Tidal current energy an overview

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 04 Issue: 07 | July-2015, Available @ http://www.ijret.org 147
TIDAL CURRENT ENERGY AN OVERVIEW
T.S. Desmukh1
, Amitkumar S. Gawas2
1
Professor, Department of civil engineering, M.A.N.I.T., M.P., India
2
M. Tech Scholar, Department of civil engineering, M.A.N.I.T., M.P., India
Abstract
Ocean currents are an enormous source of green energy. This energy from marine currents can be extracted by means of tidal
turbines. This paper explains different types of tidal current turbines. This paper discusses about tidal energy and site selection
criteria for tidal current turbine in general. This paper gives general overview about tidal current turbine design methods such as
the blade element momentum theory and computational fluid dynamics.
Keywords: Tidal energy, Tidal current turbines, Site selection, BEMT, CFD
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1. INTRODUCTION
Energy is the important driving force of world economy.
Most of the developing countries and industries are still
totally relying on the conventional energy resources that are
coal, oil and gas. Conventional energy resources will vanish
within few decades because of its limited availability.
Excessive uses of conventional energy resources are harmful
for health of living things, environment and ozone layer.
Clean renewable energy is the best alternative to avoid
further deterioration of the earth’s environment. This can be
possible only when there is solution of many technical
problems. All countries are individually or collectively
taking efforts to solve technical problems and developing
new technologies in the field of renewable energy.
Hydro power, Wind, Solar and Ocean energy are some of
the most common sources of clean renewable energies.
Hydro power energy is one of the first harnessed clean
sources of energy and hence has considerable development
over the years. Hydro power is stored in the form of
potential energy by building dams across the river. This
potential energy is converted in to kinetic energy by turbines
which are finally converted to electrical energy with the help
of generators. To get land for building dams and storage
reservoir is big environmental and people rehabilitation
issue in most of the countries. Next comes the wind energy.
Wind energy is an intermittent source of small magnitudes.
The research for improving the efficiency of wind turbine is
still under progress. Solar energy is the most abundant
source of available energy. Solar cells are used to convert
solar energy to electricity. The main issue with solar energy
is that the solar cells are very costly and hence cannot be
used for mass production. Latest addition in the field of
renewable energy is the energy from oceans. Ocean possess
many forms of energy namely – Thermal energy, tidal
energy, and energy from waves and circulating currents. The
main focus of this paper is tidal current energy. Research is
still underway for developing devices to harness the vast
potential of tidal current energy possessed by ocean. In this
paper an attempt has been made to discuss the current
scenario for tidal energy.
2. TIDAL ENERGY
The relative motion of earth and moon causes a change in
the gravitational field which results in the formation of tides.
The magnitude of tides depends upon the positions of the
moon and the sun relative to the earth, shape of the
coastline, sea floor and the rotation of the earth. Other than
gravitational pull some other causes of tidal currents are
coriolis forces created due to earth’s rotation and density
differences due to temperature and salinity variation.
Tidal energy can be utilized mainly in three forms Potential
energy, wave energy and tidal current energy. Tidal barrages
have been used for many years to utilize tidal energy in the
form of potential energy to generate electrical energy with
the help of turbines as shown in schematic diagram of tidal
barrages in Fig.1. There are two types of tidal barrages –
single basin tidal barrages and double basin tidal barrages.
Single basin scheme has one basin and it is created by
constructing barrages across the estuary. This scheme has
three methods to generate electricity by considering the fall
of tides or the rise of tides or both rise and fall of tides. In
first method when tide is rising sluice gates are opened and
water is allowed to store in to the basin. At beginning of the
ebbing when sufficient hydrostatic head is achieved between
low sea level and stored water level then sluice gates are
closed. Once the sea is at its low level after fall of tide then
water is allowed to pass through the turbine and generate
electricity. When the desired level of hydrostatic head is fall
then sluice gates are opened to drain the stored water and
maintain same level between stored water and sea water. In
this method pump can be used to store water in to the basin
when demand of electricity is less.
In second method sluice gates are kept closed during flood
and water is passed through turbine and generates
electricity. When tides start falling and the desired level of
hydrostatic head between sea level and flood level is less at
that time turbine stops generation of electricity and sluice
gates are opened to achieve the same sea level at both side
of barrage quickly.

