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THEORETICAL AND EXPERIMENTAL ANALYSIS OF
FLUTTER-BASED WIND MICROGENERATOR
BY
ABDUL HAKIM JAVID
A thesis submitted in fulfilment of the requirement for the
degree of Master of Science
(Mechanical Engineering)
Kulliyyah of Engineering
International Islamic University Malaysia
JULY 2014
ABSTRACT
In this work, we propose a novel flutter-based wind energy harvesting technique
which is capable of providing power to low power applications in remote areas. The
flutter-based wind microgenerator developed in this work uses electromagnetic
transduction to generate power with permanent NdFeB (Neodymium, Iron and Boron)
magnets used to provide intense magnetic field around the harvester. We develop a
mathematical model comprising the various physical phenomena of the energy
conversion from wind to mechanical vibrations and then electric. The mathematical
model is further compared with finite element analysis solution obtained using
NASTRAN to check its conformity. It was found that the flutter frequency and
voltage output increases with wind speed and decreases with increase in width of the
cantilever. And there is no effect of increase in span of the cantilever on flutter.
Experimental investigations were performed on two proposed designs: (1) a horizontal
cantilever harvester with embedded coils on the surface, and (2) an inverted cantilever
with coils surrounding a bluff body attached to the cantilever tip. The cantilever
attached with a square bluff body at the tip showed better flutter characteristics and
power generation than the cylindrical bluff body of same size. We found that the
proposed microgenerators can provide power from wind speeds as low as 1 m/s and
produce a peak power of 0.26 mW without any use of bluff body. Peak power of 0.21
mW and 0.16 mW were produced using square and cylindrical bluff bodies at the tip
at wind speed of 4 m/s. It also shows that there is no adverse interference between the
vibrating elements when stacked together in a row which is desirable for power
generation to support larger remote application devices. An electric circuit capable of
stepping up, conditioning and storing the energy harvested is also integrated to the
harvester and tested. The proposed design experimentally demonstrates a low wind
speed harvesting device suitable for environmental applications monitoring the
environment in rural areas such as forests or mountains.
ii
‫خالصة البحث‬
‫املتقحر ف يفذاافاعمل فذو فدراسةفحصادة فطاقةفريا فقادرةفعلىفتوفريفاعطاقةفعتطبيتقاتفاعطاقةف‬
‫املنخفضة ف ي فاملناطق فاعنائية فمن فخالل فتتقنية فجديدة‪ .‬فمت فتطوير فرفرافة فاعريا فاملستندة فإىل‬
‫‪ microgenerator‬ي فذاا فاعمل فواعيت فتستخدم فتتقنية فكهرومغناطيسية ف‪ .‬فوتستخدم ف‪NdFeB‬ف‬
‫اعدائمفعتوفريفجمالفمغناطيسيفحولفحصادة‪.‬فمتفتطويرفمنوذجفرياضيفيضمفانواعفخمتلفةففيازياف‬
‫من فحتوي فاعطاقة فمن فاعريا فإىل فاالذتزازات فامليكانيكية فومث فاجلهد فاعكهربائيف‬
‫ع ‪microgenerator‬املتقحر ف‪.‬فوكاعكفمتقارنةفاعنلوذجفاعرياضيفمعفح فحتلي فاعمنصرفاحملدودف‬
‫علتحتقق فمن فتوافتقها‪ .‬فوأجريت فاعتحتقيتقات فاعتجريبية فعلى فاعتصليم فاملتقحر فوتبني فأن‬
‫‪microgenerator‬ميكنفأنفتوفرفطاقةفمنفاعريا فبسرعةفمنخفضةفتص فإىلفمحر‪/‬ثانية‪.‬فأيضافالف‬
‫يوجد فتداخ فبني فاعمناصر فاملهتزة فعندما فتكون فمكدسة فمما ف ي فصف فواحد فواعاي فذوف‬
‫مرغوب ففيه فعتوعيد فاعطاقة فعدعم فأجهزة فاعتطبيق فعن فبمد ف‪ .‬فكلا فمت فدمج فاعدائرة فاعكهربائيةف‬
‫اعتقادرةفعلىفتكثيففوتكييففوختزينفطاقةفاحلاصدةفواختبارذا‪.‬فاعتصليمفاملتقحر فيوضحفسرعةف‬
‫اعريا فاملنخفضة فمناسبة فجلهاز فاحلصاد فىف فتطبيتقات فبيئية فمماثلة فإىل فاعغابات فأو فاجلبال‪.‬ف‬
‫اعتوصياتفواالقحراحاتفاملتقدمةف يفذاهفاألطروحةفتدعمفاعبحوثف يفاملستتقب ‪.‬ف‬
‫‪iii‬‬
APPROVAL PAGE
I certify that I have supervised and read this study and that in my opinion it conforms
to acceptable standards of scholarly presentation and is fully adequate, in scope and
quality, as a research paper for the degree of Master of Science in Mechanical
Engineering.
