International Journal of Control and Automation
Vol.9, No.2 (2016), pp.93-102
http://dx.doi.org/10.14257/ijca.2016.9.2.10
A Novel Wave Energy Harvesting System for Ocean Sensor
Network Applications
Olly Roy Chowdhury, Hong-geun Kim, Myeongbae Lee, Changsun Shin,
Yongyun Cho and Jangwoo Park
Abstract
L.
Dept. of Information and Communication Engineering,
Sunchon National University, Suncheon, 57922, Republic of Korea
{ollyroy, khg_david, csshin, yycho, jwpark}@sunchon.ac.kr
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This paper represents a solution for avoiding a common hurdle of harvesting ocean
energy using Oscillating Water Column that, though being an efficient technology OWC
based harvesters often suffers from performance disruption due to rough sea weather.
The solution reveals an idea for a completely closed, double chambered, half water filled
oscillating water column with two bidirectional air turbines inside, which shows efficient
performance in the bad weather condition as well as in general. The system has the
unique feature that, it produces electric power without any interaction with outside ocean
water or atmospheric pressure and having a simple, closed structure that can be build
using the lower vessel type part of any old ship or boat, adds extra facilities like cost
effectiveness and less maintenance effort. Offshore ocean environment monitoring
wireless sensor network or floating projects like fish farm can use this system for
powering the sensor nodes used for measuring temperature, ph buffer, salinity.
Keywords: Closed OWC, Wave energy converter, Sensor Nodes
1. Introduction
Bo
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Ocean which covers most of the surface of the world (almost 71%) is full of resources
[1]. To utilize the resources in efficient ways to be used for mankind, the ocean area needs
to be monitored first. Monitoring will include checking climate changes, detecting
pollution of the environment which effects living species under water etc. recently this is
performed by Wireless sensor networks consisting of thousands of sensor nodes, whereas
each node containing a processing unit, a transceiver, memory, a battery and sensors [2].
Besides this, floating projects like fish farm uses different sensor nodes so that these can
be remote controlled from far land. Most of these sensor nodes use batteries for power
supply that becomes troublesome for prolonged existence of the system since the batteries
have specific lifetime after which they need to be changed. To avoid this difficulty
extracting the energy for monitoring the ocean from the ocean it is the best idea.
With a view to this, different types of wave energy harvesting systems are available
now a day. Among many types of offshore and near shore wave energy harvesting
devices, Oscillating Water Column (OWC) is the most successful and extensively studied
technology for extracting energy from ocean waves [3]. But most of these are for larger
power generation which are complex and of bigger sized with high costing. And also,
they cannot perform at their maximum efficiency in rough weather condition.
Harnessing energy from ocean waves even in the harsh sea weather has always been a
challenge for off shore wave energy harvester devices. This paper describes a new idea of
closed type OWC which has been designed specially keeping in mind that it can
withstand bad sea weather and obviously be cost effective and will require less
Corresponding Author
ISSN: 2005-4297 IJCA
Copyright 2016 SERSC
International Journal of Control and Automation
Vol.9, No.2 (2016)
maintenance. This is an off shore energy harvesting device for electric power generation.
As it is closed type OWC water wouldnt enter inside it and so it will not be a matter of
keen observation or regular maintenance.
2. Oscillating Water Column
2.1. Background
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An increasing population together with its improving living standard is driving up the
consumption of energy and it is consecutively diminishing the resources like oil, coal and
natural gas. Side effect like environmental pollution have put an end to the fossil fuels and
now renewable energy is the only hope for solving the future energy crisis. Ocean has a
vast amount of energy source which can be extracted in variety of ways. Extracting
energy from wave is not a new thing. Oscillating Water Column is the most popular
among all the used technologies for harvesting ocean energy. OWC can be shoreline, near
shore or offshore type. Figure 1 shows simple classical OWC.
ok
Figure 1. Classical OWC [4]
Bo
For WSN and many other on sea projects, offshore OWC can be the most convenient
option. And this offshore OWC suffers from the troubles caused by rough sea weather.
The idea of closed type OWC discussed in this paper can overcome this difficulty and can
proceed the system one step further toward perfection.
2.2. Related Works
Scientists all over the world is trying to make wave energy extraction more easy, cost
effective, enduring and most importantly efficient. A study (before 2010) showed that
there were over 1000 patented ideas for wave energy conversion. Among different
technology groups OWC is a very straightforward device consisting of two basic part: 1)
the chamber; and 2) the turbine [5] Multi chamber or segmented OWC has been studied
briefly in [6] Performance of the turbines like savonious or wells and effect of guided
vanes on wells turbine used in OWC, have also been matter of research. [6,7,8]
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2.3. The Closed Type OWC
Figure 2. Closed Type OWC Device
ok
An oscillating water column device harnesses the motion of ocean waves as they
pressurize a pocket of trapped air. Figure 2 shows an off shore floating OWC device
which comprises a partially submerged, rectangular shaped, half water filled chamber
with air trapped above the water surface.
