Tidal energy
Monoki Ákos
Definition:
Tidal means structure
changing in a planet, caused by the gravitational interaction with another
planet.
The gravitational
pull of the moon and the sun cause the changing of the sea-level
twice a day on the Earth.
The period from the
smallest water-level, to the highest sea-level called upstream tides, while
the period between the highest and the smallest sea-level is called downstream
tides.
The sea-level changing
between the rising and the falling of tides is called tidal range.
History:
In the Middle Ages small tidal mills were used on tidal
estuaries for grinding corn and sawing wood. One of the restored tidal
mills of Britain is the Carew Castle Tidal Mill, shown on the picture below.
The source of the picture: http://www.pembrokeshirecoast.org.uk/english/entercarew.htm
The simplest tidal power device is a tidal mill, a variety
of water wheel utilizing the inward and outward flow of water. Doomsday
Book in the 11th century mentions a tidal mill plan to the Port of Dover
in England. There were other tidal mills built from
time-to-time along the coast of Great Britain and the west coast of Europe.The
first tidal mill of the USA was constructed in 1635, in Salem, in Massachusetts.
Tidal power was used in Hamburg, Germany in 1880 for
sewage pumping.
The use of tidal energy for energy generation is recent.
There were many plans to build a tidal power plant through the Severn estuary,
but none of them were achieved. A plan were done by Thomas Telford in 1849,
to build a barrage through the estuary, than in 1920 there was the first
plan for a power plant, to generate electricity.
There was a tidal power plant in France built. It was
constructed between 1961 and 1967 in Brittany near St. Malo. They achieved
their 240 MW turbine capacity in 1966. Since than it has operated successfully.
There were many schemes to built tidal power plants in
Canada and in the former USSR, but they couldn`t achieve any.
There is a power plant in the estuary of the La Rance
with a 18 MW single unit and using a rim generator, Annapolis Royal in
Nova Scotia, which was executed in 1984.
There were two other completed power plants in the East
China Sea , at Jangxia Creek with a 500 kW unit and in the Bay of Kislaya
with a 400 kW unit.
Theoretical background:
The rise and fall of
tides can be exploited by tidal mills, on the tidal section of rivers.
At low tide the head of water which was topped up into a pond at the high
tide , drive a water wheel. The rapidly flowing tidal streams can be exploited
too. These are strong surface currents created due to the effects of concentration
in narrow chanels for example between islands.The upstream tidal
flow called in an estuary flow tide, and that can be trapped
behind a barrage.
Tidel barrages built
across suitable estuaries, extract the energy from the rise and fall of
tides, by the help of turbines located in water passages in the barrage.
The source of the picture: http://www.greentie.org/records/TUS03113.HTM#
The changing of sea
level across the barrage, is the potential energy which is converted into
kinetic energy in the form of the fast-moving water passing through the
turbines. This power is converted into rotational kinetic energy, by the
blades of the turbine. The spinning turbine drives the generator , which
produce the electricity.
The energy output
approximately proportional to the area of the water trapped behind the
barrage.
At least 5m of tidal
range is needed for the minimum for viable power generation.
The first tidal barrage
of the world is on the Rance Estuary in Brittany on the west coast of France,
producing 240 MW (megawatt).
There are many small
projects all around the world, for example in Russia and China.
Great Brittain has
approximately half the European total potential for tidal energy. The Severn
Estuary thought to be the best potential site of the world, but there are
many smaller scale sites, for example Convy, Mersey.
Theoretically the
total tidal potential of the UK is thought to be about 53 TWh/year which
is approximately 17% of the current UK electricity generation.
Physics:
The variation in tidal
height is caused by the gravitational interaction between the earth and
the moon.
The earth rotates
on its axis, gravitational forces produced at any particular point on the
planet twice a day, rise and fall in sea level, this being modified in
height and gravitational pull of sun, and by topography of land masses
and oceans.
Gravitational forces are determined by Newton`s law of
gravitation which states that the force of atraction (F) between two bodies
is:
where m1 and m2 are the masses of the
bodies, r is the distance between the centres of mass of the bodies.
