International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013
WAVE ENERGY CONVERTER
ADITHYA B.V.S#1,VIJAY PALLIPOGU,#2
#1
b.Tech(1v/1v),Mechanical,K.L.University
Vaddeswaram, India.
#2
assistant Professor
Mechanical, K.L.University
Vaddeswaram, India
and it is much more regular than wind energy. The
various types of devices used to tap the wave energy
are:
1. Ocean flow energy converters:
a. Tidal lagoon
b. Tidal dam
2. Ocean wave energy converters
a. Oscillating water column type
b. Overtopping wave energy converter
c. Buoy type WEC.
Abstract: OWC wave energy converters work on the
principle of wave induced air pressurization. They usually
consist of a tube or some sort of closed containment placed
above the water surface and the passage of waves changes
the water level within the housing. If the housing is
completely sealed, the rising and falling of the water level
will increase and decrease the air pressure respectively
within the housing or containment. We can place a turbine
on the top of the housing through which air may pass in
and out of the housing. Air will flow into the housing
during a wave trough and out of it during a wave crest.
Due to this bidirectional flow of air, the turbine must be
designed in such a way that it will rotate in only one
direction no matter what the direction of air flow. The
wells turbine is designed for this purpose and it is
commonly used in OWC devices. Our aim is to study and
modify the housing or tube by retrofitting a convergentdivergent nozzle and increase the efficiency of the wave
energy converter.
Keywords: OWC , induced air
pressurization ,housing ,convergent-divergent nozzle
INTRODUCTION:
Due to the ever growing population, the energy
requirement is increasing at an exponential rate. The fossil
fuel based energy is becoming scarce and it would come to an
end soon. Due to this fact, there is a need to develop
renewable energy resources in order to quench the energy
thirst of the ever growing population. Some of the renewable
energy resources are:
1. Solar energy: this is the free and perennial source of
clean energy. But tapping solar energy is very
expensive since it uses devices like solar panels etc.
2. Wind energy: this is also a clean energy. Even
though it is not as expensive as the solar energy, it is
highly local, that is the availability is limited to
places where there is abundant amount of wind
energy available.
3. Wave energy: this is the energy possessed by the
waves. It is available wherever there is a coastline.
This is the most prevalent type of renewable energy
ISSN: 2231-5381
This project deals with the study and modification of
oscillating water column type WEC to suit for small scale
energy production in harbor.
OSCILLATING WATER COLOUMN:
The oscillating water column operates much like a
wind turbine via the principle of wave induced air
pressurization. Some sort of closed containment housing (air
chamber) is placed above the water and the passage of waves
changes the water level within the housing. If the housing is
sealed completely, the rising and falling of the water level will
increase and decrease the air pressure respectively within the
housing. With this concept in mind, we can place a turbine on
top of the housing through which air may pass into and out of
it. Air will flow into the housing during a wave trough and
will flow out of the housing during a wave crest. Because of
this bidirectional air flow, the turbine must be designed to
rotate in only one direction no matter what the direction of the
flow is. The Wells turbine is designed for this type of
application and is used in most OWC
devices.
OWC DESIGN:
The air chamber within the OWC housing must be
designed with the wave period, significant wave height and
wave length characteristics of the local ocean climate. If the
housing is not sized correctly, waves could resonate within the
air chamber. This resonating effect causes a net zero passage
of air through the turbine. Ideally, the air chamber dimensions
will be designed to maximize energy capture in the local wave
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climate while research has shown that the generator design
(generator size and generator coefficient) is almost
independent of wave climate such that only areas of extreme
wave energy benefit from larger generators. In addition to
sizing the air chamber with respect to the wave climate, the air
chamber must also be conductive to air flow through the
turbine. This is best achieved with a funnel shaped design
such that the chamber narrows from the water surface level to
the turbine.
OWC PLACEMENT:
Shore and shoreline placements have their both
advantages and disadvantages. Of foremost concern is that the
wave energy is greater offshore than at the shoreline, so more
energy is available for capture in a near shore OWC. Both
near shore and shoreline OWCs are eye sores since they are
visible over the ocean surface. But the shoreline device will
interfere with the beachgoers more directly and will therefore
be met with most public resistance. With the need for the
public acceptance and decent available energy, one may
conclude that the near shore OWC is the better device.
