comparative study for energy harvesting

INTERNATIONAL JOURNAL OF R&D IN ENGINEERING, SCIENCE AND MANAGEMENT

Vol.2, Issue 4, June 2015, Impact Factor-0.439, p.p.125-139, ISSN 2393-865X

Available at :www.rndpublications.com/journal Page 125 R&D Publications

A Comparisonal Study: Vibrational Energy Harvesting Techniques

Ashwani Kumar

Department of Mechanical Engineering, University Institute of Engineering & Technology

Maharshi Dayanand University, Rohtak, Haryana, India

ABSTRACT

___________________________________________________________________________________________________________

In our advance and high-tech world the issue of increasing electric demand and its fulfillment has attracted the focus of researchers. There

are various resources for the extraction of electricity, out of them main resources from which major demand of electricity is fulfilled are:

thermal power plants, hydro power plants and nuclear power plants. The whole world is looking towards the eco-friendly and never ending

energy resources like: solar energy, wind energy, geothermal energy, water energy, vibrational energy etc. to become independent from non-

renewable energy resources. Harvesting the electric energy from vibrating systems, is a significant step in this direction. This paper puts light

on the basic three vibrational energy harvesting techniques: piezoelectric, electrostatic and electromagnetic harvesters.

Keywords: Energy harvesting, piezoelectric materials, electrostatic materials, electromagnetic materials

__________________________________________________________________________________

1. INTRODUCTION

We all are surrounded by vibrating bodies/systems like: vehicles, industrial and household machines,

small electronic devices such as phone, music systems, wrist watch even our body is also produces

vibrations in different ways: heart beats, blood circulatory, walking movements. So we can understand the

universal availability of vibrations in our surrounding atmosphere, hence the energy harvesting from

vibrations can be a better option to produce electricity. Due to increasing demand of low power portable

electronic devices (MEMS) and requirement of micro level energy harvesters, researchers have focused on

the vibrational energy harvesting techniques.

Energy Harvesting, Power Harvesting or Energy Scavenging is the process of converting ambient energy,

which would otherwise go waste or lost, into usable electrical energy (electrical power). Electrical energy

thus extracted can be stored either for later use into the batteries or can be used instantaneously through an

efficient circuit. Energy harvesters can be broadly classified into two categories: macro level and micro

level energy harvesters, solar and wind energy harvesters comes under macro level energy harvesting

systems while the field of micro level energy harvesting is still not deeply touched by the researchers that

is vibrational energy harvesting. As the portability in electronics is growing size of electronic devices is

trying to be minimized(portable mobile phones, ear phones, Bluetooth phones, music systems, pacemakers

etc.) and thus their power requirement is also decreasing. Vibrational energy harvesting systems can be a

better power supply option for these low powered electronic devices. In this paper we have studied all

three basic techniques of vibrational energy harvesting techniques, their fundamental principles of

operation, their applications, their inter-comparison to choose the best one for energy harvesting

applications from human foot steps.

Figure(1) shows that energy harvesting from ambient energy sources can be used as an alternative for

micropowering.


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Figure: 1 Energy harvesting as an alternative for micropowering

In the last few years, the topic of energy harvesting or energy scavenging from ambient mechanical

vibrations has attracted the attention of researchers. Many techniques have been analyzed by research

groups to find out their capabilities to harvest electric energy from vibrations. There are three basic

techniques which can be used to convert vibrations into electric energy: piezoelectric, electrostatic and

electromagnetic generators. In electrostatic generator there are two conductors separated by a dielectric

which vibrate relative to each other behaving like a capacitor. In an electromagnetic-energy harvester a

coil attached to an oscillating mass traverses a magnetic field that is established by a stationary magnet.

The coil travels through a varying amount of magnetic flux, inducing an AC voltage according to

Faraday's law. Piezoelectric generators have gained most attention out of these three techniques because

of their ability to directly convert applied strain energy into electrical energy due to their crystalline

molecular structure which exhibits a local charge separation known as electric dipole. When the material

is strained deformation of dipoles takes place as a result of which en electric charge is produced with in

the materials that can be removed through a rectifying circuit and can either be used to power portable

electronic devices or can be used to recharge the batteries for later use. In this paper we will discuss all

these three techniques in detail and compare their utilities in different formats. Figure(2) shows different

vibration-energy harvesting techniques.