The interaction between
the earth, moon and sun is quite complex. The gravitational pull of the
moon draws the seas on the side of the earth nearest to the moon
into a bulge towards the moon, while the seas furthest from the moon experience
a less than average lunar pull and bulge away from the moon. That gives
two tides per day (in every 24.8 hour), occuring approximately 12.5 hours
apart. This timing of of the high tides will vary occuring approximately
50 minutes later each day.
The kinetic energy of a solid sphere of radius, R (m)
with a mass M (kg), rotating with an angular velocity W (rad/sec), is given
by:
where the Ek is the Kinetic Energy
(Joules).
With this relationship plus the fact that the mass
of the earth is 5.98x1024 kg and the tidal action
has slowed the rate of rotation by one second in 120.000 years, it can
be calculated, that the average power of the tides is 1.570.000 Mw.
That basic mechanism
is modified by the sun. Although the sun is much larger than the moon,
it is much further away from the earth, and the influance of the moon on
the seas is about twice of the sun.
If
the sun and the moon are in the same line, they pull together, the high
"spring tides" are resulted.
If
the sun and the moon are at 90 degrees at each other, the result is low
"neap tide".
There are about 14
days between the neap and spring tides, which is half of the 29.5 days
lunar
cycle.
There
are other factors, which make the tidal patterns more complicate.The
first is the centrifugal force, which is the result of the rotation of
the earth and the moon around each other. The earth and the
moon is moving around the centre of gravity of the earth-moon system. That
motion produces centrifugal force which must be equal and opposite to the
gravitational forces at the centres of the planets. In the case we neglect
the diurnal rotation of earth, each point of the surface of the earth experiences
a centrifugal force acting on the centre of the earth.
The mutual rotation around the center of the earth-moon system produces
a large outward centrifugal force acting on the seas on the side of the
earth furthest from the moon.A smaller centrifugal force directed towards
the moon, acting on the seas facing the moon. The gravitational pull of
the moon draws the seas on the side of the earth nearest to the moon into
a bulge towards the moon. Whilst the seas furthest from the moon experience
less than average lunar pull and bulge away from the moon.
That gravitational
force which is exerted by the moon on the surface of the earth varies with
the position of the planets. It depends on their distance from each other.
The tide-generating
force is the vector sum of the gravitational and the centrifugal forces
at any location of the surface of the earth.
The tide generating
force is directly proportional to the tide generating body (which is the
sun or the moon) and it is inversely proportional to
the cube of the distance
between the earth and the tide generating body.
Sometimes the weather
can effect the tides with its stormy winds. The
tides are modified in some locations by the Coriolis force. The Coriolis
force deflect tidal currents from the paths that they would otherwise have
taken.
To summerise:
The primary mechanism in the tide generation based on the gravitational
pull of the earth and the moon but the energy in tidal flow comes from
the rotation of the earth, which draws the tidal bulges across the seas.
The rotation of the earth is slowed about one-fiftieth of a second every
1000 years by friction, but this won`t effect the energy generation.
To estimate the amount of energy that may be available
from a given tidal project, consider the energy stored by the water held
in a depth, H, behind a dam of height, H0. If the surface area
of this body of water is A and the area doesn`t change as the water level
drops. The total weight of water stored, W, is given if the dam is completely
filled, by this relationship:
where 
is the density of water. As the water runs out of the reservoir (or tidal
basin) the height H, of the water drops. The amount of energy released,
dE, per unit of decrease in the height of water stored is then
To find the total energy stored the
equation above may be integrated between two limits, which are empty (H=0)
and full (H=H0).
The total energy available depends on the square of the height
of the dam. In a similar way, the total energy available from a tidal energy
scheme will depend on the square of the tidal range, R. To figure out the
maximum possible average power available from a given tidal scheme the
total energy available must be divided by the total cycle time between
low and high tides, T. In most cases T would be approximately 6 hours (21,600
seconds).
The average power that could be produced is given by
Using opportunities
and technical factors:
Tides have a 12.4-hour cycle, so the rising and falling
of sea level have got a sinusoidal pattern.
