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University of Thi-Qar

College of Engineering

Mechanical Engineering Department

"Theoretical study about Tidal Energy"

By

Balqees Hmode Abdullah

Supervised by

Asst. Prof. Dr.Mushtaq I. Hasan

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Abstract

This article is about tidal power. It describes tidal power

and the various methods of utilizing tidal power to

generate electricity. It briefly discusses each method

and provides details of calculating tidal power generation

and energy most effectively. The paper also focuses

on the potential this method of generating

electricity has and why this could be a common

way of producing electricity in the near future.

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1. Overview:

Tidal power exploits energy drawn from the movement of ocean tides to

produce electricity. There are two scenarios in which tides can be tapped for

energy. The first is in changing sea levels. This phenomenon is responsible for

the advancing and receding tides on shorelines. With the help of turbines,

incoming tides can be manipulated to generate electricity. The second way to

exploit tidal energy is by sinking turbines to the sea floor where fast-flowing

currents turn generator blades much like wind does with a wind turbine.

Tidal energy is considered renewable because the tides move on a predictable,

daily schedule, depending only on the orbits of the Earth, Moon, and Sun, and

are essentially inexhaustible [1]. Though tidal energy is carbon free, it is not

environmentally benign. Concerns over the health of shoreline and aquatic

ecosystems mar this otherwise clean source of energy. Older tidal barrage

technology can devastate fish populations [2].

In the past, large-scale barrage systems dominated the tidal power scene. But

because of increasingly evident unfavorable environmental and economic

drawbacks with this technology, research into the field of tidal power shifted

from barrage systems to tidal current turbines in the last few decades. This

new technology leaves a smaller environmental footprint than tidal barrages,

as turbines are placed in offshore currents avoiding the need to construct dams

to capture the tides along ecologically fragile coastlines. Harnessing tidally-

driven coastal currents cannot yet deliver the sheer amount of power that

barrage style facilities can, like at the 240 MW barrage generating station at

La Rance, France [3]. However, the technology is quickly evolving with

numerous test plants popping up around the globe.

Canada hosts two test sites, one tidal barrage and one tidal current power

station. With one new and one old, both a history and a newfound interest in

tidal power is apparent. The Annapolis Royal tidal barrage built in Nova

Scotia's Bay of Fundy in 1984, with its world-famous tides, operates as the

third largest tidal power plant in the world, with 20 MW [4]. The smaller Race

Rocks facility in British Columbia, installed in 2006, uses tidal current

technology to generate 65 kW of power [5]. Studies have estimated a potential

4,000 MW of untapped energy flowing along the coasts of BC [6]. Canada,

and the shores of British Columbia, are home to some of the world's most

attractive locations for tidal power development.

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2. Brief History of Tidal Power:

The energy stored in tides been known to people for many centuries. The earliest

records of tidal mills are dated back to the 8th Century CE [7]. The tidal mills were

mainly used for grain grinding and were of similar design to the conventional water

mills with the exception of the addition of a dam and reservoir. The industrial

revolution increased demand for power but tidal energy never got off the ground,

undercut by cheap fossil fuels and other developments which offered easier access to

power generation. Existing tidal mills became as obsolescent as pre-industrial water-

mills. The first large scale modern tidal electric plant started to operate in La Rance

Estuary, St. Malo, France in the 1960s and has been operating ever since. In recent

years the search for renewable, non-polluting energy sources and the increase in fossil

fuel prices has encouraged renewed interest in tidal power.

3. Tides:

The interaction of the sun-moon-earth system causes ones of the strangest

phenomena: tides. Tides rise and fall is the product of the gravitational and centrifugal

forces, of primarily the moon with the earth. The gravitational forces maintain the

moon on it is positions with respect to the earth, forcing to pull the earth and the moon

together, see figure 1. The centrifugal forces acts on the opposite direction pulling the

moon away from the earth. These two forces acts together to maintain the equilibrium

between these two masses.

The influence of the sun can be included on the balance of the entire system. The

distance plays an important role on the development of tides. Based on the newton

law, the gravitational force is proportional to the square of the distance of two bodies,

but tidal force is proportional to the cube of the distance. For this reason although the

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moon has a much smaller mass than the sun it is much closer to the earth. The moon

effect is 21

4 greater than that of the sun on the generation of tides [8].