Figure: 2 Vibrational energy harvesting techniques


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2. ENERGY HARVESTING VIA PIEZOELECTRIC HARVESTER

In the last few years, there has been an increasing demand for low-power and portable-energy sources due

to the development and mass consumption of portable electronic devices. Furthermore, the portable-

energy sources must be associated with environmental issues and imposed regulations. These demands

support research in the areas of portable-energy generation methods. In this scope, piezoelectric materials

become a strong candidate for energy generation and storage in future applications. In this section we will

discuss the energy generation using nano technology by means of piezoelectric materials to convert the

mechanical energy into the electric energy.

The electric power generation using the piezoelectric element is one of the techniques utilizing the

piezoelectric effect. The wasted energy in the natural phenomenon, e.g., wind and tidal energy, and the

generated vibration on the structure such as a bridge, can be reutilized by this energy harvesting technique.

Several groups have been investigating the energy harvesting techniques using the piezoelectric material

PZT ceramics are suitable for the energy harvesting system since the conversion efficiency from the

mechanical to the electrical energy is governed by the piezoelectric constants d and g and the PZT

ceramics have high piezoelectric constants and quality factor. . Ashwani et al.(2014) used the direct

piezoelectric effect to harvest the energy from cantilever beam and inverse effects to control the vibrations

of the plate through making FEM model in ANSYS and using Block Lanczos solver to find out the

optimal placement of different shapes actuator patches in the region of maximum strain[1].

2.1 Principle of Operation

Piezoelectric materials are those which have the ability to produce an electric charge when a mechanical

stress is applied (the material is sretched or compressed) and conversely they get strained when an electric

field is applied on them. The former phenomena is called direct effect while the second one is called

inverse effect. Figure(3) shows the phenomena of charge separation in piezoelectric materials.

Figure: 3 The piezoelectric effect causes crystal materials like quartz to generate an electric charge

when crystal material is compressed, twisted or pilled. The reverse also is true, as the crystal

material compresses or expands when an electric voltage is applied


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Pierre and Jacques Curie were the first to discover the piezoelectric effect in 1880 by measuring surface

charges which were demonstrated on specially arranged crystalline salts of naturally occurring materials

such as cane sugar, Rochelle salt and quartz. The term piezoelectricity was subsequently coined by the German mathematician Hermann Hankel, coming from the Greek word piezen which means to press. Figure(4) shows the piezoelectric effect in a cylindrical body under different loading conditions.

Figure: 4 Piezoelectric effect in a cylindrical body (a) no load, (b) compressed, (c) stretched,

(d) shorten, (e) lengthen, (f) grow and shrink. Shah, A. A. (2011) [2].

To produce piezoelectric effect a poly-crystal is heated above the curie temperature under the influence of

a strong electric field. The raised temperature inside the crystal speeds up the random movement of its

molecules while the applied electric field causes to align the dipoles in the same direction (shown in

figure:5).

Figure: 5 Alignment of dipoles in a piezoelectric material

2.2 Piezoelectric Materials (Material with Piezo properties)

Piezoelectric materials can be broadly classified into three categories: 1. Naturally occurring crystals:

Berlinite (AlPO4), Cane sugar, Quartz, Rochelle salt, Topaz, Tourmaline Group Minerals, and dry bone

(apatite crystals). 2 Man-made ceramics: Barium titanate (BaTiO3), Lead titanate (PbTiO3), Lead

zirconate titanate (Pb[ZrxTi1-x]O3 0


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2.3 Modes of Operation and Governing Equations

Potentially different magnitude of stress, strain can be produced in the piezo material by applying the

electric field along its different surfaces. Hence to facilitate the ability to apply electric potential in three

directions, piezoelectric properties must contain a sign convention. To under this in a simple way

piezoelectric materials can be generalized into two cases, one is stack configuration which operates in the

-33 mode and another is bender configuration which operates in -31 mode (shown in figure:6).

Assumption of sign convention is that poling is always in 3 direction. In the -33 mode , electric field is applied in 3 direction and material is strained in poling or 3 direction. In -31 mode electric field is applied in 3 direction and material is strained in the 1 direction or perpendicular to the poling direction.

Figure: 6 The piezoelectric transduction modes Figure: 7 Direction of forces affecting

Kubba et al.(2014) [3] a piezoelectric element

Chopra (2002) explained the coupled electromechanical behavior of piezoelectric materials using

following two linearized constitutive equations [4]:

Direct piezoelectric effect:

iD = md

imjij dEe

Converse piezoelectric effect:

m

E

kmj

c

jkk SEd

where vector iD is the dielectric displacement in N/mV or C/m2 , k is the strain vector, jE is the applied

electric field vector in volts/meter, and m is stress vector in N/m2 . The piezoelectric constants are the

piezoelectric coefficients dimd and

c

jkd in m/V or C/N, the dielectric permittivity ije in N/V

2 or F/m, and

E

kmS is the elastic compliance matrix in m2 /N. The superscripts c and d refer to the converse and direct

effects, respectively, and the superscript and E indicate that the quantity is measured at constant stress and constant electric field, respectively.


