ASSIGNMENT for some locations with large tides. On most

ASSIGNMENT
ON TIDAL ENERGY

Introduction:

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Gravitational
forces between the moon, the sun and the earth cause the constant rising and
lowering of ocean waters around the world those results in Tide Waves. The moon
poses more than twice as great a force on the tides as the sun due to its much
nearer position to the earth. As a result, the tide almost follows the moon
during its rotation around the earth, creating tide and ebb cycles at any
particular ocean surface. The amplitude or height of the tide wave is very
small in the open ocean where it measures several centimeters in the center of
the wave distributed over hundreds of kilometers.

 However, the tide can increase suddenly when
it reaches continental shelves, bringing huge masses of water into narrow bays
and river  along a coastline. For
instance, the tides in the Bay of Fundy in Canada are the greatest in the
world, with amplitude between 16 and 17 meters near shore. High tides close to
these Rgures can be observed at many other sites worldwide, such as the Bristol
Channel in England, the Kimberly coast of Australia, and the Okhotsk Sea of
Russia. Table 1 contains ranges of amplitude for some locations with large tides.
On most coasts tidal Suctuation consists of two Soods and two ebbs, with a
semidiurnal period of about 12 hours and 20 minutes.

 However, there are some coasts where tides are
twice as long  or are mixed, with a
diurnal inequality, but are still diurnal or semidiurnal in period. The
magnitude of tides changes during each lunar month. The highest tides, called
spring tides, occur when the moon, earth and sun are positioned close to a
straight line (moon syzygy).

 The smallest tides, called neap tides, occur
when the earth, moon and sun are at right angles to each other (moon
quadrature). Isaac Newton formulated the phenomenon Rrst as follows: ‘The ocean
must Sow twice and ebb twice, each day, and the highest water occurs at the
third hour after the approach of the luminaries to the meridian of the place’.
The Rrst tide tables with accurate prediction of tidal amplitudes were
published by the British Admiralty in 1833. However, information about tide
Suctuations was available long before that time from a fourteenth century
British atlas, for example. Rising and receding tides along a shoreline area
can be explained in the following way.

 A low height tide wave of hundreds of
kilometers in diameter runs on the ocean surface under the moon, following its
circulation around the earth, until the wave hits a continental shore. The
water mass moved by the moon’s gravitational pull  narrow bays and river estuaries where it has
no way to escape and spread over the ocean. This leads to interference of waves
and accumulation of water inside these bays and estuaries, resulting in
dramatic rises of the water level (tide cycle). The tide starts receding as the
moon continues its travel further over the land, away from the ocean, reducing
its gravitational inSuence on the ocean waters (ebb cycle).

 

 

 

 

                                            Table
1

Highest
tides (tide ranges) of the global ocean

 Country                                    Site                                                  Tide range (m)

Canada                             Bay of Fundy                                                     16.2

England                            Severn Estuary                                                  14.5    

France                              Port of
Ganville                                                 14.7

France                               La Rance                                                          13.5

Argentina                   Puerto Rio Gallegos                                                13.3

Russia                        Bay
of Mezen (White Sea)                                      10.0

Russia                          Penzhinskaya Guba                                               13.4 (Sea of Okhotsk)

The above
explanation is rather accordingly since only the moon’s gravitation has been
taken into account as the major factor inSuencing tide Suctuations. Other
factors, which affect the tide range are the sun’s pull, the centrifugal force
causing from the earth’s rotation and, in some cases, local resonance of the
gulfs, bays or estuaries.

Energy of
Tides:

 The energy of the tide wave contains two
components, namely, potential and kinetic. The potential energy is the work
done in lifting the mass of water above the ocean surface. This energy can be
calculated as:

E”goAP zdz”0.5goAh2 ,

where E is the
energy, g is acceleration of gravity, o is the seawater density, which equals
its mass per unit volume, A is the sea area under consideration, z is a vertical
coordinate of the ocean surface and h is the tide amplitude. Taking an average
(go)”10.15 kN m~3 for seawater, one can obtain for a tide cycle per square
meter of ocean surface:

E”1.4h2 ,
watt-hour

or

E”5.04 h2 ,
kilo joule

The kinetic
energy T of the water mass m is its ability to do work by virtue of its
velocity V. It is denoted by T”0.5 mV2 . The total tide energy is the sum
of its potential and kinetic energy components.