The gravitational force of attraction of the moon causes that the oceans waters bulge

on the side of the earth that faces the moon. The centrifugal force produce the same

effect but in the opposite side of the earth. On these two sides it can be observe the

maximum amplitudes of the tides (high tides) and on midways of it occur the

minimum amplitudes of the tides (low tides). As the earth rotates these two bulges

travel at the same rate as the earth`s rotation. The moon rotates around the earth with

respect to the sun approximately 29.5 days (lunar month) in the same direction that

the earth rotates every 24 hours. The rotation of the earth with respect to the moon is

approximately 24.48 hours (24 hours and 50 minutes) and is called lunar day. This is

the reason of why the tides advance approximately 50 minutes each day [9].

Fig.2: High and low tides.

In the same manner that the ocean waters bulges towards the moon, the gravitational

force of the sun causes that the ocean waters bulges too but in a lesser degree. Twice a

month, when the earth, the moon and the sun are aligned (full and new moon) the tide

generating forces of the sun and the moon are combined to produce tide ranges that

are greater than average knowing as the spring tides [10]. At the half moon (first and

third quarters) the sun and the moon are 90° with respect to the earth and the tide

generating forces tend to produce tidal ranges that are less than the average knowing

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as the neap tides, see figure 3 [10]. Typically the spring tides range tend to be twice

the neap tides range.

The tidal movements can be reflect and restrict by the interruption of masses of land,

the bottom friction can reduce it is velocity and the depth, size and shape of the ocean

basins, bays and estuaries altered the movements of the tidal bulges and generate

different types of tides [8]. There are three types of tides: diurnal, semidiurnal and

mixed, see figure 4 [11].

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Diurnal tides (daily) present one single high and low water during a period of a lunar

day of 24 hours and 50 minutes and occur in the Gulf of Mexico, southeast Asia and

the coast of Korea, semidiurnal tides (twice a day) present two high an two low waters

during a lunar day with periods of 12 hours and 50 minutes and is common along the

Atlantic coast of North America and the mixed tides that presents two unequal high

and two unequal lows waters and generally have a periods of 12 hours and 50

minutes. In a lunar month this type of tide that is common on the pacific Ocean coast

of the United states can experience semidiurnal and diurnal tides characteristics. In

1964 Davis classified the tidal ranges as: micro-tidal with tidal range less than 2

meters, meso-tidal with tidal range between 2 and 4 meters and macro-tidal with tidal

range of more than 4 meters [12].

4. How Tidal Power Generation Systems Work?

In very simple terms a barrage is built at the entrance of a gulf and the water levels

vary on both sides of the small dam. Passages are made inside the dam and water

flows through these passages and turbines rotate due to this flow of water under head

of water. Thus, electricity is created using the turbines. A general diagram of the

system is shown in Fig 5. What follows will be a description of a general tidal power

station with its components. Also, many systems of power generation will be

described.

General scheme of the tidal power station. Fig.6:

The components of a tidal power station are:

4.1. A barrage : a barrage is a small wall built at the entrance of a gulf in order to

trap water behind it. It will either trap it by keeping it from going into the gulf when

water levels at the sea are high or it will keep water from going into the sea when

water level at the sea is low.

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4.2. Turbines: they are the components responsible for converting potential energy

into kinetic energy. They are located in the passageways that the water flows through

when gates of barrage are opened. There are many types of turbines used in tidal

power stations.

A. Bulb turbines: as shown in Fig.7 these are difficult to maintain as water flows

around them and the generator is in water.

B. Rim turbines: as shown in Fig. 8 these are better maintained than the bulb

turbines but are hard to regulate as generator is fixed in barrage.

C. Tabular turbines: as shown in fig.9 these turbines are fixed to long shafts and

thus solve both problems that bulb and rim turbines have as they are easier to

maintain and control.

Fig.7: A Bulb turbine.

Fig.8: Rim turbines.

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Fig.9: Tabular turbines.

4.3. Sluices: sluice gates are the ones responsible for the flow of water through the

barrage they could be seen in Fig.6.