Power can be generated from the passing of incoming tide,
which drive the turbines, mounted in the barrage. This is called flood
generation. There can be power generated by another way. The incoming tide
pass through sluices, and develops behind the barrage and the sluices are
closed at high tide. At low tide than the head of water pass through the
turbines, this is called ebb generation. In
the case of a one basin system, power can be generated only during the
tidal cycle.
Power can be generated twice in a 24.8-hour period by
the using of flow and ebb generation. The power generation can be made
more uniform over a longer period by systems with two basins. With the
outgoing low tide, the low pool became empty and then get sealed. The next
incoming tide fill the high pool, while the low pool remains empty.
When this high tide starts to ebb, the high pool is isolated
from the sea and allowed to empty through the turbines into the low pool.
This process continues until the level in the low pool is equal to that
of the decreasing level of the tide. The low pool is then opened and it
level falls with the outgoing tide.
The main tidal-power utilisation technologies:
- One-way, single-basin generation
This is the simplest way of power generation. There
is single basin closed off the estuary by a barrage. During the high tide
period the water fills the basin by passing through the sluiceways.
At low tide, when the water level in the basin is higher
than the sea level, power can be generated by emtying the basin through
turbine generators.
That type of systems can allow power generation for about
five hours, which is followed by the refilling of the basin.
-Two-way, single basin generation
That system allow power generation from the water moving
from the sea to the basin and than at low tide moving back to the sea.
this process require bigger and more expensive turbines.
The Rance power plant is a two wayed system.
-Multiple-Basin Schemes
There are two one-way, single basin installations which
are interconnected electrycally.
The multiple-basin system allow continuous power generation.
The rising tide fills the high-level basin through the
sluiceways. When the falling sea level is equal to the water level in the
high-level basin, by the closing of the sluiceways they prevent the outflowing
to the sea. The water than flows from the high-level basin to the low-level
basin through turbine generators. When the falling seawater level becames
lower than the rising water level in the low-level basin, the sluiceways
are opened to allow water to flow into the sea from the low-level basin.
This process continues untill the water level in the low-level basin equals
to the rising sea level. Than the sluiceways are closed to prevent the
filling of low-level basin from the sea.
This process allow continuous power generation.
Main turbine types:
- Bulb turbine
At La Rance there is a this system in use. That type of
turbine have a horizontal-axis propeller with variable pitch blades.
There are turbine generators sealed in a bulb-shaped enclosure mounted
in the flow. In the bulb system the water has to flow around a large bulb
and access to the generator involves cutting off the flow of water.They
can be run either as power generating turbines or in reverse as pumps.
These reversible pump generators can be used to overempty the tidal basin
at low tide or to overfill it at high tide.
-Straflo turbine
The problems mentioned above are reduced, in the `Straflo`
turbine, which is used at Annapolis Royal, where the generators are
mounted radially around the rim and only the runner (that is the turbine
blades) in the flow.
-Tubular turbine
In that system the runner set is at an angle so that a
long (tubular) shaft can take a rotational power out to an external generator.
The rotational speeds of the turbine is about 50-100
revolutions per minute and the wear of it reduced compared with a high-head
hydro plant. In a large-scale barrage large numbers of turbines are
required because large volumes of water have to pass through
in a short time.
In a simple ebb or flood generation the large installed
capacity is used only for a relatively short period (3 to 6 hours at most)
in each tidal cycle.
-Reversible-pitch turbine
By the use of reversible-pitch turbines there can power
generated both the ebb and flow, but these are most complex and expensive.
Although the energy generation will be distributed in time, there will
be a net decrease in power output for each phase compared with a simple
ebb generation scheme. This is because, in order to be ready to the next
cycle, neither the ebb nor the flow generation phases can be taken to completion:
it is necessary to open the sluices and reduce water levels ready for the
next flood cycle and for ebb generation.
The blades cannot be designed optimally for flow in both
directions, and would have to be compromised for two-way operation.
-Rim generator turbine
Other systems:
-Flood pumping
In that system the turbine generators are run in reverse
as motor-pump sets, powered with electricity from the grid. water is pumped
behind the barrage into the basin, to provide extra water for the ebb generation
phase.