Knowledge of the
potential energy of the tide is important for designing conventional tidal
power plants using water dams for creating artiRcial upstream water heads. Such
power plants exploit the potential energy of vertical rise and fall of the
water. In contrast, the kinetic energy of the tide has to be known in order to
design Soating or other types of tidal power plants which harness energy from
tidal currents or horizontal water Sows induced by tides. They do not involve
installation of water dams.

 Extracting Tidal Energy:
Traditional Approach:

People used the
phenomenon of tides and tidal currents long before the Christian era. The
earliest navigators, for example, needed to know periodical tide Suctuations as
well as where and when they could use or would be confronted with a strong
tidal current. There are remnants of small tidal hydromechanical installations
built in the Middle Ages around the world for water pumping, watermills and
other applications. Some of these devices were exploited until recent times.
For example, large tidal waterwheels were used for pumping sewage in Hamburg,
Germany up to the nineteenth century. The city of London used huge tidal
wheels, installed under London Bridge in 1580, for 250 years to supply fresh
water to the city. However, the serious study and design of industrial-size
tidal power plants for exploiting tidal energy only began in the twentieth
century with the rapid growth of the electric industry.

Electrification
of all aspects of modern civilization has led to the development of various
converters for transferring natural potential energy sources into electric
power. Along with fossil fuel power systems and nuclear reactors, which create
huge new environmental pollution problems, clean renewable energy sources have
attracted scientists and engineers to exploit these resources for the
production of electric power. Tidal energy, in particular, is one of the best
available renewable energy sources. In contrast to other clean sources, such as
wind, solar, geothermal etc., tidal energy can be predicted for centuries ahead
from the point of view of time and magnitude. However, this energy source, like
wind and solar energy is distributed over large areas, which presents a difficult
problem for collecting it. Besides that, complex conventional tidal power
installations, which include massive dams in the open ocean, can hardly compete
economically with fossil fuel (thermal) power plants, which use cheap oil or
coal, presently available in abundance.

These thermal
power plants are currently the principal component of world electric energy
production. Nevertheless, the reserves of oil and coal are limited and rapidly
dwindling. Besides, oil and coal cause enormous atmospheric pollution both from
emission of greenhouse gases and from their impurities such as sulfur in the
fuel. Nuclear power plants produce accumulating nuclear wastes that degrade
very slowly, creating hazardous problems for future generations. Tidal energy
is clean and not depleting. These features make it an important energy source
for global power production in the near future. To achieve this goal, the tidal
energy industry has to develop a new generation of efRcient, low cost and
environmentally friendly apparatus for power extraction from free or ultra-low
head water Sow. Four large-scale tidal power plants currently exist. All of
them were constructed after World War II.

They are the La
Rance Plant (France, 1967), the Kislaya Guba Plant (Russia, 1968), the
Annapolis Plant (Canada, 1984), and the Jiangxia Plant

 

Table 2

 Extant large tidal power
plants

Country          Site        Installed power    Basin
area      Mean tide (MW) (km2 ) (m)     

 France     
 La Rance                 240                    22                 8.55

Russia        Kislaya Guba          0.4                     1.1                   2.3                  

Canada      Annapolis                18                       15                   6.4

China          Jiangxia                   3.9                     1.4                   5.08

 

(China, 1985).
The main characteristics of these tidal power plants are given in Table 2. The
La Rance plant is shown in Figure 1. All existing tidal power plants use the
same design that is accepted for construction of conventional river hydropower
stations. The three principal structural and mechanical elements of this design
are: a water dam across the Sow, which creates an artiRcial water basin and
builds up a water head for operation of hydraulic turbines; a number of
turbines coupled with electric generators installed at the lowest point of the
dam; and hydraulic gates in the dam to control the water Sow in and out of the
water basin behind the dam. Sluice locks are also used for navigation when
necessary. The turbines convert the potential energy of the water mass
accumulated on either side of the dam into electric energy during the tide. The
tidal power plant can be designed for operation either by double or single
action. Double action means that the turbines work in both water Sows, i.e.
during the tide when the water Sows through the turbines, Rlling the basin, and
then, during the ebb, when the water Sows back into the ocean draining the
basin.