4.4. Embankments: they are caissons made out of concrete to prevent water from

flowing at certain parts of the dam and to help maintenance work and electrical wiring

to be connected or used to move equipment or cars over it. These embankments are

shown in Fig.10 [13].

Fig.10: Embankments.

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5. Tidal Energy Generators:

There are currently three different ways to get tidal energy which are:

5.1. Tidal Fences:

Tidal fences are composed of individual, vertical axis turbines which are mounted

within the fence structure, known as a caisson. Kind of like giant turn styles which

completely block a channel, forcing all of the water through them. Unlike barrage

tidal power stations, tidal fences can also be used in unconfined basins, such as in the

channel between the mainland and a nearby off shore island, or between two islands.

Since they do not require flooding of the basin, tidal fences have much less impact on

the environment, and are significantly cheaper to install. Unlike barrage generators,

tidal fences have the advantage of being able to generate electricity once the initial

modules are installed [14].

Fig.11: Tidal fences.

5.2. Tidal lagoons:

Tidal lagoons are an adaptation of the barrage system. Similar to standard barrage

models, tidal lagoons retain a head pond and generate power via conventional hydro-

turbines.

The difference is that the conventional barrage designs exploit the natural coast line to

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minimize barrage length. However, this entails blocking the estuary regardless of how

deep it is. This raise the costs considerably. However, a lagoon, for a low cost can

pretty much be built anywhere that there is a high tidal range. The lagoon has

relatively little visual impact, as it is below the high water tide mark and appears like

a normal sea wall at low tide.

The lagoon can be built using loose aggregates found in quarries or demolished

structures. This rubble would be „dumped‟ until an impound wall was complete. As

any aggregate can be used, it is possible to restrict construction costs by implementing

the cheapest materials available. This construction technique also has the added

benefit of creating an artificial reef. As well, a calm water lake would be created in

the middle where smaller fish and birds could flourish. Migrating fish can swim

around unimpeded and without the danger of sluices or negotiating turbines [14].

Fig.12: Tidal lagoons.

5.3. Tidal Turbines

For most tidal energy generators, turbines are placed in tidal streams. A tidal stream is

a fast-flowing body of water created by tides. A turbine is a machine that takes energy

from a flow of fluid. That fluid can be air (wind) or liquid (water). Because water is

much more dense than air, tidal energy is more powerful than wind energy. Unlike

wind, tides are predictable and stable. Where tidal generators are used, they produce a

steady, reliable stream of electricity.

Tidal turbines utilize tidal currents that are moving with velocities of between 2 and 3

m/s (4 to 6 knots) to generate between 4 and 13 kW/m2. Fast moving current (>3 m/s)

can cause undue stress on the blades in a similar way that very strong gale force winds

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can damage traditional wind turbine generators, whilst lower velocities are

uneconomic.

Placing turbines in tidal streams is complex, because the machines are large and

disrupt the tide they are trying to harness. The environmental impact could be severe,

depending on the size of the turbine and the site of the tidal stream. Turbines are most

effective in shallow water. This produces more energy and allows ships to navigate

around the turbines. A tidal generator's turbine blades also turn slowly, which helps

marine life avoid getting caught in the system [15].

Fig.13: Tidal turbine.

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6. Tidal energy to electric energy conversion:

The technology that is used to produce electricity using the difference between the

low and high tides is very similar to the one use on the generation of electricity on the

traditional hydroelectric power plants. The use of the tidal energy requires a dam or

barrage across a shallow area preferably an estuary, bay or gulf of high tidal range

where the difference on the low and high tide have to be at least 5 meters [8]. The tide

basins are filled and empty every day with the flood tides when water level falls. On

the barrage there are low-head turbines and sluice gates that allow the water to flow

from one side of the barrage to inside the tidal basin. This difference on elevation of

the water level creates a hydrostatic head that generates electricity. There are different

modes to generate electricity using the barrage systems:-

6.1. Ebb generation: Incoming water (flood tide) is allowed to flow freely to fill

the basin until high tide, then the sluices are close and water are retained on one side

of the barrage. When level of the water outside of the barrage decreased (ebb tide)

sufficiently to create a hydrostatic head between the open waters and tide basin, the

sluices are open and water flows through the turbines and generate electricity [16].

6.2. Flood generation: During the flood tide the sluices gates and low-head

turbines are kept closed to allow the water level outside of the barrage to increase.