The changing mean ocean levels accompanying tides
may pose problems for a fixedly moored OWC device.
Nonetheless, a fixed moored OWC device maintains its
position better than a slack moored device so as to provide
more resistance to the incoming waves and therefore produce
more energy. Another trade off between the fixedly and slack
moored OWC is that while the fixedly moored device collects
more energy, the slack moored device provides more
flexibility in rough seas which might damage a fixedly
moored device.
WELLS TURBINE:
The Wells turbine is a low-pressure air turbine that rotates
continuously in one direction in spite of the direction of the air
flow. Its blades feature a symmetrical airfoil with its plane of
symmetry in the plane of rotation and perpendicular to the air
stream.
It was developed for use in oscillating-watercolumn wave power plants, in which a rising and falling water
surface moving in an air compression chamber produces an
oscillating air current. The use of this bidirectional turbine
avoids the need to rectify the air stream by delicate and
expensive check valve systems.
AIR PRESSURE AND FLOW CONTROL:
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Well’s turbine efficiency is lower than that of a
turbine with constant air stream direction and asymmetric
airfoil. One reason for the lower efficiency is that symmetric
airfoils have a higher drag coefficient than asymmetric ones,
even under optimal conditions. Also, in the Wells turbine, the
symmetric airfoil is used with a high angle of attack (i.e., low
blade speed / air speed ratio), as it occurs during air velocity
maxima in volatile flow. A high angle of attack causes a
condition known as "stall" in which the airfoil loses lift. The
efficiency of the Wells turbine in oscillating flow reaches
values between 0.4 and 0.7.
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HOUSING:
The housing forms the oscillating water column. It is
placed just above the water surface. It may be used as a
shoreline device or a near shore device. The housing is
designed in such a way that the cross-section decreases as we
go from the base to the top. There is a further increase in the
air stream velocity by using a convergent-divergent nozzle.
Clamps and brackets are provided in order to stabilize and
support the structure to the harbor wall. Bypass valves are
provided for the release of excessive pressure buildup due to
the occurrence of waves whose amplitude is more than the
designed conditions.
TURBINE:
The turbine used is a low pressure wells turbine and it is
modified by placing an aerodynamic nose over the turbine
rotor which concentrates the air flow onto the fins which in
turn increase the efficiency. The wells turbine always rotate in
the same direction no matter in what direction the air flow is
in.
MY DESIGN:
I have made changes in the OWC housing by placing
a convergent divergent nozzle at the housing-turbine interface.
Modification includes an upright housing instead of a slanting
housing. The various components in the OWC are discussed
below:
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The turbine and the generator are placed in a separate housing.
This housing is made with up off a convergent-divergent
nozzle which acts when the air flows from outside into the
housing, that is during a wave trough.
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TURBINE HOUSING:
The generator is directly coupled to the turbine. An
aerodynamic nose is provided at the rear of the generator in
order to reduce the loses due to the drag force as in case of a
generator with a flat rear surface.
CONVERGENT-DIVERGENT NOZZLE:
The convergent-divergent nozzle is a device to
increase the velocity of the air stream. This nozzle works on
the Bernoulli’s principle. A de Laval nozzle (or convergentdivergent nozzle, CD nozzle or con-di nozzle) is a tube that is
pinched in the middle, making a carefully balanced,
asymmetric hourglass-shape. It is used to accelerate a hot,
pressurized gas passing through it to a supersonic speed, and
upon expansion, to shape the exhaust flow so that the heat
energy propelling the flow is maximally converted into
directed kinetic energy. Because of this, the nozzle is widely
used in some types of steam turbines, it is an essential part of
the modern rocket engine, and it also sees use in supersonic jet
engines.
