon the timing of
Tides. Therefore some arrangements (double basin or double basin with pump
storage) must be made to supply the continuous power. These also further
increases the capital cost of the plant.
4) The capital cost of the plant (Rs.5000/kw) is considerably l arge compared
with conventional-power plants (hydro, thermal)
5) Sedimentation and siltration of the basins are some of the added problems
with tidal power plants.
6) The navigation is obstructed.
7) It is interesting to note that the output of power from tidal p ower plant
varies with lunar cycle, because the moon largely influences the tidal
rhythm, where as our daily power requirement is directly related to solar
cycle.
In addition to all the above mentioned (imitations of tidal power, the utilization
of tidal energy on small scale has not yet proved economical.
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Ecological and environmental impact of tidal barrages
The installation of a tidal barrage has a major impact on both the
environment and ecology of the estuary and the surrounding area for
the following reasons.
1. The barrage acts as a major blockage to navigation and requires the
installation of locks to allow navigation to pass through.
2. Fish are killed in the turbines and impeded from migrating to their spawning
areas.
3. The intertidal wet/dry habitat is altered, forcing plant and animal life
to adapt or move elsewhere.
4. The tidal regime may be affected downstream of a tidal barrage. For
example, it has been claimed that a proposed barrage for the Bay of
Fundy in Canada could increase th e tidal range by 0.25 m in Boston,
1300 km away.
5. The water quality in the basin is altered since the natural flushing of
silt and pollution is impeded, affecting fish and bird life.
On the positive side, there are the benefits arising from carbon -free
energy, improved flood protection, new road crossings, marinas, and
tourism.
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Economics and prospects for tidal power
Large tidal barrages have the economic disadvantages of large capital
cost, long construction times, and intermittent operation. On the other
hand, they have long plant lives (over 100 years for the barrage
structure and 40 years for the equipment) and low operating costs. An
alternative idea is to create a closed basin in the estuary known as a
tidal lagoon. The wall of a tidal lagoon does not extend across the
whole channel so the environmental effects are lessened and the impact
on fish and navigation is reduced. Also, by restricting the tidal lagoons to
shallow water, the retaining wall can be low and cheap to build. The
global tidal resource has been estimated as 3000 GW, but only 3% of
this is in areas where the tidal range is enhanced and hence suitable for
power generation. So far, large barrage schemes have not been
pursued.
The economics of small tidal current devices (kineti c energy
absorbers) has the attraction that they can be installed on a piecemeal
basis, thereby reducing the initial capital outlay. They also have a more
predictable output than wind turbines and there is no visual impact. The
danger to fish is minimal because the blades rotate fairly slowly
(typically about 20 revolutions per minute). The potential for tidal stream
generation around the UK has been estimated as possibly 10 GW, a
contribution of about a quarter to the UK electricity demand.
The long-term economic potential and environmental impact of such
devices will become clearer after trials on various designs, notably in the
UK, Canada, Japan, Russia, Australia, and China. The engineering
challenge is to design reliable and durable equipment capable o f
operating for many years in a harsh marine environment with low
operational and maintenance costs.
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TIDAL ENERGY 15 Vaibhav Vithoba Naik (BE. Mech)
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Advantages
1. Exploitation of tidal energy will in no case make demand for large area of
valuable land because they are on bays.
2. It is free from pollution as it does not use any fuel.
3. It is much superior to hydro -power plant as it is totally independent of rain which
always fluctuates year to year. Therefore, there is certainty of power supply a the
tide cycle is very definite.
4. As in every form of water power, this will also not produce any unhealthy waste
like gases, ash, atomic refuse which entails heavy removal costs.
5. Tidal Power is superior to conventional hydro power as the hydro plants are know
for their large seasonal and yearly fluctuations in the output of energy because they
are entirely dependent upon the natures cycle of rainfall, which is not the case with
tidal as monthly certain power is assured. The tides are totally independent on
natures cycle of rainfall.
6. Another notable advantage of tidal power is that it has a unique capacity to meet
the peak power demand effectively when it works in combination with thermal or
hydroelectric system.
7. It can provide better recreational facilities to visitors and holiday makers, i n
addition to the possibility of fish forming in the tidal basins.
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TIDAL ENERGY 17 Vaibhav Vithoba Naik (BE Mech)
Disadvantages
1. These Power plants can be developed only if natural sites are available.
2. As the sites are available on the bay which will be always far away from the load
centers. The power generated must be transported to long distances. This
increases the transportation cost.
3. The supply of power is not continuous as it depends upon the timing of tides.
Therefore some arrangements (double basin or double basin with pump st orage)
must be made to supply the continuous power. This also further increases the
capital cost of the plant.
4. The capital cost of the plant (Rs.5000/kw) is considerably large compared with
conventional-power plants (hydro, thermal)
5. Sedimentation and siltration of the basins are some of the added problems
with tidal power plants.
6. The navigation is obstructed.
7. It is interesting to note that the output of power from tidal power plant varies with
lunar cycle, because the moon largely influences the tid al rhythm, where as our
daily power requirement is directly related to solar cycle. In addition to all the above
mentioned (imitations of tidal power, the utilization of tidal energy on small scale
has not yet proved economical.