Once a hydrostatic head is created the sluices gates are opened and the water flows

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through the turbines into the basin. This mode is less efficient than the ebb generation

[8,17].

6.3. Two ways generation: This mode permits to generate electricity using the

ebb generation and the flood tide. The main problem with this type mode is that the

turbines must work both ways, when water enters or exits the basin. This requires

move expensive turbines and at this time computer simulations do not indicate that

this mode increases significantly the energy production [17].

6.4. Pumping: On the ebb generation the hydrostatic head can be increases

reversing the power and turning the turbine-generation into a pump motor. During the

generation the energy that was use is returned [17].

Fig.14: Power output of two way single basin tidal power station with pumping [12].

6.5. Double basin: All of the modes discuss above use one tide basin. Using tow

basins, the turbines are placed between the basins. The main basin will going to use

the ebb generation mode to operate and pump water with part of the energy that is

generated to and from the second basin to generated electricity continuously. This has

the disadvantage that is very expensive [17].

Fig.15: Schematic diagram of two basin tidal power station [12].

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Fig.16: A diagram showing transformation of tidal energy to electric energy [13].

7. 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 = 𝑔𝜌𝐴 𝑧𝑑𝑧 = 0.5𝑔𝜌𝐴ℎ2

Where E is the energy, g is acceleration of gravity, ρ is the sea water density, which

it`s 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

ρg = 10.15 KN 𝑚−3 for sea water, one can obtain for a tide cycle per square meter

of ocean surface:

𝐸 = 1.4ℎ2 ,𝑤𝑎𝑡𝑡 − ℎ𝑜𝑢𝑟 = 5.04ℎ2 ,𝑘𝑖𝑙𝑜𝑗𝑜𝑢𝑙𝑒

The kinetic energy T of the water mass m is its capacity to do work by virtue of it`s

velocity V. It is defined by 𝑇 = 0.5 𝑚 𝑉2 . The total tide energy equals the sum of

it`s 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 artificial upstream water heads. Such

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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 floating 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 [18].

*Example calculation of tidal power generation:

Assumptions:

Let as assume that the tidal range of tide at a particular place is 32 feet =10m

(approx.).

The surface of the tidal energy harnessing plant is 9 2 (3 3 ) =3000

3000 = 10 2.

Density of sea water = 1025.18kg/ 3.

Mass of the sea water = .

=

= 10 2 10 1025.1 3

= 2 10 .

Potential energy content of the water in the basin at high tide = 1

2

.

P.E. = 1

2 10 2 1025.1 3

. 1

10 2

=4.5 1012 .

Now we have 2 high tides and 2 low tides every day. At low tide the potential energy

is zero.

Therefore the total energy potential per day = ℎ ℎ 2.

= 4.5 1012 2

= 1012

Thus the mean power generation potential=energy generation potential / time in 1 day

Power = 1012 6400

=104MW.

Assuming the power conversion efficiency to be 30% : The daily-average power

generated = 104 30 = 31 . .

Because the available power varies with the square of the tidal range, a barrage is

placed in a location with a very high-amplitude tides. Suitable locations are found in

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Russia, U.S.A, Canada, Australia, Korea, and the U.K. Amplitudes of up to 17 m

(56ft) occur for example in The Bay of Fundy, where tidal resonance amplifies the

tidal range [19].

8. Tidal power around the world:

There are places that have large tidal ranges. Some of these places are The Bay of

Fundy Canada with a mean tidal range of 10m, Severn Estuary between England and

Wales with a mean range of 8m and the northern of France with a mean range of 7m.

The first large-scaled tide generation plant is located in Brittany on the La Rance

River on France. It was completed in 1966 at a cost of $100 million. The generation

plant has a capacity of 240 MW. The plant consists of 24 bulb-type turbine generators

of 5.35m (17.55ft.) diameter with 4 mobile pales and a rated capacity of 10 MW. The

barrage has a length of 910m (2986ft.) and serves as a four-lane highway that

connects Saint Malo and Dinard. The bulb turbines were design to operate on ebb or

flood generation mode and pump water either into or out of the basin when there are

slack tides periods. These turbines have the disadvantage that the water flows around

them and make the maintenance difficult and expensive. The plant is operated almost

of the time on the ebb generation mode because operate on the two-way generation

mode (ebb and flood tides) was prove not to be successful. Only when high spring

tides are present the plant operates on two-way generation mode. The plant average

generation was about 64 GW per year (0.012% France energy consumption). On 1996

the plant passes to a 10 years refurbishment plan for it is 24 bulb turbines [17].