…………..………………………………
Raed Ismail Mohmoud Kafafy
Supervisor
…………..………………………………
Moumen Idres
Co Supervisor
I certify that I have read this study and that in my opinion it conforms to acceptable
standards of scholarly presentation and is fully adequate, in scope and quality, as a
research paper for the degree of Master of Science in Mechanical Engineering.
…………..………………………………
Asan Gani Bin Abdul Muthalif
Internal Examiner
…………..………………………………
Shuhaimi Mansor
External Examiner
This dissertation was submitted to the Department of Mechanical Engineering and is
accepted as a fulfilment of the requirement for the degree of Master of Science in
Mechanical Engineering.
…………..………………………………
Meftah Hrairi
Head, Department of Mechanical
Engineering
This dissertation was submitted to the Kulliyyah of Engineering and is accepted as a
fulfilment of the requirement for the degree of Master of Science in Mechanical
Engineering.
…………..……………………………
Md. Noor bin Hj Salleh
Dean, Kulliyyah of Engineering
iv
DECLARATION
I hereby declare that this dissertation is the result of my own investigations, except
where otherwise stated. I also declare that it has not been previously or concurrently
submitted as a whole for any other degrees at IIUM or other institutions.
Abdul Hakim Javid
Signature ____________________
Date ____________________
v
INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA
DECLARATION OF COPYRIGHT AND AFFIRMATION
OF FAIR USE OF UNPUBLISHED RESEARCH
Copyright © 2014 International Islamic University Malaysia. All rights reserved.
THEORETICAL AND EXPERIMENTAL ANALYSIS OF
FLUTTER-BASED WIND MICROGENERATOR
No part of this unpublished research may be reproduced, stored in a retrieval
system, or transmitted, in any form or by any means, electronic, mechanical,
photocopying, recording or otherwise without prior written permission of the
copyright holder except as provided below.
1.
Any material contained in or derived from this unpublished research
may only be used by others in their writing with due acknowledgement.
2.
IIUM or its library will have the right to make and transmit copies (print
or electronic) for institutional and academic purposes.
3.
The IIUM library will have the right to make, store in a retrieval system
and supply copies of this unpublished research if requested by other
universities and research libraries.
Affirmed by Abdul Hakim Javid
__________________
__________________
Signature
Date
vi
ACKNOWLEDGEMENTS
In the Name of Allah, the Most Gracious, the Most Merciful.
I start with thanking the Almighty for providing me an opportunity to do my masters. I
thank my beloved parents and family members for their support, encouragement and
prayers.
I am grateful to my supervisor Dr. Raed I. Kafafy and co-supervisors
Dr.Moumen Idres for their guidance, suggestions, encouragement and support
throughout this research work. I thank Dr. Erwin Sulaeman, IIUM for his support in
performing FEA Analysis using Nastran.
My special thanks are to Prof. Waqar and Ida Rahayu, Technical Assistant,
Aerodynamics Lab for their support in conducting experiments. I thank all my
colleagues at ECTMRG lab for all the technical support, and assistance.