Operation: Its operation relies on the different properties of
gases flowing at subsonic
and supersonic speeds. The speed of a subsonic flow of gas
will increase if the pipe carrying it narrows because the mass
flow rate is constant. The gas flow through a de Laval nozzle
is isentropic (gas entropy is nearly constant). At subsonic flow
the gas is compressible; sound, a small pressure wave, will
propagate through it. At the "throat", where the cross sectional
area is a minimum, the gas velocity locally becomes sonic
(Mach number = 1.0), a condition called choked flow. As the
nozzle cross sectional area increases the gas begins to expand
and the gas flow increases to supersonic velocities where a
sound wave will not propagate backwards through the gas as
viewed in the frame of reference of the nozzle
ASSEMBLY:
The assembly consists of the OWC housing, turbine
and generator coupled together and placed in the turbine
housing, convergent-divergent nozzles (one at the turbinehousing interface and the other at the turbine-outside
environment interface).
GENERATOR:
In electricity generation, an electric generator is a
device that converts mechanical energy to electrical energy. A
generator forces electric charge (usually carried by electrons)
to flow through an external electrical circuit. It is analogous to
a water pump, which causes water to flow (but does not create
water). The source of mechanical energy may be a
reciprocating or turbine steam engine, water falling through a
turbine or waterwheel, an internal combustion engine, a wind
turbine, a hand crank, compressed air or any other source of
mechanical energy.
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013
OPERATION:
The whole assembly is clamped to a wall near a
harbor. The generator is connected to a grid in order to
transmit the electricity produced. The housing is placed such
that the bottom is submerged to a certain level and the mean
sea level is above the submerged level. When a wave crest hits
the housing, the water level inside the housing is increased
which pushes the air present in the column out of the housing
through the convergent-divergent nozzle and through the
turbine. The energy of the compressed air is transferred to the
turbine which in turn rotates the generator producing
electricity.
When a wave trough hits the housing, the water level
inside the housing is decreased. This leads to the development
of low pressure inside the housing and the air from the
atmosphere is sucked into the housing through a convergentdivergent nozzle and through the turbine, once again rotating
it in the same direction (as it is a wells turbine). Thus
electricity is produced during both a wave crest and a wave
trough.
The housing is made up of fiber reinforced composite with
concrete as the matrix. This makes the housing strong and
durable even in the extremely hostile and corrosive
environment of the ocean.
IMPACT ON ENVIRONMENT:
The impact on environment is very low when
compared to other conventional power plants like thermal and
hydel power plants. However, the aquatic flora and fauna are
affected by the wave energy converters to some extent.
IMPACT ON AQUATIC FAUNA:
Fish reproduction may be affected in case of near
shore devices although the shoreline devices do not cause this
problem. There is a danger of aquatic life being swept into the
housing of the wave energy converters. This may be avoided
by using special meshes or nets that would not allow the
animals to be swept into the devices. Animals may be affected
by the noises underwater. Electromagnetic fields and
vibrations may affect mammals like whales. The plants that
grow on the ocean floor may lose their natural habitat due to
the moorings that are used to hold the wave energy converters.
CONCLUSION:
The wave energy converter is an ingenious device
taking the present energy crisis into consideration. The need
of today is to provide sufficient energy to feed the needs of the
ever growing population and at the same time to provide clean
and green energy. Environmental pollution and global
warming are the greatest threats the earth is facing today.
Therefore the wave energy converter would serve the needs if
the slight setbacks are overcome. Efforts are being put into the
improvisation of the housing design and turbine design
worldwide. If the environmental impact is reduced by the
means of better design or better placement, the OWC wave
energy converters can be setup in the coastal areas all around
the world which would provide at least a fraction of the total
energy requirement in those areas.
ACKNOWLEDGMENT:
I owe a debt of gratitude to Asst.Prof. VIJAY
PALLIPOGU (Asst.Prof, in Mechanical Engineering,
K.L.UNIVERSITY,Vaddeswaram, A.P) for his valuable
advice and excellent supervision.
REFERENCES
1. “Wave Energy”, World Energy Council.
2. “Reliable Resourceful Renewable”, Sea volt
Technologies.
3. C. Carroll and M. Bell, Wave energy converters
utilizing pressure differences
4. http://www.google.co.in
5. http://www.wikipedia.org
ADVANTAGES:
1. It is environmental friendly.
2. Its initial cost is low when compared to wind and
solar energy systems
3. It does not add to the green house gases.
4. Efficiency is increased due to the inclusion of the
convergent-divergent nozzle.
5. More often, the land used comes under land
reclamation and thus there is no need for evacuation
of the public as in case of thermal and hydal power
plants
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