Fig.17: Ebb generation with a bulb turbine.

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9. Tidal giants - the world‟s five biggest tidal power plant:

The Swansea Bay tidal lagoon project in the UK and the MeyGen tidal array project

in Scotland stand out among the few large-scale tidal power projects currently under

development. Power-technology.com lists five of the world‟s biggest tidal power

plants, including those both operational and under construction.

9.1. Sihwa Lake Tidal Power Station, South Korea - 254MW:

With an output capacity of 254MW, the Sihwa Lake tidal power station located on

Lake Sihwa, approximately 4km from the city of Siheung in Gyeonggi Province of

South Korea, is the world's biggest tidal power plant.

The project, owned by Korea Water Resources Corporation, was opened in August

2011 and utilises a 12.5km long seawall constructed in 1994 for flood mitigation and

agricultural purposes. Power is generated on tidal inflows into the 30km2 basin with

the help of ten 25.4MW submerged bulb turbines. Eight culvert type sluice gates are

used for the water outflow from the barrage.

The $355.1m tidal power project was built between 2003 and 2010. Daewoo

Engineering & Construction was the engineering, procurement and construction

(EPC) contractor for the project. The annual generation capacity of the facility is

552.7GWh.

Fig.18: Sihwa lake tidal power station.

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Fig.19: Project status by Google earth [20].

Fig.20: Operation of Sihwa Tidal power plant [20].

9.2. La Rance Tidal Power Plant, France - 240MW:

The 240MW La Rance tidal power plant on the estuary of the Rance River in

Brittany, France, has been operational since 1966 making it the world's oldest and

second biggest tidal power station. The renewable power plant, currently operated by

Électricité de France (EDF), has an annual generation capacity of 540GWh.

The La Rance tidal power facility, built between 1961 and 1966, involved the

construction of a 145.1m long barrage with six fixed wheel gates and a 163.6m-long

dyke. The basin area covered by the plant is 22km2. Power is produced through 24

reversible bulb turbines with a rated capacity of 10MW each.

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The plant site features an average tidal range of 8.2m, the highest in France.

Electricity is fed into the 225kV national transmission network serving the needs of

approximately 130,000 households every year.

Fig.21: La Rance Tidal Power Plant, France.

9.3. Swansea Bay Tidal Lagoon, United Kingdom - 240MW

The 240MW Swansea Bay Tidal Lagoon project, to be built at Swansea Bay in the

UK, is the world's biggest tidal power project and will become the world's third

biggest tidal power project upon completion. The planning application for the £850m

($1.4bn) project was approved in March 2013.

The plant will be located at a site with average tidal range of 8.5m and will involve

the construction of a 9.5km-long sea wall or breakwater facility to create a lagoon

cordoning off 11.5km2 of sea. The plant will use reversible bulb turbines to generate

power as water passes in and out of the lagoon with the rise and fall of tides.

The ground breaking for the tidal power project is scheduled for 2015 while full

commissioning is expected in 2018. The tidal lagoon, with an estimated annual power

generation capacity 400GWh, will power over 120,000 homes for 120 years.

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Fig.22: Swansea Bay Tidal Lagoon, United Kingdom.

9.4. MeyGen Tidal Energy Project, Scotland - 86MW:

MeyGen Tidal Energy Project located in the Inner Sound of the Pentland Firth off the

north coast of Caithness, Scotland, is currently the world's biggest underwater tidal

turbine power project under development.

The tidal array project received offshore planning consent for its 86MW first phase

development from the Scottish Government towards the end of 2013. The second

phase development of the project is expected to raise the total installed capacity to

398MW by 2020.