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2.4 Energy Harvesting Circuitry

An input vibration applied on to a piezoelectric material as shown in Figure 3 causes mechanical

strain to develop in the device which is converted to electrical charge. Lead- zirconate-titanate (PZT) is a

commonly used piezoelectric material for power generation. The equivalent circuit of the

piezoelectric harvester can be represented as a mechanical spring mass system coupled to an

electrical domain as shown in figure( 8 ) . Here, LM represents the mechanical mass, CM the

mechanical stiffness and RM takes into account the mechanical losses. The mechanical domain is

coupled to the electrical domain through a transformer that converts strain to current. On the electrical

side, Cp represents the plate capacitance of the piezoelectric material. At or close to resonance, the

whole circuit can be transformed to the electrical domain, where the piezoelectric element when excited

by sinusoidal vibrations can be modelled as a sinusoidal current source in parallel with a capacitance Cp

and resistance Rp. One of the challenges in a power generator of this type is the design and construction

of an efficient power conversion circuit to harvest the energy from the PZT membrane. Another

unique characteristic of this power source is that it outputs relatively low output voltages for the low

levels of input vibration typically encountered in ambient conditions. This low output voltage makes

it challenging to develop rectifier circuits that are efficient since many diode rectifiers require non zero

turn-on voltage to operate.

Figure: 8 Equivalent circuit of a piezo-harvester. Dhingra et al.(2013) [5]

A piezoelectric harvester is usually represented electrically as a current source in parallel with a

capacitor and resistor. The current source provides current proportional to the input vibration

amplitude. The power output by the piezoelectric harvester is not in a form which is directly usable

by load circuits such as micro-controllers, radios etc. which the harvester powers. The voltage and

current output by the harvester needs to be conditioned and converted to a form usable by the load-

circuits as shown in the figure(9). The power conditioning and converting circuits should also be able to

extract the maximum power available out of the piezoelectric energy harvester. Commonly used analog

and digital circuits require a regulated supply voltage to operate from. Since the piezoelectric


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harvester outputs a sinusoidal current, it first needs to be rectified before it can be used to power circuits.

Figure: 9 Piezoelectric mechanical energy harvesting circuit

3. ENERGY HARVESTING VIA ELECTROSTATIC HARVESTER

Electrostatic devices: they use a variable capacitor structure to generate charges from a relative motion

between two plates.

This method depends on the variable capacitance of vibration-dependent varactors. A varactor, or

variable capacitor, which is initially charged, will separate its plates by vibrations; in this way,

mechanical energy is transformed into electrical energy. Constant voltage or constant current achieves

the conversion through two different mechanisms. For example, the voltage across a variable capacitor

is kept steady as its capacitance alters after a primary charge. As a result, the plates split and the

capacitance is reduced, until the charge is driven out of the device. The driven energy then can be

stored in an energy pool or used to charge a battery, generating the needed voltage source. The most

striking feature of this method is its IC-compatible nature, given that MEMS (Micro-electromechanical

system) variable capacitors are fabricated through relatively well-known silicon micro-machining

techniques. This scheme produces higher and more practical output voltage levels than the

electromagnetic method, with moderate power density.

3.1 Conversion Principle

Electrostatic converters are capacitive structures made of two plates separated by air, vacuum or any

dielectric materials. A relative movement between the two plates generates a capacitance variation

and then electric charges. These devices can be divided into two categories:

Electret-free electrostatic converters that use conversion cycles made of charges and discharges

of the capacitor (an active electronic circuit is then required to apply the charge cycle on the structure

and must be synchronized with the capacitance variation).

Electret-based electrostatic converters that use electrets, giving them the ability to directly convert

mechanical power into electricity. Figure: 10 shows the principle of operation of the electrostatic

transducer (constant charge type).