Finally I thank all my teachers and friends back home and in Malaysia for their
help in making this a successful research and helping in various other engagements.
vii
TABLE OF CONTENTS
Abstract .................................................................................................................... ..ii
Abstract in Arabic ...................................................................................................... .iii
Approval Page ............................................................................................................ .iv
Declaration Page ........................................................................................................ ..v
Copyright Page ........................................................................................................... .vi
Acknowledgements .................................................................................................... vii
List of Tables ............................................................................................................. .xi
List of Figures ........................................................................................................... xii
List of Symbols ……………………………………………………………………..xv
CHAPTER ONE: INTRODUCTION ...................................................................... 1
1.1 Introduction ............................................................................................. 1
1.2 Problem Statement and its Significance .................................................. 2
1.3 Research Objectives ................................................................................ 3
1.4 Research Methodology............................................................................ 3
1.5 Scope of research .................................................................................... 4
1.6 Thesis organization ................................................................................. 4
CHAPTER TWO: LITERATURE REVIEW ......................................................... 5
2.1 Introduction On Energy Harvesting ......................................................... 5
2.2 Sources For Energy Harvesting ............................................................... 5
2.3 Energy Harvesting Techniques ................................................................ 8
2.3.1 Photonic Energy Harvesting .......................................................... 8
2.3.2 Magnetic Energy Harvesting.......................................................... 8
2.3.3 Acoustic Energy Harvesting .......................................................... 9
2.3.4 Pyroelectric Energy Harvesting ..................................................... 9
2.3.5 Thermoelectric Energy Harvesting ................................................ 9
2.3.6 Piezoelectric Energy Harvesting .................................................. 10
2.3.7 Electrostatic Energy Harvesting................................................... 10
2.3.8 Electromagnetic Energy Harvesting ............................................ 11
2.3.9 Magnetostrictive Energy Harvesting ........................................... 11
2.3.10 Electroactive Polymers................................................................ 11
2.3.11 Radio Frequency Energy Harvesting .......................................... 12
2.4 Wind Energy Harvesting ........................................................................ 12
2.4.1 Micro Turbine Energy Harvesting ............................................... 12
2.4.2 Non Turbine Energy Harvesting from Artificial Fluid Sources .. 15
2.4.3 Non Turbine Energy Harvesting from Natural Sources ............... 16
2.4.4 Limit Cycle Fluttering Operation ................................................. 20
2.4.5 Broadband Energy Harvesting ..................................................... 21
2.4.6 Augmenting Wind Funnel ............................................................ 22
2.4.7 Bluff Body Extension................................................................... 23
2.4.8 Power Management Circuit ......................................................... 24
2.5 Summary ................................................................................................ 25
viii
CHAPTER THREE: ANALYTICAL MODELING OF FLUTTER
MICROGENERATOR ............................................................................................ 27
3.1 Introduction ............................................................................................ 27
3.2 Aerodynamic Flow Model ..................................................................... 27
3.3 Electromagnetic Generator Model ......................................................... 32
3.4 Time Domain Analysis Of Flutter Microgenerator ................................ 36
3.4.1 Introduction .................................................................................. 36
3.4.2 Modeling The Flutter Microgenerator ......................................... 37
3.4.3 Simulation .................................................................................... 39
3.4.4 Results .......................................................................................... 40
3.5 Flutter Estimation Of Flow Induced Vibration In Thin Cantilever Plate 41
3.5.1 Introduction .................................................................................. 41
3.5.2 Frequency Estimation Model ....................................................... 42