The MyGen project was initiated in 2006 by the Scottish company MeyGen, a joint

venture between the tidal technology company Atlantis Resources and Morgan

Stanley. Atlantis Resources acquired full ownership of the tidal array project in

December 2013. Construction is expected to start for a demonstration array involving

up to six AR1000 single-rotor tidal turbines in 2014 with final commissioning

expected in 2015. The first 1MW prototype of the 22.5m tall AR1000 tidal turbine

with 18m rotor diameter was deployed at the European Marine Energy Centre in2011.

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Fig.23: MeyGen Tidal Energy Project, Scotland.

9.5. Annapolis Royal Generating Station, Canada - 20MW:

The Annapolis tidal power generating station located in the Annapolis Basin, a sub-

basin of the Bay of Fundy in Canada, has an installed capacity of 20MW making it

the world's third biggest operating tidal power plant. It generates 50GWh of electricity

annually to power over 4,000 homes.

The plant, operated by Nova Scotia Power, came online in 1984 after four years of

construction. The plant utilises a causeway built in the early 1960s, which was

originally designed to serve as a transportation link as well as a water control structure

to prevent flooding.

The power plant comprises of a single four blade turbine and sluice gates. The gates

are closed as the incoming tides create a head pond in the lower reaches of the

Annapolis River upstream of the causeway. The gates are opened and the water

rushing into the sea drives the turbine to generate power when a head of 1.6m or more

is created between the head pond and sea side with the falling of the tide [21].

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Fig.24: Annapolis Royal Generating Station, Canada.

10. Environmental and Ecological Concerns of Tidal Energy:

Tidal power generation can offer significant advantages, including improved

transportation due to the development of traffic or rail bridges across estuaries and

reduced greenhouse gas emissions by utilizing tidal power in place of fossil fuels.

However there are also some significant environmental disadvantages which make

tidal power, particularly barrage systems less attractive than other forms of renewable

energy.

The construction of a tidal barrage in an estuary will change the tidal level in the

basin. This change is difficult to predict, and can result in a lowering or raising of the

tidal level. This change will also have a marked effect on the sedimentation and purity

of the water within the basin. In addition, navigation and recreation can be affected as

a result of a sea depth change due to increased sedimentation within the basin. A

raising of the tidal level could result in the flooding of the shoreline, which could have

an effect on the local marine food chain.

Potentially the largest disadvantage of tidal power is the effect a tidal station has on

the plants and animals which live within the estuary. As very few tidal barrages have

been built, very little is understood about the full impact of tidal power systems on the

local environment. What has been concluded is that the effect due to a tidal barrage is

highly dependent upon the local geography and marine ecosystem.

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Fish may move through sluices safely, but when these are closed, fish will see

turbines and attempt to swim through them. Also, some fish will be unable to escape

the water speed near a turbine and will be sucked through [22].

*Fish Mortality:

There are two categories of threats to fish

*Direct: injury and mortality due to blade strike and water conditions (for example

water pressure) resulting in damage or disorientation.

*Indirect: loss and degradation of habitat which may be important for feeding and

spawning; and disruption to movement (such as completion of migration).

Mortality due to blade strike is the most studied source of fish mortality. Estimates

depend on the type and operation of a turbine, and species of fish. In order to reduce

levels of blade strike, the Oak Ridge National Laboratory (ORNL) produced a set of

criteria for the design of „fish-friendly‟ turbines. These criteria are based on an

extensive literature review of studies on single fish passage through turbines at hydro-

power schemes. In recent years, levels of survival greater than 90% have been

achieved. While high survivability is possible, no field trials of turbines in an

estuarine environment have been carried out. In addition, most studies of fish

mortality have been carried out on small-sized salmon, a notoriously robust species;

therefore reports of „negligible‟ mortality levels may not be applicable to the diversity

of fish, crustacean and invertebrate species common in estuaries such as adult salmon,

shad, flounder, brown shrimp. In addition, estimates of fish mortality are based on

fish making a single pass through a turbine: fish living in an estuary may make

multiple passes in a day, increasing their risk of mortality.

Less studied is mortality due to sub-lethal injuries, predation or indirect impacts.