 In single action systems, the turbines work
only during the ebb cycle. In this case, the water gates are kept open during
the tide, allowing the water to Roll the basin. Then the gates close,
developing the water head, and turbines start operating in the water slow from
the basin back into the ocean during the ebb. Advantages of the double action
method are that it closely models the natural phenomenon of the tide, has least
effect on the environment and, in some cases, has higher power efRciency.
However, this method requires more complicated and expensive reversible
turbines and electrical equipment. The single action method is simpler, and requires
less expensive turbines.

The negative
aspects of the single action method are its greater potential for harm to the
environment by developing a higher water head and causing accumulation of
sediments in the basin. Nevertheless, both methods have been used in practice.
For example, the La Rance and the Kislaya Guba tidal power plants operate under
the double-action scheme, whereas the Annapolis plant uses a single-action
method. One of the main parameters of a conventional hydropower plant is its
power output P (energy per unit time) as a function of the water Sow rate Q
(volume per time) through the turbines and the water head h (difference between
upstream and downstream water levels). Instantaneous power P can be deRned by
the expression: P”9.81Qh, kW, where Q is in m3 s~1 , h is in meters and
9.81 is the product (og) for fresh water, which has mass density o”1000 kg
m~3 and g”9.81 m s~2 . The (og) component has to be corrected for
applications in salt water due to its different density (see above).

The average
annual power production of a conventional tidal power plant with dams can be
calculated by taking into account some other geophysical and hydraulic factors,
such as the effective basin area, tidal Suctuations, etc. Tables 2 and 3
contain some characteristics of existing tidal power plants as well as
prospects for further development of traditional power systems in various
countries using dams and artiRcial water basins described above.

Extracting
Tidal Energy: Non-traditional Approach

As mentioned
earlier, all existing tidal power plants have been built using the conventional
design developed for river power stations with water dams as their principal
component. This traditional river scheme has a poor ecological reputation
because the dams block the migration, destroying their population, and damage
the environment by Sooding and swamping adjacent lands. Flooding is not an
issue for tidal power stations because the water level in the basin cannot be
higher than the natural tide. However, blocking migration of Rsh and other
ocean inhabitants by dams may represent a serious environmental problem.

Table 3 Some
potential sites for tidal power installations (traditional approach)

 Country       Site                        Potential power     Basin
area        Mean tide (MW) (km2 ) (m)

USA         Passamaquoddy                          400                        300                                 5.5      

USA           Cook Inlet Up to                      18 000                     3100                               4.35

Russia            Mezen                                 15
000                       2640                              5.66

 Russia          Tugur                                   6790                           1080                               5.38

UK             Severn                                     6000                                490                                8.3       

UK             Mersey                                      700                                    60                                  8.4

Argentina     San Jose                              7000
                                 780                                6.0             

 Korea       
 Carolim Bay                           480                                     90                                  4.7

Australia         Secure                               570                                    130                                8.4

Australia         Walcott                             1750                                  260                                8.4

More, even the
highest average global tides, such as in the Bay of Fundy, are small as compared
with the water heads used in conventional river power plants where they are
measured in tens or even hundreds of meters. The relatively low water head in
tidal power plants creates a difRcult technical problem for designers. The fact
is that the very efRcient, mostly propellertype hydraulic turbines developed
for high river dams are inefRcient, complicated and very expensive for low-head
tidal power application. These environmental and economic factors have forced
scientists and engineers to look for a new approach to exploitation of tidal
energy that doesnot require massive ocean dams and the creation of high water
heads.

The main
component of such an approach is using new unconventional turbines, which can
efficiently extract the kinetic energy from a free unconstrained tidal current
without any dams. One such turbine, the Helical Turbine, is shown in. This
cross-Sow turbine was developed in 1994. The turbine consists of one or more
long helical blades that run along a cylindrical surface like a screw thread,
having a so-called airfoil or ‘airplane wing’ proRle. The blades provide a
reaction thrust that can rotate the turbine faster than the water Sow itself.
The turbine shaft (axis of rotation) must be perpendicular to the water
current, and the turbine can be positioned either horizontally or vertically.