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Third method generates power during both the flood and ebb
tides. The direction of flow through the turbines is opposite
during the flood and ebb tides. Hence the machine used for
power generation can act as a turbine for both direction of
flow. During ebb tide sluice gates are closed when tide level
and basin level is same. When the sea level falls and
required minimum head for turbine is reached then the
turbine operates to generate power by emptying the
reservoir. When available minimum head to operate turbine
is utilized then the sluice gates are opened and equalized the
level of reservoir with the sea level during flood tide. After
equalizing the level sluice gates are closed and flow is
allowed to pass through turbine to fill the basin and hence
turbine generates power during the flood tide. Just before
the end of flood tide turbine shut down and sluice gates are
opened to fill the basin quickly.
Double basin scheme has two basins. Main basin is used to
operate similar to that of the single basin operating with ebb
motion as explained earlier. During this type of electricity
generation small portion of electricity is used to pump water
in to second basin. This will fulfill fluctuating energy
demand.
There are very less sites available for tidal barrages because
it can be built only across natural estuary. La Rance power
facility in France is the largest operating tidal power plant.
Its capacity of power generation is about 240MW. Another
tidal power plant facilities are – Annapolis tidal generation
at Bay of Fundy, Canada (20MW); Kislaya Guba power
plant facility in Russia (400KW); Jangxia Creek at east
China sea (500KW).
Fig -1: Schematic diagram of tidal barrage [7]
Another form of ocean energy is wave energy. When wind
passes over the sea surface then waves are generated. These
waves can be utilized to generate electricity by using
specially designed devices which can convert wave energy
in to mechanical energy and finally in to electrical energy.
There are mainly four type of devices are in use to convert
wave energy in to electrical energy that are - overtopping
devices, point absorbers, attenuators and terminators.
However the technology to utilize wave energy is in
developing stage.
Third form of tidal energy is Tidal current energy which is
the form of kinetic energy which can be utilized by tidal
current turbines to generate electricity through generator.
One study estimated that India has about 8000 MW potential
of tidal energy. Gujarat state of India has tidal energy
potential of 7000 MW in the Gulf of Cambay and 1200 MW
in Gulf of kutch. West Bengal state of India has tidal energy
potential of 100 MW in the Gangetic Delta in Sunderbans
region. Gujarat power corporation Ltd. Govt. of Gujarat and
M/S Atlantis Resource Corporation (U.K.) had signed on
MoU for commissioning of 50MW Tidal power project [16].
3. SITE SELECTION
To select best site designer should study physics of tides,
topology of seabed, Temperature, salinity and pH level,
environmental importance of sites and socio-economic value
of site.
The study of physics of tides mainly includes current
velocity and wave patterns of the tides. While selecting a
site it should be kept in mind that high current velocities (2
m/sec & above) should be available for major period of the
year at the site; so as to ensure sufficient energy production
throughout the year. Low velocity current (1.2 m/s to 2 m/s)
can also be utilized if these current are available
continuously like river current flows.
Available power in tidal current is –
Where P is power, ρ is density of sea water, V is the current
velocity and A is the swept area of the rotor.
Sites which have high environmental importance such as
reserved sanctuary, fishing zones, area reserved for
endangered sea species should be avoided because
construction and installation of turbines may cause
environmental damage. Also the rotation of rotor and
turbulence created by rotation may disturb the biological
cycle of any sea species. Selected site must not be in the
vicinity of any busy sea route of ship. Else ship activity may
create hurdles for installation, operation and maintenance of
tidal current turbines.
Topology of seabed is also an important factor from turbine
installation point of view. If seabed topology has large plane
surface area then a number of tidal current turbines can be
installed similar to the wind turbine farm.
Salinity, temperature and pH level also need to be kept in
mind while selecting a site as they affect the cavitation
characteristic of the turbine rotor blade. If the temperature,
salinity and pH level are high at particular site then the

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chances of cavitation on the blade and corrosion rate of rotor
and its structure is also high. Hence to avoid corrosion
surface coating of very high quality is required. If the
surface finish of turbine blades is very smooth then
cavitation can also be avoided.
The selected site should be nearer to the land, so that the
erection cost of electrical transmission lines from turbine to
the grid and maintenance cost of the electrical transmission
lines is minimized.
4. TIDAL CURRENT TURBINE
Tidal current turbines are devices which convert kinetic
energy of tidal currents in to mechanical energy and finally
generate electricity. These turbines are designed in such a
way that it can generate electricity during both flood current
and ebb current.
Tidal current turbines can be classified according to the
direction of flow of tidal current passing over the turbine
are as i) Horizontal axis tidal current turbine (HATCT) ii)
Vertical axis tidal current turbine(VATCT).
4.1 HATCT
In this type of tidal current turbine tidal currents flow
parallel to the axis of rotation of the rotor. HATCT rotates
due to lift force generated by the airfoil section of the blade
when tidal currents pass over the rotor. Fig. 2 explains the
working principle of HATCT.
Fig -2: Schematic diagram to explain working principle of
HATCT
Rotor of a HATCT has 2 or 3 blades. Mostly 3 blade rotor
design is used because of its structural stability. The blades
are connected to the hub. Hub is connected to the shaft and
the shaft is finally connected to the gearbox and generator
assembly. Gearbox and generator are placed at the rear end
of the hub of the HATCT.
This whole assembly is then installed on a base structure.
The type of base structure to be used depends upon seabed
topology and depth of seawater at the site. If the selected
site has shallow water depth then gravity base, monopole
and piled jacket is used as base structure of the HATCT. If
the water depth is large then floating type of structure can be
used to support the tidal current turbine rotor assembly. In
floating type structure rotor assembly is installed below the
floating platform and this platform is connected by a chain
to the seabed.
Fig -3: Horizontal axis tidal current turbine [7]
Horizontal axis tidal current turbine has a simple design and
it is easy to manufacture as the rotor shape is not complex.
The rotor speed is very low hence it does not harm the
marine life. The turbine rotor can be lifted above sea water
hence its maintenance can be done easily as compared to a
vertical axis tidal current turbine.
The main disadvantage of such turbines is that - due to the
horizontal axis of rotation shaft output cannot be transferred
above sea water on floating platform and hence the gearbox
and generator have to be accommodated in the limited space
behind the hub.
4.2 VATCT
Vertical axis tidal current turbines have a vertical axis of
rotation vertical perpendicular to the flow of tidal currents.
They are also called as cross flow turbines because the flow
of tidal currents crosses through the turbine rotor.
Some of the common types of the vertical axis tidal current
turbines are Squirrel cage Darrieus, H-Darrieus, Darrieus,
Gorlov, Savonius (Fig.4) are the vertical axis tidal current
turbines. Squirrel cage Darrieus turbine has vertical blades
fixed between two circular plates. Gorlov turbine blades has
helical shaped blades fixed between two horizontal circular
plates, Whereas Savonius turbine blade is made up of two
semi cylinder fixed along their length to the rotor axis, at
1800
to each other and with their curvature in opposite
directions. These blades are fixed between 2 horizontal
circular plates. H-Darrieus and Darrieus turbine blades are
separately connected to the single axis as shown in fig.4.