3.5.3 Simulation .................................................................................... 46
3.5.4 Results .......................................................................................... 47
3.6 Parametric Study Of A Cantilever Fluttering Beam .............................. 48
3.6.1 Introduction .................................................................................. 48
3.6.2 Parametric Study Model............................................................... 49
3.6.3 Simulation .................................................................................... 50
3.6.4 Results .......................................................................................... 51
CHAPTER FOUR: FINITE ELEMENT ANALYSIS OF FLUTTER
HARVESTER ........................................................................................................... 55
4.1 Introduction ............................................................................................ 55
4.2 Modeling In Nastran .............................................................................. 55
4.3 Results .................................................................................................... 56
CHAPTER FIVE: EXPERIMENTAL STUDY OF FLUTTER
MICROGENERATOR ............................................................................................ 60
5.1 Introduction ............................................................................................ 60
5.2 Design And Fabrication ......................................................................... 60
5.2.1 Fluttering Cantilever .................................................................... 60
5.2.2 Electromagnetic Generator........................................................... 62
5.2.3 Power Management Ciruit ........................................................... 63
5.3 Experimental Setup And Procedure ....................................................... 64
5.3.1 Experimental Setup ...................................................................... 64
5.3.2 Experimental Procedure ............................................................... 66
5.3.3 Experimental Results And Analysis ............................................ 67
CHAPTER SIX: CONCLUSION ........................................................................... 82
6.1 Conclusion.............................................................................................. 82
6.2 Main Contribution .................................................................................. 83
6.3 Recommendation.................................................................................... 83
REFERENCES ......................................................................................................... 84
ix
PUBLICATIONS ..................................................................................................... 88
APPENDIX A – NASTRAN INPUT FILES ............................................................. 89
APPENDIX B – MATLAB CODES ....................................................................... 105
x
LIST OF TABLES
Table No.
Page No.
3.1
Microgenerator parameters
40
‎5.1
Magnet and coil properties
63
xi
LIST OF FIGURES
Figure No.
2.1
Page No.
The cross section of axial flow micro turbine with integrated axialflux electromagnetic microgenerator (Holmes et al., 2004)
12
The three propellers tested experimentally (1-46o stagger, 2-26o at the
tip, 3-19o at the tip); (Rancourt et al., 2007)
13
Design of a centimetre-scale shrouded wind turbine for energy
harvesting (a) schematic drawing showing stator and rotor (b) cutsection view (c) six-blade rotor fixed with circular magnets
14
(a) Schematic diagram of electromagnetic resonator; (1) Bolt (2) Coils
(3) Housing (4)Magnet (5)Spring (6)Base (b) experimental setup of
the modified windbelt with electromagnetic resonator; (1) belt
supporting beam (2) electromagnetic resonator (3) power management
circuit (4) replaceable supercapacitor (5) oscilloscope (6) junction of
belt and resonator
17
Schematic diagram of T-shaped piezoelectric cantilever under a fluid
flow
18
Experimental setup of Park et al. showing T-shaped cantilever with
magnets and coils placed externally.
19
2.7
T-shaped cantilever harvester placed inside a wind contracting funnel
23
2.8
Experimental setup showing cylindrical body attached to the tip of
a PZT cantilever (Gao, 2011)
24
Schematic diagram of the power management circuit proposed by Fei
et al.
25
Illustration of the displacements and forces acting on a cross section
of a fluttering wind belt.
28
Real and imaginary parts of theodorsen circulatory function
C(K)=F(K)+i G(K).
29
*
*
Aeroelastic coefficients Ai and H i plotted vs 1/K for thin plate
31
Proposed flutter-based with pick up coils embedded on to the surface
of belt: (a) arrangement of three different sets of cantilever with a
magnet; (b) cantilever film placed with respect to magnet; (c)micro
square spiral coil on the film
35
‎2.2
2.3
‎2.4
2.5
2.6
2.9
3.1
‎3.2
3.3
‎3.4
xii
‎3.5
Design of flutter-based microgenerator harvester. (a) Schematic of coil
placed around the magnet (b) Coil topography
36
‎3.6
Simulation Model of the Flutter-based Micro generator in Simulink
38
3.7
Sub model of the Aeroelastic forces in Simulink
38
3.8
Sub model of the structure dynamics in Simulink
39
3.9
Sub model of the electromagnetic generator in Simulink
39
3.11
Plot showing the behaviour of two dimensionless parameters
discussed in equation 33 and 34.