In the classical OWC, water waves enter inside the chamber and this water column
behaves like a piston, continuously moving up and down that compresses or
decompresses the air trapped above the water surface inside the chamber. The
compression or decompression of air inside OWC chamber, generate an alternating
stream of high-velocity air through a bidirectional air turbine placed in an exit blow hole.
This turbine is coupled with a generator to produce electricity [3,9].
The proposed structure of closed OWC utilizes the motion of waves in the sea and
works with the same principle of operation as classical OWC except the fact that the sea
water waves doesnt enter or exit the chamber. It works with a fixed amount of water
inside the chamber moving from one chamber to other with the ups and downs of the
waves of the sea. The air trapped above the water surface moves in the reverse direction
with the water, through the two air turbines placed inside a tunnel at the upper portion of
the device. The movement of turbine rotor due to the air passing through these, in turn
moves the generator which produces electricity.
Bo
3. Theory of Operation
The invented structure is a rectangular double chambered vessel with buoyancy
material around the down part, floating on water. The two chambers are separated not
fully by a barrier which has open portions at the bottom part and at the upright position of
the structure. The bottom aperture acts like a channel between the chambers keeping the
two chambers in fluid contact with each other. At upper aperture there is a tunnel within
which stays two self-rectifying air turbines and a power generator for producing
electricity. The whole device is closed type and half filled with water.
On the arrival of water waves, the device gets tilted. The heave motion of the wave that
is the ups and downs of the waves causes the water to move from one chamber to other
through the down aperture. The amount of water that leaves one chamber, the same
amount of water enters another chamber. The rise and fall of water levels in the chambers
due to this moving water creates pressure difference in the two chamber which makes an
alternating stream of high-velocity air through the upright tunnel. The amount of water
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Figure 3. Device Position with Incoming Wave
L.
moves from 1st to 2nd chamber, the same amount of air leaves the 2nd chamber and
enters the first chamber.
As the air moves through the turbine, the turbine goes under movement and rotates a
power generator along with it to generate electric power.
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Figure 3 represents the five different position of the device at five different wave
condition in time t1 to t5. From steps 1-5, the device gets tilted in one direction from
horizontal equilibrium position and then again returns to the same equilibrium position as
before. During this time, water passes from one chamber to another chamber and returns
to the first chamber at time t5 through the aperture down the chamber barrier.
Bo
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Figure 4. Triangle of Moving Water
Figure 5. Device at Maximum Titled Position
According to figure 4, let triangle abc amount of water passes once during t1 to t3. If
the tilt angle is , then,
tan = ()/
= tan
(1)
Where, a = l/2, length of a single chamber
b(t) = instantaneous wave height inside the chamber
t =angle of tilt
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L.
T = wave period
= inclined angle of wave
d = chamber height
A1 = area of the triangular portion of water
v1 = velocity of air just above the water surface
A2 = area of turbine rotor
v2 = velocity of wind at the turbine rotor
L = deep water wavelength
x = horizontal distance in time T/4
H = wave height
rt = radius of the turbine rotor
= air density
From figure 4 and using (1),
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1 = 1 2 = 1 2 2 tan
= 1 2 2 tan [From fig: 4]
(2)
According to figure 5, the device gets maximum tilted at time t3. So,
3 1 = 4
(3)
Considering the maximum tilted position of the device, water inside the chamber
crosses distance, b = d/2, in time T/4 . So, at t = T/4, b(t) = b. Then,
1 =
4
=
(4)
1 = 0
1 = 1 =
(5)
(6)
Again from equation of continuity,
1 1 = 2 2
(7)
= /4
(8)
Bo
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From figure 5,
tan =
2 = 2
(9)
So, from (7) wind speed at the rotor,
2 =
1 1
2 =
2 2
42
=
2
3 2
2
2
1 = 1 2 2 tan =
=
2
(10)
10
(11)
Power available from the turbine:
= 1 2 2 2 3
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4. Analysis
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For a device of length 3m, the length of a single chamber is, a = 1.5m and for chamber
height d=1m we get the draft height, b = 0.5m. Taking the parameters into consideration,
the graph in fig.6 shows the variation of air velocity obtainable at the turbine, with wave
period for three different wave height condition.
