Ocean Wave Energy
Wave energy:
Ocean Waves: An oscillatory movement of body of water caused
as the wind flowing over the sea exerts a tangential stress on the
water surface resulting in alternate rise and fall of the surface.
• Waves get their energy from the wind, which comes from solar
energy.
• Wave power is renewable, pollution free and environmental
friendly.
• Though, it varies in intensity, it is available twenty four hours a
day all round the year.
• Power produced is much steadier and more predictable day to day
and season to season.
• Wave energy can be considered as a concentrated form of solar
energy.
Ocean Waves: An oscillatory movement of body of water manifested by
an alternate rise and fall of the surface.
Wave parameters:
Period (T): The time it takes two successive crests to pass a fixed point.
Wavelength (λ): The distance between two consecutive crests.
Height (H): The vertical distance between a trough and a crest.
Wavelengh (λ)
Speed of wave (C) =
Period (T)
Fig. : Characteristics of an idealised wave [5]
Water waves can be considered to travel with an approximate sinusoidal profile.
Difficulties in development of wave power are encountered mainly due to the
following reasons:
• Irregularity of wave patterns in amplitude, phase and direction, which
makes it difficult to extract power efficiently.
• The power extraction system is exposed to occasional extreme stormy
wave amplitude which may reach as high as 10 times the normal value and
the associated power is about 100 times the normal value.
• The peak power of deep water waves is available in open sea, where it is
difficult to construct, operate and maintain a system and transmit power
to the shore.
• The slow and irregular motion of a wave is required to be coupled to an
electrical generator requiring high and constant speed motion.
Wave Energy Technology:
Energy in the waves is harnessed in the form of mechanical energy using
wave energy converters, also known as wave devices or wave machines.
A wave device may be placed in the ocean in various possible situations and
locations.
The fluctuating mechanical energy obtained is modified/smoothed out to
drive a generator.
Wave energy devices may be classified as:
(i). Off-shore or deep water (water depth of 40m or more)
(ii). Shoreline device
Shoreline Devices
• These devices are fixed to or embedded in the shoreline itself, which has
the advantage of easier maintenance and/or installation.
• In addition, these would not require deep-water moorings or long lengths
of underwater electrical cable.
• However, they would experience a much less powerful wave regime.
Shoreline Devices:
1. Oscillating water column:
One major class of
shoreline device is the
oscillating water column
having partly submerged
concrete or steel structure,
which has an opening to
Fig.: Oscillating water column device [3]
the sea below the water
line, thereby enclosing a
column of air above a
column of water.
As waves impinge on the device, they cause the water column to rise and
fall, which alternately compresses and depressurize the air column.
The air is allowed to flow to and from the atmosphere through a turbine,
which drives an electric generator.
The axial flow Wells turbine
invented in the 1970, is the best
known turbine for this kind of
application.
Fig.: Oscillating water column device [3]
Main advantages :
1. The air velocity can be increased by reducing the cross section area of the air
channel. Thus, the slow wave motion can be coupled with the high-speed
turbine motion.
2. Generating equipment's are kept away from immediate saline water
environment.
2. Tapered Channel Device (TAPCHAN):
• The TAPCHAN comprises a gradually narrowing channel with wall heights
typically 3 to 5 m above mean water level.
• Waves enter the wide end of the channel and, as they propagate down
the narrowing channel, the wave height is amplified until the wave crests
spill over the walls to the reservoir.
• The water in the reservoir returns to the sea via a conventional low head
turbine, which generates a stable output due to the storage effects of the
reservoir which provides a stable water supply to a low head turbine.
• A demonstration device with rated output of 350 kW began operating in
1985 at Toftesfallen, in Norway.
.
Tapered Channel Device [3]
3. Pendulor Device:
• A 5 kW Pendulor test device has been operating in Hokkaido (Japan) since
1983.
• It consists of a rectangular box, which is open to the sea at one end.
A pendulum flap is hinged over this opening, so that the actions of the
waves cause it to swing back and forth.
• This motion is then used to power a hydraulic pump and generator.
Pendulor Device [3]
Offshore Devices
• This class of device exploits the more powerful wave regimes available in deep
water (> 40 m depth) before energy dissipation mechanisms have had a
significant effect.
• In order to extract energy from the waves, the devices need to be at or near the
surface (i.e. floating) and so they require flexible moorings and electrical
transmission cables.
• There are many different types of offshore device, some of which are shown in
Fig.:
Fig.: Float type device [1]
• A float placed on the surface of water heaves up and down
with waves due to the rise and fall of the water level.
• The resulting vertical motion is used to operate the piston of
an air pump through linkage.
• The pump may be anchored or moored to the sea bed. Several
float-operated air pumps are used to store energy in a
compressed air storage.
• The compressed air is used to generate electricity through an
air turbine coupled to a generator.
• In a Swedish design, a submerged flexible tube attached to a buoy is
used as an air pump.
Present Status:
• Wave energy is passing through R and D phase and has not reached
maturity from several points of view.
• Main countries involved in its development are Danmark, India, Ireland,
Japan, Norway, Portugal, UK and USA.
• These efforts are largely been uncoordinated and so a wide variety of
technologies have emerged.
• Some of them have been deployed in the sea as demonstration scheme.
• Large-scale offshore devices are likely to remain uneconomical in the near
future.
• The development of small-scale modular shoreline devices at shore-based
sites seems to become more economical and competitive.
• The wave energy program was started in India in 1983 with
preliminary studies at IIT Madras.
• Initial research concluded that oscillating water column is the
most suitable for Indian conditions.
• A 150 kW pilot OWC plant was built onto the breakwater of the
Vizhingam Fisheries Harbour, near Trivandrum (Kerala) in 1991.
• A plant of total capacity of 1.1 MW comprised of 10 caissons is
being developed at the same site.
References :
1. Non-Conventional Energy Resources by B. H. Khan, Tata McGraw Hill Education
Private Limited, second edition, 2011
2. https://nptel.ac.in/courses/1081
3. https://nptel.ac.in/courses/105107059/module6/lecture4/lecture4.pdf08078/
pdf/chap8/teach_slides08.pdf
4. Introduction to Fluid Mechanics and Fluid Machines by S K Som, Gautam
Biswas, S Chakraborty, Third Edition, McGraw Hill Education (India) Private
Limited
5. https://www.open.edu/openlearn/ocw/mod/oucontent/view.php?id=73764&e
xtra=thumbnailfigure_idm45676359806720 [Accessed on 22-02-2021]