46
Plot showing the pattern of frequency for a thin cantilever plate over a
wide range of wind speed (a) width of the cantilever varied between 13cm (b) length of the cantilever varied between 3-5cm
48
Effect of change in span of the cantilever on the response frequency at
various wind speeds
51
Effect of change in width of the cantilever on the response frequency
at two wind speeds
52
Effect of change in length of coil winding on the cantiever surface
against frequency response at two wind speeds
53
Effect of change in load resistance connected to the coils on the
cantilever on the current
54
‎4.1
The schematic of the cantilever modeled in Nastran
56
‎4.2
(a) Grid elements of FEA model generated using Nastran
(b)(c)(d)(e)(f) first five mode shapes of structural vibration occurring
on the plate denoted by Eigen values of displacement
57
Damping force offered by the structure at various wind speeds for the
first five mode shapes of vibration
58
Frequencies of flutter at various wind speeds for the first five mode
shapes of vibration
59
5.1
Flutter-based harvester with coils-on cantilever.
61
‎5.2
Flutter-based harvester with coils-around body.
62
5.3
Schematic of magnet placed around the fluttering cantilever (a) Coil
on surface arrangement (b) Coil on body arrangement
62
N48 grade Nd-FeB magnets (2cm x 2cm x 0.4cm) Stacked 8 pieces
and 2 pieces.
63
‎3.12
3.13
‎3.14
‎3.15
3.16
‎4.3
4.4
5.4
xiii
5.5
Power management circuit (EH4295, EH301 and LED from left to
right).
64
5.6
Flutter-based harvester - cantilever setup.
65
5.7
Flutter-based harvester with tip-attached
cantilever setup (a) square (b) cylinder.
5.8
bluff
body-inverted
66
Output of the coil on surface flutter-based microgenerator at various
loads and wind speeds.
68
Output of the flutter-based microgenerator at various loads and wind
speeds.
70
Output of flutter-based harvester at 4 m/s for 2 magnets and 8 magnets
at top.
72
5.11
Output of flutter-based harvester at 4 m/s for 50m and 100m coils.
73
‎5.12
Output of flutter-based harvester at 4 m/s for 50m coil with top and
back magnets.
74
Output of flutter-based harvester at 4 m/s for 50m coil with horizontal
and vertical 8 magnets.
76
Output of flutter-based harvester at 4 m/s for 50m coil for various
span of the cantilever
77
Output of flutter-based harvester at 4 m/s for 50m coil for various
span of the cantilever
78
Experimental setup of 3 cantilevers with copper coils wound on its
surface
79
5.17
Capacitor charging voltage over time.
80
5.18
Plot showing the raw induced voltage
81
5.9
‎5.10
‎5.13
5.14
5.15
5.16
xiv
LIST OF SYMBOLS
Acoil
Aw
B
Bmag
Br
b
f
h
I
i
k
L
Lc
LMT
Lw
M
m
N
q
Rc
RL
r0
ri
S
t
U
VT
wd


rotational derivatives; i= 1, 2, 3, 4
area of coil, m2
area of wire (conductor only), m2
Width of the cantilever, m
magnetic field strength of the magnet, Tesla
residual magnetic flux density, Tesla
half chord width = B/2, m
coil fill factor
vertical displacement derivatives; i= 1, 2, 3, 4
vertical displacement, m
section polar moment per unit length, m4
induced current through the microgenerator circuit, Ampere
reduced frequency = b/U
vertical aerodynamic force per unit length, N
effective coil length, m
length of wire, m
length of wire, m
aerodynamic pitching moment per unit length, Nm
mass per unit length, Kg
number of turns
dynamic pressure of the wind
microgenerator coil resistance, Ohm
load resistance of the microgenerator circuit, Ohm
coil outer radius, m
coil inner radius, m
Span of the cantilever, m
coil thickness, m
uniform wind velocity, m/s
coil volume, m3
wire diameter, m
induced electromotive force (e.m.f.) on coil, V
magnetic flux density, Tesla
resistivity of copper, Ohm
angular displacement, deg
density of air, Kg/m3
circular frequency of oscillation, rad/s
xv
1. CHAPTER ONE
INTRODUCTION
1.1 INTRODUCTION
A wireless sensor network (WSN) consists of spatially distributed autonomous sensors
to monitor physical or environmental conditions, such as temperature, sound,
vibration, pressure, motion or pollutants and to cooperatively pass their data through
the network to a main location. Monitoring includes remote locations in air pollution
monitoring, forest fires detection, landslide detection, machine health monitoring and
structures such as bridges, flyovers, embankments, tunnels, dams etc.