 As its axial symmetry, the turbine always
develops unidirectional rotation, even in reversible tidal currents. This is a
very important advantage, which simpliRes design and allows exploitation of the
double-action tidal power plants. A pictorial view of a Soating tidal power
plant with a number of vertically aligned triple-helix turbines is shown in
Figure 3. This project has been proposed forthe Uldolmok Strait in Korea, where
a very strong reversible tidal current with Sows up to 12 knots (about 6m s~1 )
changes direction four times a day. The following terms can be used for
calculating the combined turbine power of a Soating tidal plant (power
extracted by all turbines from a free, unconstrained tidal current):
Pt”0.5goAV3 , where Pt is the turbine power in kilowatts, g is the turbine
efRciency (g”0.35 in most tests of the triple-helix turbine in free Sow),
o is the mass water density, A is the total effective frontal area of the
turbines in m2 (cross-section of the Sow where the turbines are installed) and
V is the tidal current velocity in m s~1 . Note, that the power of a free water
current through a cross-Sow area A is Pw”0.5oAV3 .

The turbine efficiency
g, also called power coefficient, is the ratio of the turbine power output Pt
to the power of either the water head for traditional design or unconstrained
water current Pw, i.e. g”Pt/Pw. The maximum power of the Uldolmok tidal
project shown in Figure 3 is about 90 MW calculated using the above approach
for V”12 knots, A”2100m2 and g”0.35. Along with the Soating
power farm projects with helical turbines described, there are proposals to use
large-diameter propellers installed on the ocean Soor to harness kinetic energy
of tides as well as other ocean currents. These propellers are, in general,
similar to the well known turbines used for wind farms.

 

 

Utilizing Electric Energy from Tidal Power Plants:

 A mean issue that must be addressed is how and
where to use the electric power generated by extracting energy from the tides.
Tides are cyclical by their nature, and the corresponding power output of a
tidal power plant does not always coincide with the peak of human activity. In
countries with a well-developed power industry, tidal power plants can be a
part of the general power distribution system. However, power from a tidal
plant would then have to be transmitted a long distance because locations of
high tides are usually far away from industrial and urban centers.

An attractive
future option is to utilize the tidal power in situ for year round production
of hydrogen fuel by electrolysis of the water. The hydrogen, liquefied or
stored by another method, can be transported anywhere to be used either as a
fuel instead of oil or gasoline or in various fuel cell energy systems. Fuel
cells convert hydrogen energy directly into electricity without combustion or
moving parts, which is then used, for instance, in electric cars. Many
scientists and engineers consider such a development as a future new industrial
revolution. However, in order to realize this idea worldwide, clean hydrogen
fuel would need to be also available everywhere.

Right now, most
hydrogen is produced from natural gases and fossil fuels, which emit greenhouse
gases into the atmosphere and harm the global ecosystem. From this point of
view, production of hydrogen by water electrolysis using tidal energy is one of
the best ways to develop clean hydrogen fuel by a clean method. Thus, tidal
energy can be used in the future to help develop a new era of clean industries,
for example, to clean up the automotive industry, as well as other
energy-consuming areas of human activity.

Conclusion:

Tidal waves 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. Depleting oil
reserves, the emission of greenhouse gases by burning coal, oil and other
fossil fuels, as well as the accumulation of nuclear waste from nuclear
reactors will inevitably force people to replace most of our traditional energy
sources with renewable energy in the future. Tidal energy is one of the best
candidates for this approaching revolution.

Development of
new, efficient, low-cost and environmentally friendly hydraulic energy
converters suited to free-Sow waters, such as triple-helix turbines, can make
tidal energy available worldwide. This type of machine, moreover, can be used
not only for multi-megawatt tidal power farms but also for mini-power stations
with turbines generating a few kilowatts. Such power stations can provide clean
energy to small communities or even individual households located near
continental shorelines, straits or on remote islands with strong tidal
currents.

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