During turbine passage, blade strike and hydraulic conditions can result in injuries

(for example scale loss, eye loss or abrasions) which may not cause immediate

mortality, but will reduce survival through disease or decreased fitness. Fish

commonly suffer disorientation during turbine passage which increases predation risk

by other fish, fish-eating birds and aquatic mammals; this has been observed at La

Rance. In a recent study, mortality because of predation of juvenile salmon was found

to account for between 46-70% of total mortality, indicating it is potentially a

considerable source of mortality. However, at present studies on mortality levels due

to predation pressure and sub-lethal injuries are limited [23].

*Very Low-Head Turbines:

Very Low-Head (VLH) turbines are a new technology that reportedly has a smaller

impact on fish than existing technologies. A recent trial on a VLH prototype achieved

100% survivability of both large and small eels. However, very few trials of VLH

turbines have been undertaken. In response to growing interest in VLH turbines, the

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Canadian government published guidelines for the testing of these turbines. The

report comments that “mortality, although easy to define and measure, is simply one

way to evaluate the biological effectiveness of a turbine yet the majority of studies

focus only on mortality as an endpoint”. It suggests that a “suite of endpoints should

be examined which incorporate relevant metrics that have the potential to influence

long-term survival, health, condition and fitness” [23].

11. Advantages of Tidal energy:

There are a number of advantages to tidal energy. Because the force behind tidal

energy comes from the pull of the moon, it is an inexhaustible energy source. As long

as the moon continues to orbit the earth, there will be energy in the tides.

This relationship to the moon also makes tidal energy a predictable energy source.

Other forms of renewable energy, such as wind and solar energy, are dependent on

random weather patterns. But tidal energy is based on the rise and fall of tides, which

is more uniform and reliable.

It is a clean energy source because, unlike the burning of fossil fuels, it does not

release greenhouse gases or other pollutants into the air. It is also a cheap energy

source. After the initial investment is paid off, the cost of generating electricity is very

low. Tidal energy has a high energy density, meaning that the tides store a larger

amount of energy than most other forms of renewable energy, such as the wind [24].

12. Disadvantages of Tidal energy:

Despite this list of advantages, there are a number of disadvantages to tidal energy.

Tidal energy development is hampered by high upfront costs. For example, one study

noted that ocean power generation can cost more than $400 per MWH compared to

other renewable energy sources, such as wind, biomass, hydroelectric and geothermal

energy, that cost about $150 per MWH.

There are also limited suitable locations for tidal energy. A suitable location must

have sizable tides to justify the cost of constructing a power plant. Environmentalists

are concerned that tidal energy can be detrimental to marine life. Power plants can

disrupt the movements and migration of fish and other marine life in the oceans. Fish

can also be killed by the turbines.

Tidal energy can only be captured during the tides, so it is an intermittent energy

source. Because tides occur two times a day, in order for tidal energy to reach its full

potential, it must be paired with an efficient energy storage system [24].

26

13. Future outlook:

Although sustainable energy resources produce limited amounts of carbon dioxide

emissions, they are, by nature, reliant on the natural environment and therefore are

vulnerable to the effects of climate change. While sea level and wind pattern changes

are expected, tidal energy is less likely to be affected. This industry also has the

advantage of being predictable and quantifiable, both spatially and temporally.

It is also hoped that with future development of tidal current turbine technology, the

impact upon marine life can be reduced. In case of malfunction these type of facilities

do not impose any major catastrophic damage to the surroundings, compared to, say,

nuclear or hydroelectric dam failure [25].

Fig.25: Tidal Generation Emissions Savings.

27

14. Conclusion:

Tidal power has the potential to generate significant amounts of electricity at certain

sites around the world. Although our entire electricity needs could never be met by

tidal power alone, it can be a valuable source of renewable energy to an electrical

system. The negative environmental impacts of tidal barrages are probably much

smaller than those of other sources of electricity, but are not well understood at this

time. The technology required for tidal power is well developed, and the main barrier

to increased use of the tides is that of construction costs. The future costs of other

sources of electricity, and concern over their environmental impacts, will ultimately

determine whether humankind extensively harnesses the gravitational power of the

moon [13].

28

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2. Clark, Nigel. 2006. Tidal barrages and birds. British Ornithologists'

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3. Aubrecht, Gordon. 2006. Energy: Physical, Environmental, and Social

Impact. Third Edition. Pearson Education Inc. San Francisco, CA.

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http://en.wikipedia.org/wiki/Tidal_power., Wikipedia14.

29

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