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Darrieus type of tidal current turbine is lift driven turbine
i.e. rotation of this turbine is due to lift force of airfoil of the
blade when tidal current flows around the blades. Whereas
Savonius type of tidal current turbine is rotated due to drag
force created by the blade in tidal currents.
The main advantage of the vertical axis tidal current turbine
is that the shaft power can be vertically transmitted over the
sea surface which can be connected to the gearbox and
generator. The gearbox and generator can be kept on a
floating platform or vessel [1].
However due to the complex nature of its rotor and its
weight the vertical axis turbines are more costly as
compared to a horizontal axis tidal current turbine.
Moreover due to the nature of flow through the turbine rotor
these turbines are more prone to cavitation as compared to
horizontal axis tidal current turbine. It is not a self-starting
machine; it needs to be driven up.
5. TIDAL CURRENT TURBINE DESIGN
CONSIDERATION
Designing methods and technologies used for tidal current
turbines are similar to that of wind turbine designing
methods and technologies. However there are lots of
differences in design considerations; major differences are
density and Reynolds number of flowing fluid and
cavitation. The fluid flowing around tidal current turbine is
sea water which is about 800 times denser than air. There is
no cavitation problem in wind turbines whereas in tidal
current turbine due to the high density flowing fluid there is
cavitation problem.
Blade element momentum theory (BEMT) and
computational fluid dynamics (CFD) are two main
approaches for numerically analyzing the performance of
HATCT [10]. BEMT comprises of two theories that are
momentum theory and blade element theory.
Momentum theory includes a control volume analysis of the
forces at the blade. This analysis is based on the
conservation of linear and angular momentum. Blade
element theory includes an analysis of forces at a section of
the blade and these forces depend on blade geometry. The
outcome of these two theories is combined and used in
BEMT for performance analysis of HATCT.
CFD is used as a tool to analyze the flowing fluid around the
rotor 3 dimensionally. CFD analysis can predict the
performance of tidal current turbine. It shows pictorial view
of pressure and velocity variation over surface of the blade.
It is also used to predict the cavitation (i.e. local pressure on
the surface of the blade falls below the vapor pressure of
flowing fluid) inception on the surface of blade of tidal
current turbine.
6. CONCLUSION
With the machine demand of energy and fast depletion of
fossil fuels there is necessity to explore the renewable
source of energy. Tidal energy is promising, predictable and
very clean source of energy. There is abundance of potential
sites which need to be explored world over for setting up of
tidal current turbines. The basic design of HATCT is similar
to that of wind turbines while that of vertical axis turbines is
similar to cross flow hydraulic turbines. However due to
their location on seabed their construction becomes
different. Due to the difference in the working environment
as well as flowing fluid there is a need for much research for
improving the design of tidal current turbine.
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[14]. Ahmad Safwan Sakmani, Wei-HaurLam, Roslan
Hashim, Heap-Yih Chong; Site selection for tidal turbine
installation in the Strait of Malacca. Renewable and
Sustainable Energy Reviews 21(2013)590–602
[15]. Ali Al-Abadi,Ozg ur Ertun, Florian Beyer, & Antonio
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[16]. http://pib.nic.in/newsite/erelease.aspx?relid=70685
BIOGRAPHIES
Specialization in Hydro Power, Water
Resoures, CFD. Life member of following
bodies: ISTE; Institution of Engineers;
National Society for Fluid Mechanics and
Fluid Power; Indian Association of
Hydrologists
Industrial experience of piping
engineering; currently working on design
and CFD analysis of HATCT.

Tidal current energy an overview

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