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Figure: 10 The electrostatic transducer, showing the charges on the electrodes +qand q, the electric field E, a constant voltage V and time varying voltage Vt. Mitcheson et al.(2008) [6]

Electret-based converters are electrostatic converters, and are therefore based on a capacitive structure made of two plates (electrode and counter-electrode (Figure 11). The electret induces charges on electrodes and counter-electrodes to respect Gausss law. Therefore, Qi, the charge on the electret is

equal to the sum of Q1 and Q2, where Q1 is the total amount of charges on the electrode and Q2 the total amount of charges on the counter-electrode (Qi=Q1+Q2). A relative movement of the

counter- electrode compared to the electret and the electrode induces a change in the capacitor geometry (e.g. the counter-electrode moves away from the electret, changing the air gap and then the electret's influence on the counter-electrode) and leads to a reorganization of charges between the electrode and the counter-electrode through load R (Figure 12). This results in a current circulation through R and one part of the mechanical energy (relative movement) is then turned into electricity.

Figure: 11 Electret based electrostatic conversion-Concept. Boisseau et al.(2012) [7]


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Figure:12 Figure: Electret based electrostatic conversion-Charge circulation. Boisseau et al.(2012)

[7]

3.2 Types of Electrostatic Vibration Energy Harvesters

Electrostatic energy harvesters are based on variable capacitors. There are two sets of electrodes in the

variable capacitor. One set of electrodes are fixed on the housing while the other set of electrodes are

attached to the inertial mass. Mechanical vibration drives the movable electrodes to move with respect

to the fixed electrodes, which changes the capacitance. The capacitance varies between maximum and

minimum value. If the charge on the capacitor is constrained, charge will move from the capacitor to a

storage device or to the load as the capacitance decreases. Thus, mechanical energy is converted to

electrical energy. Electrostatic energy harvesters can be classified into three types as shown in Fig. 13,

i.e. In-Plane Overlap which varies the overlap area between electrodes, In-Plane Gap Closing

which varies the gap between electrodes and Out-of-Plane Gap which varies the gap between two

large electrode plates.

Figure: 13 Three types of electrostatic energy harvesters (a) In-Plane Overlap (b) In-Plane Gap

Closing (c) Out-of-Plane Gap Closing. Zhu, D. (2011) [8]

3.3 Power Management Control Circuits (PMCC) Dedicated to Electrostatic VEH (eVEH)

Sometimes electrostatic vibration energy harvesters are characterized by a high output voltage that may

reach some hundreds of volts and a low output current (some 100nA). Obviously, it is impossible to

power any application, any electronic device with such a supply source. This is the reason why a power

converter and an energetic buffer are needed to develop autonomous sensors. Following figure presents


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the conversion chain.

Power Management Control Circuits (PMCC, Figure: 14) can have many functions: changing eVEH

resonant frequency, controlling measurement cycles.

Figure: 14 Power management control circuit to develop viable VEH, Boisseau et al.(2012) [7]

In conclusion we can say that electrostatic energy harvesters have high output voltage level and low

output current. As they have variable capacitor structures that are commonly used in MEMS devices,

it is easy to integrate electrostatic energy harvesters with MEMS fabrication process. However,

mechanical constraints are needed in electrostatic energy harvesting. External voltage source or pre-

charged electrets is also necessary. Furthermore, electrostatic energy harvesters also have high output

impedance.

4. ENERGY HARVESTING VIA ELECTROMAGNETIC HARVESTER

This technique uses a magnetic field to convert mechanical energy to electrical energy. A coil

attached to the oscillating mass is made to pass through a magnetic field, which is established by a

stationary magnet, to produce electric energy. The coil travels through a varying amount of magnetic

flux, inducing a voltage according to Faraday's law. The induced voltage is inherently small and

therefore must be increased to become a viable source of energy. Techniques to increase the induced

voltage include using a transformer, increasing the number of turns of the coil, or increasing the

permanent magnetic field . However, each of these parameters is limited by the size constraints of the

microchip as well as its material properties.

4.1 Working Principle

Electromagnetic induction is based on Faraday's Law which states that an electrical current will be induced in any closed circuit when the magnetic flux through a surface bounded by the conductor

changes. This applies whether the magnetic field changes in strength or the conductor is moved through it. In electromagnetic energy harvesters, permanent magnets are normally used to produce

strong magnetic field and coils are used as the conductor. Either the permanent magnet or the coil is

fixed to the frame while the other is attached to the inertial mass. In most cases, the coil is fixed while

the magnet is mobile as the coil is fragile compared to the magnet and static coil can increase lifetime

of the device. Ambient vibration results in the relative displacement between the magnet and the coil,

which generates electrical energy. According to the Faradays Law, the induced voltage, also known as electromotive force (e.m.f), is proportional to the strength of the magnetic field, the

velocity of the relative motion and the number of turns of the coil.