Figure 6. Variation of Air Velocity at Turbine with Wave Period
Using the same parameters and with a turbine of radius 5cm the variation of extractable
mechanical power for different outside wave height is given in the table below:
Table 1. Power Available From the Turbine for A=1.5m and B=0.5m
Hm
0.2
P
W
T=1.7s
2.009
at
P
W
T=1.8s
1.2
0.3
4.055
1.57
0.35
10.77
6.44
at
Bo
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Figure 7 shows the variation of extractable mechanical power from turbine, with
wave period T.
Calculation was also done to study the effect of device height on power generation.
Fig.8 shows the power variation with draft height b.
This is the calculated mechanical power obtainable from the device. Generated electric
power will be factor of the efficiency of turbine and generator.
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Vol.9, No.2 (2016)
Figure 7. Variation of Power with Wave Period
Figure 8. Power Variation with Draft Height
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5. Conclusion
Bo
From the above analysis, it is clear that both wave height and wave period effects the
performance of OWC. For speed of wind at the turbine as well as mechanical power from
the turbine, wave height works as a proportionality factor. That is the power from the
system increases as the wave height goes high. Again the reverse happens with the wave
period. Maximum power can be obtained for waves of minimum period. Device height
also plays proportional role on the power production.
The completely closed, floating, offshore, rectangular shaped, double chambered OWC
structure which is half filled with water can produce electric power without any
interaction with the outside ocean water or atmospheric pressure by using two
bidirectional air turbines inside the device. With the system parameters used in this paper,
a range of 1-10 W of power, on average can be got from this system depending on
different ocean wave condition, enough to meet up the power requirement of the sensor
nodes used in Wireless Sensor Network. Its an innovative, simple, stable, cost effective
structure with buoyancy material, able to work in the harsh ocean wave climate. As it is
completely closed device, it will need less observation and maintenance which is a
beneficial characteristic for any off shore on sea floating WEC. The criteria that the lower
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part of any old boat or ship can be used to build the structure and thus a totally new
structure is indeed not a compulsory condition, makes the system more efficient in further
ways.
Acknowledgments
This work was supported by the Industrial Strategic technology development program,
10041766, Development of energy management technologies with small capacity based
on marine resources funded by the Ministry of Knowledge Economy (MKE, Korea).
References
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[3] Dresser-Rand HydroAir Variable Radius Turbine, Available online: http://www.dresserrand.com/products/hydroair/
[4] Mpower Available online: http://www.mpoweruk.com/hydro_power.htm
[5] D. G. Dorrell, M. F. Hsieh and C. C. Lin, A Multichamber Oscillating Water Column Using Cascaded
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[6] D.G. Dorrell, M.F. Hsieh and C.C. Lin, A small segmented oscillating water column using a savonius
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Bo
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Authors
Olly Roy Chowdhury, She received the BS and MS degrees in
Applied Physics Electronics and Communication Engineering, University
of Dhaka, Bangladesh in 2006 and 2007, respectively. She is an Assistant
Professor of the Department of Physics and Mechanical Engineering, in
Patuakhali Science and Technology University, Bangladesh. Currently,
she is a PhD student in Sunchon National University. Her research area is
renewable energy harvesting.
Hong-geun Kim, He received the BS and MS degrees in Information
& Communication engineering from Sunchon National University in 2011
and 2013, respectively. Currently, he is a PhD student in Sunchon
National University. His research focuses on Localization and renewable
energy harvesting and Embedded System and RFID/USN technologies.
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International Journal of Control and Automation
Vol.9, No.2 (2016)
Myeongbae Lee, He is a PhD student of Sunchon National
University. He is also a researcher of Go-Lab at Sunchon National
University too. His main research interests include Renewable energy,
Ubiquitous Computing and Middleware.
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Changsun Shin, He received the PhD degree in computer
engineering at Wonkwang University. Currently, he is an assistant
professor of the Department of Information & Communication
Engineering in Sunchon National University. His main research
interests include Distributed Real-Time Computing, Distributed
Object Modeling, Ubiquitous Agriculture, and Ubiquitous Sensor
Network (USN).
Yongyun Cho, He received the PhD degree in computer
engineering at Soongsil University. Currently, he is an assistant
professor of the Department of Information & Communication
Engineering in Sunchon National University. His main research
interests include System Software, Embedded Software and
Ubiquitous Computing.
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Jangwoo Park, He received the BS, MS and PhD degrees in
Electronic engineering from Hanyang University, Seoul, Korea in
1987, 1989 and 1993, respectively. In 1995, he joined the faculty
member of the Sunchon National University, where he is currently a
professor in the Department of Information & Communication
Engineering. His research focuses on Localization and SoC and
system designs and RFID/USN technologies.
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Vol.9, No.2 (2016)
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