Powering the remote sensors is a challenge, they were initially powered by
batteries. These batteries powered sensors only for a certain period of time. This
caused the sensors to go offline when they are running out of battery life. Battery life
is crucial to the whole WSN. Replacing the batteries of every sensor after its life time
is not an option, since a WSN involves several hundred sensors in complex structures
and along with that these sensors are placed in remote locations which are often very
difficult to reach periodically. Research is being done to explore the possibility of
having an energy harvesting system integrated to the sensors of WSN so that they can
operate continuously without running out of power.
Wind energy which is a renewable energy source comes clean and has a power
density proportional to cube of the wind speed; it promises a very good deal of energy
when it is rightly converted. Wind energy harvesting shows a good sign in
autonomous supply of power to the sensor nodes at raised locations for example
bridges, dams, mountains etc. and remote locations such as wildlife sanctuaries. Since
1
these nodes need only a small amount of power, micro generators are suitable for this
purpose. Conventional method of harvesting wind with the help of wind turbine is
suitable only for larger applications. For smaller applications, when wind turbines
were made in small sizes it reduces its lesser efficiency because of down scaling of the
moving parts which had more friction compared to the rotational motion generated by
the micro wind turbine.
1.2 PROBLEM STATEMENT AND ITS SIGNIFICANCE
Energy harvesting from natural wind sources is an area of interest for researchers
since it promises power at a variety of locations for powering low power remote
monitoring devices. Also, the power density of a natural wind source like breeze is
considered enough and more suitable for micro energy harvesting applications to
support remote monitoring. Usually small amounts of energy harvested are collected
over a period of time in a capacitor, and then the accumulated energy is used by the
sensor and wireless transmission device since they work once in every few hours.
There have been numerous studies done on natural wind energy harvesting
techniques using various energy transduction methods. This study aims on providing
research data on flutter-based wind energy harvesting technique including
mathematical model for design optimization, which can be later optimized for a
specific application.
Windbelt microgenerator using embedded magnets suffer from a decrease in
the flux linkage between the magnets and the coil at high wind speeds. Thereby,
reducing the efficiency of the microgenerator (Fei & Li, 2009). In addition, the
windbelt microgenerator is designed for a specific natural frequency which is decided
based on the most likely wind speed, whereas in real time the wind speed may vary
2
widely. Therefore, the windbelt microgenerator will be able to harvest wind energy
practically only within a particular narrow range of wind speeds. At wind speeds other
than the design speed, the output signal generated by the microgenerator is
considerably lesser than its design output.
1.3 RESEARCH OBJECTIVES
1.
To develop a multiphysics mathematical model for non turbine wind
harvesting
microgenerator
involving
aeroelastic
flutter
and
electromagnetic transduction.
2.
To design a non turbine wind harvesting microgenerator.
3.
To experimentally investigate the behavior of non turbine wind harvesting
microgenerator at varied span, width and incident wind speeds .
1.4 RESEARCH METHODOLOGY
This research is mainly focused on developing and evaluating a new method of wind
energy harvesting viable for a variety of applications. A comprehensive study was
carried out in analytical, numerical and experimental stages of design and evaluation.
A number of parameters were changed during the study and its effects are studied and
discussed. Numerical simulations using FEA (Finite Element Analysis) through a
commercial software Nastran were carried out as an evaluation for mathematical
model proposed.
The following list contains the key steps of this study.

Literature survey and review

Mathematical modelling of the non-turbine wind harvesting microgenerator
o Time domain analysis of flutter-based harvester
3
o Frequency estimation of flutter-based harvester

FEA analysis of the proposed design using Nastran

Experimental construction and testing of microgenerator at various parameters.