-Double-basin system
The double-basin systems are useful for power generating,
when power is required. The turbines of the first basin can be used for
pumping water into the second basin.
The turbines of the power plant are usually located in
large units. Turbines can be mounted into caisson
structures.
Sluices allow the tide to flow through the turbines for
flow or ebb generation, and they also can be mounted into caissons. There
are several types of sluice caisson, just like flap
gate sluice, vertical lift
gate sluice and radial gate sluice.
The embankment of the barrage can be rock-filled, like
in La Rance or can be sand-filled with rock protection, as it was planned
in the case of Severn barrage.
At the entrance of the estuary the maximum tidal range
is approximately 13 m. To effectively utilize that tidal range, the plant
has an installed electric capacity of 240 MW.
Site-selection criteria for tidal-power developments.
These datas are based on New Sources of Energy and Economic Development,
United Nations Department of Economics and Social Affairs, New York, 1957.
| Country and site |
Tidal range (m) |
Basin area (km2) |
Potential annual electrical-energy production (106
kwhe/y) |
| France: |
|
|
|
| Lorient |
4.5
|
16.0
|
97
|
| Brest |
6.4
|
92.0
|
1,130
|
| Alber-Benoit |
7.4
|
2.9
|
48
|
| Alber -Vrach |
7.4
|
1.1
|
18
|
| Arguenon and Lancieux |
11.4
|
28.0
|
1,090
|
| La Frasnaye |
11.4
|
12.0
|
470
|
| Rance |
11.4
|
22.0
|
860
|
| Rotheneuf |
12.0
|
1.1
|
48
|
| Chausey |
12.4
|
610.0
|
28,140
|
| Somme |
9.3
|
49.0
|
1,270
|
| United Kingdom: |
|
|
|
| Severn |
11.5
|
44.0
|
1,750
|
| United States: |
|
|
|
| Passamaquoddy |
7.5
|
120.0
|
2,025
|
|
|
|
|
Environmental factors:
Building barrages in estuaries affect the local
ecosystem. The result of it would be higher minimum water level and slightly
lower high water level behind the barrage. There will be changes in sediment
characteristics and in the salinity and quality of water. These factors
will have a major effect on the ecology and the environment of the estuary.
Estuaries are important for the migrating birds and fishes.
The estuaries of the UK play host approximately 28% of European swans and
ducks and to 47% of European geese. Migrating fishes and birds rely on
the estuaries for food and access to that supply might be affected by a
tidal barrage. The higher water level behind the barrage would destroy
the mud flats where the mudwading birds could feed on worms.
Tidal barrages could effect the silt and sediment suspended
in the water, which make the water impenetrable to sunlight. With a barrage
the tidal ebbs and flows would be reduced, most of the silt would drop
out which would make the water clearer. This change in the turbidity of
water would allow the sunlight to penetrate further down, increasing the
biological productivity of the water.
The changings in the characteristic of water can
be positive for many species, but also can be negative for other species.
The construction of a barrage in an estuary will hold up the shipping.
Tidal barrages can be useful in the protection against
flooding and storm damages and would have some effects on the local economy.
It can be useful from the viewpoint of employment generation, turism, water
sport opportunities. There can be a new road or railway created through
the barrage, just like in the case of Rance Barrage.
The electricity produced by tidal barrages have
to be integrated with the electricity produced by other power plants. The
main problem is that the electricity produced by a tidal barrage can be
connected into the national grid network with difficulity, because the
tidal energy inputs come in relatively short bursts, at about 12 hours
intervals.
Power can be produced for five or six hours at spring
tides and three hours during neap tides in a 12.4 hours tidal cycle.
Literature:
Open University - Renewable Energy
M.L. Shepard, J.B. Cliaddock,
F.H. Cocks, C.M.Harman - Introduction to energy
technology
1976. Ann Arbor Science Publishers,
Inc.
Jerrold H. Krenz - Energy conservation
and utilization 1984. Allyn and Bacon Inc.
S.S. Penner, L. Icerman - Energy
Non Nuclear Technologies 1975. Addison - Wesley
publishing company Inc.