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4.2 Types of Electromagnetic Harvester

Generally, there are two types of electromagnetic energy harvesters in terms of the relative

displacement. In the first type as shown in Fig. 15(a), there is lateral movement between the magnet

and the coil. The magnetic field cut by the coil varies with the relative movement between the magnet

and the coil. In the second type as shown in Fig. 15(b), the magnet moves in and out of the coil. The

magnetic field cut by the coil varies with the distance between the coil and the magnet. In contrast, the

first type is more common as it is able to provide better electromagnetic coupling.

Figure: 15 Two types of electromagnetic energy harvester. Zhu, D. (2011) [8]

Electromagnetic energy harvesters have high output current level at the expense of low voltage. They

require no external voltage source and no mechanical constraints are needed. However, output of

electromagnetic energy harvesters rely largely on their size. It has been proven that performance of

electromagnetic energy harvesters reduce significantly in micro scale (Beeby et al., 2007a) [9].

Furthermore, due to the use of discrete permanent magnets, it is difficult to integrate electromagnetic

energy harvesters with MEMS fabrication process.

Figure (16) shows the comparison of normalized power density of some reported electromagnetic

vibration energy harvesters. It is clear that power density of macro-scaled electromagnetic vibration

energy harvesters is much higher than that of micro-scaled devices. This proves analytical results

presented by Beeby et al.(2007 a) [9].

Figure: 16 Comparison of normalized power density of some existing electromagnetic vibration

energy harvesters. Beeby et al.(2007 a) [9]


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5. Comparison Between Vibrational Energy Harvesting Techniques

In the last few years we have seen significant growth in the field of energy harvesting because of

ever-increasing desire to produce portable and wireless electronics with extended lifespans. Energy

harvesting from ambient energy sources for powering these portable electronic devices can

eliminate the need of conventional batteries. Ambient energy is omnipresent in our environment in

the form of solar and radio frequency radiation, thermal energy, energy from chemical and

biological sourcessuch as salinity gradient and blood sugar levels, respectivelyand mechanical energy from natural phenomena such as waves or from vibrations generated by man-

made activity. Ambient vibrations, in particular, were until recently considered of little interest as

an energy source due to their very low energy level. Past attempts to harvest them have focused on

physical energy conversion methods, such as micro-electro-mechanical systems, which do not

require fossil fuels. Today, the main challenge is to harvest enough energy to power electronic

devices. Since these devices are becoming smaller in size and consume less power, they may be

well suited for ambient vibration conversion systems for charging batteries or supplying power

directly. The integration of multidisciplinary research produced mechatronic harvesting systems:

fruits of the synergistic integration of mechanics, electronics, control theory, and computer science

within product design and manufacturing to improve functionality. These can be used as

autonomous source of electrical energy for remote or wireless applications powered by ambient

mechanical vibrations from machines, aircraft, ships, bridges, buildings, and so forth.

As we have discussed earlier in above sections there are mainly three methods of harvesting

energy from vibrations: piezoelectric, electrostatic, electromagnetic energy harvesters. These

have their own limitations, requirements, advantages, disadvantages and working conditions. It

is always not possible to implement any of these techniques for a particular application because

of their specific circuitry and input energy sources requirements. In this section we will

compare advantages and disadvantages of each of these three techniques and their capabilities

to harvest energy.

Roundy et al.(2003) performed a qualitative comparison study of three basic vibration energy

harvesting methods. They found in their analysis that a piezoelectric harvester can directly

convert vibrations into electrical energy without any need of additional voltage source but

electrostatic harvesters require a separate voltage source to operate. Electromagnetic harvester

dont require any separate voltage source to function but produce relatively low output voltages. They also noticed that it is more easy to integrate the electrostatic harvesters into

micro-systems than piezoelectric harvesters. After this analysis they performed the modeling

experimental testing of cantilever piezoelectric harvester and an electrostatic harvester and

found that piezoelectric harvester can generate more power per unit volume than electrostatic

harvester however electrostatics are more suited for integration into Microsystems[10].

Roundy(2005) continued his study of comparison of vibration energy harvesting techniques

conducted in 2003 and presented some more effective and practical conclusions. He stated that

selection of a particular energy harvesting technique depends upon the physical constraints of

the system and environmental conditions. He analyzed that piezoelectric harvesters produce

high voltage and low currents, also the current generation decreases in both electrostatic and

piezoelectric harvesters as the size of device decreases. While electromagnetic harvester


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produce low voltages and the voltage output decreases as the size of device decreases. He

observed some critical implementation issues regarding electrostatic generators because they

require oscillation level of hundred of microns while maintaining minimum capacitive gap

(about 0.5 m or less) and providing power at a level similar to other harvesters[11].