1.5 SCOPE OF RESEARCH
This research develops a non-turbine wind harvesting microgenerator which aims to
harvest energy from wind sources mainly in environmental wind speeds. Although
tests are conducted at higher wind speeds for evaluation and future design.
Characterization of the proposed microgenerator is performed to conduct a detailed
study which may be useful in selecting application or environment specific parameters
during‎ implementation.‎ Also,‎ a‎ demonstration‎ of‎ microgenerator’s‎ real‎ time‎
performance is carried out with suitable circuits and loads.
1.6 THESIS ORGANIZATION
The first chapter is an introduction to the thesis wherein objectives, problem
statement, research philosophy, research methodology and scope of the study has been
described. The second chapter contains the literature review discussing various
methods of energy harvesting and wind energy harvesting in particular. The third
chapter explores the analytical modeling of flutter-based wind microgenerators and
fourth chapter contains numerical analysis performed using Nastran. The fifth chapter
presents the experimental setup and results. Conclusion and recommendations are
presented in the sixth chapter.
4
2. CHAPTER TWO
LITERATURE REVIEW
2.1 INTRODUCTION ON ENERGY HARVESTING
Energy harvesting which is performed to support remote application sensors mainly
harvest energy from available sources of energy but in unusable form i.e., in electric
voltage. These abundantly available sources of energy are converted into useful
electrical energy with a proper transduction mechanism for storage and then use. This
chapter describes the various energy sources both natural and artificial available
around us, which can be harvested. Although the energy available at source is not
large, it is still considered in many of the applications as a promising energy source
because this small amount of energy is enough for harvesting to support a miniature
remote monitoring application or its like.
The energy harvesting is usually performed by converting the source energy
into electrical voltage through a variety of steps. These steps convert the energy in one
form to another which finally leads it to electrical form. Energy harvesting also
involves the use of power conditioning and power storage circuits which usually
support the remote device completely as a power source.
2.2 SOURCES FOR ENERGY HARVESTING
The energy harvesting sources can mainly be broadly classified at the root level into
two, natural and artificial. The natural sources are simply those which are available in
the environment without any human interference. These are always driven by Mother
5
Nature’s‎powers.‎The‎two‎main‎and‎large‎sources‎of‎energy‎provided‎by‎the‎nature‎in‎
many or most of the environment are radiation and wind sources. Solar radiations and
wind breezes are the two forms of energy provided by these sources respectively.
These sources depend on the concentration per area, radiations from the sun are
usually classified by its energy density or W/m2 and it also depends on the location.
Few locations have a very good exposure to sun during summer and do not have
enough exposure during the winter. Also sun exposure is only during the day, which
means the energy storage should be high enough to support the remote application for
the night. Wind breezes are affected by this, but the microgenerators are provided with
wind speeds of varying magnitude all throughout the day and every day a year.
On the other side, the artificial sources are further classified into pressurized
fluid, magnetic, noise, thermal, vibration and radiation sources. These artificial
sources are man-made and mostly found in a majority of applications.
These
pressurized fluid sources are often created using fans, blowers and compressors. In an
application where long lines of pneumatic lines or air ducts are running,
microgenerators nearby can support themselves with this as an energy sources. This
kind of application is also seen in air conditioning ducts where remote sensors are
placed to monitor air quality and flow rate.
Magnetic sources are available nearly everywhere today, majority of electronic
components and home appliances have a magnetic influence to its neighbouring. In
places where electromechanical devices are installed, a higher magnetic field is
expected. For instance, an electric motor has a varying pole magnetic field around it,
this can been harvested into usable electric energy.
Acoustic energy or noise is available everywhere, it is of high intensity and
repetitive nature from artificial sources, although natural noise sources are present.
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Noise from trains, bus and other locomotives are few of the major sources of noise.
Noise harvesting and reduction technique has been demonstrated suitable for trains
(Kralov, Terzieva, & Ignatov, 2011).
Thermal sources which radiate heat are also considered as a promising source
of energy for remote harvesting. Heat is radiated from a majority of consumer
applications, including computers, home appliances, cars and as large as industrial
machines and chimneys. This heat which is usually wasted or lost into the
environment can be converted using suitable methods into useful energy.
Energy in the form of vibration is available everywhere similar to thermal and
acoustic sources. This vibration varies in its amplitude and frequency from one source
to another. Few sources like electric motors or other industrial machineries vibrate at a
very high frequency and small amplitude, and other sources like buildings and bridges
vibrate with small amplitudes and low frequency. These vibration sources are
promising due to their availability as an energy source for remote applications. These
vibration sources which are usually undesirable are dampened with dampers or other
energy dissipating devices to reduce. This energy which is lost can be harvested into
usable energy.
With the advent of communication devices, the radio frequency spectrum has
covered the entire globe with high concentration in the city areas. This radio
frequency energy is present everywhere around us. These sources with little
modification can also act as wireless energy providing techniques.
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2.3 ENERGY HARVESTING TECHNIQUES
2.3.1
Photonic Energy Harvesting
This is a method of converting solar radiation into direct current electricity using
photovoltaics. Photovoltaics use semiconductors which exhibit photovoltaic effect. It
is often electrically connected in multiples as solar photovoltaic arrays to convert
energy from the sun into electricity. Photons from sunlight knock electrons into a
higher state of energy, creating electricity. All photovoltaic devices are a derivate of
photodiode. Radiations from the sun can be harvested using Photovoltaic cells. A thin
film solar cell to power microelectromechanical systems (MEMS) electrostatic
actuator was developed(Lee, Chen, Allen, Rohatgi, & Arya, 1995). The array
contained 100 single cells connected in series with total area of only 1 cm2. Under
incandescent lighting situations, an area of 1 cm 2 produced‎around‎60‎μW‎of‎power‎
(Van der Woerd, 1998). Energy harvesting from light using biomedical
microelectromechanical systems (Bio-MEMS) is also being carried out. Altering the
dendrimer structure yields precise placement of chromophores that can serve as
energy harvesters, mimicking photosynthesis.
2.3.2
Magnetic Energy Harvesting
Magnetic energy harvesting is carried out by making use of flux leakage from
magnetic applications, such as an electric motor. The alternating magnetic field in a
three phase induction motor is used as the energy source. Here, a ferritic core with
windings is designed to harvest magnetic flux around a 3 phase induction motor (Xue
& Argueta, 2007). It was noted that different sizes and the materials for the core are
parameters that demonstrate significant results. Also, the winding pattern and the
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number of turns are characteristics that contribute to the amount of energy generated
from the magnetic flux leakage of the motor.
2.3.3
Acoustic Energy Harvesting
An acoustic energy harvester in demonstrated, this acoustic energy harvester uses a
circular, piezoelectrically active diaphragm to covert acoustical energy to mechanical
energy and then mechanical energy to electric energy. In that case, it is a
micromachined piezoelectric diaphragm. An output of 0.34 W/ cm2 were achieved
for an acoustic input of 149 dB. (Horowitz, Sheplak, Cattafesta, & Nishida, 2006)
2.3.4
Pyroelectric Energy Harvesting
Pyroelectric scavenger and thermoelectric scavenger are used to harvest energy from
thermal sources. Pyroelectric materials demonstrate a phenomenon of generating
voltage a temporary voltage when they are heated or cooled. The position of atoms are
modified within the crystal structure due to the change in temperature and it changes
the polarization of the material. This polarization change induces a voltage potential
across the material / crystal. A pyroelectric device can reach efficiency up to 50% of
Carnot efficiency. Power peaks up to 0.2 mW cm−3 were found and a mean power of 1
µW cm−3 on average was generated (Sebald et al., 2009).
2.3.5
Thermoelectric Energy Harvesting
Thermoelectric effect is the conversion of temperature differences
into electric
voltage or vice versa. This device creates a voltage when there is a different
temperature on either side. On the other side, a thermoelectric material creates a
temperature difference on either sides of it when an electric input is given.
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