Sterken et al(2004) conducted the the comparative study to analyze the size constraints

imposed by the application for different vibrational energy harvesters in order to make it easy

for designers to select a particular energy harvesting method for a given application. They

compared the all three vibartional energy harvesting techniques through their mathematical

modeling and declared that all three techniques can be used under different conditions but they

are more effective under some particular domains. They made some conclusions on the behalf

of this comparative study like: in electrostatic generators small gap between the capacitors

should be maintained for their effective functioning hence their application is limited in small

size systems, while the application of electromagnetic generators is best suited for large

systems in which large size harvesters can be fitted. They also found that maximum power

generation capability of electrostatic generators was lesser than other two harvesters but they

can be used for any application without any size constraints[12].

Poulin et al(2004) compared analytically the energy harvesting capabilities of electromagnetic

and piezoelectric harvesters from human movement to power portable electronic devices.

Electromagnetic device under consideration was composed of coil containing a translating

magnet and the piezoelectric device was made from a cantilever piezoceramic bar. They

compared the typically sized devices of each type and concluded that maximum volumetric

power output of both devices was relatively close while the difference in their resonant

frequency was considerable. The resonant frequency of electromagnetic device was the order of

few hertz while it was on the order of few hundred kilohertz for piezoelectric device. Also a

considerable difference was seen in matching load resistance, it was in the k range fo r electromagnetic system and in the M range for piezoelectric system. Finally it was concluded that piezoelectric harvesters are more useful for micro-level power harvesting applications

because of their ability to yield high power density while electromagnetic harvester are best

suited for macro-level applications because of large size device requirements[13].

Niu et al.(2004) studied the electric power generation capabilities of electrostatic,

electromagnetic and piezoelectric harvesters from heel strike during human walking. They

found that electrostatic generators require additional voltage source to operate hence they are

not suited for heel strike energy harvesting applications but they can give better results than

electromagnetic generators in the case when small displacements are involved. It was found

that piezoelectric generator do not give significant power output during compression mode,

while in the bending mode the cantilever beam mounted in the heel was not capable of utilizing

the energy of heel strike excitation. Regarding electromagnetic devices it was observed that

they have low efficiency of energy conversion under low frequency vibrations produced by the

heel strike but the efficiency can be improved by converting heel strike excitation into

rotational excitation[14]. Maezencki(2005) showed a comparisonal view of vibration energy

harvesting techniques depending upon their reliability and feasibility study ( As shown in

Table:1)[15].


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Table: 1 Comparison of Vibration Energy Harvesting Techniques. Maezencki(2005) [15]

6. CONCLUSIONS

A vibration energy harvester is an energy harvesting device that couples a certain transduction

mechanism to ambient vibration and converts mechanical energy to electrical energy. Ambient

vibration includes machinery vibration, human movement and flow induced vibration. For energy

harvesting from machinery vibration, the most common solution is to design a linear generator that

converts kinetic energy to electrical energy using certain transduction mechanisms, such as

electromagnetic, piezoelectric and electrostatic transducers. Electromagnetic energy harvesters have

the highest power density among the three transducers. However, performance of electromagnetic

vibration energy harvesters reduces a lot in micro scale, which makes it not suitable for MEMS

applications. Piezoelectric energy harvesters have the similar power density to the electromagnetic

energy harvesters. They have simple structures, which makes them easy to fabricate. Electrostatic

energy harvesters have the lowest power density of the three, but they are compatible with MEMS

fabrication process and easy to be integrated to chip-level systems.

Electromagnetic energy harvesters dont require any additional voltage source to operate and have highest power density among three but these are more suited for macro-scale applications

not for micro-level applications like MEMS.

Electrostatic energy harvesters require a separate voltage source to operate which make it useless for powering independent portable electronic devices however it is easy to integrate

them into small size microsystems to produce significant amount of output power.

Piezoelectric energy harvesters have the capability to directly convert strain energy into electrical energy without any requirement of additional voltage sources and can be integrated

into any microsystem without any size issue and hence best suited for human powered

applications. They have the ability to yield a high power density to power portable electronic

devices.

In this way we have compared three different vibrational energy harvesting techniques in

different aspects. This study helps the designers and engineers to select a particular energy

harvesting technique for a particular application under different constraints.


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