A review of renewable power generation using piezoelectric materials

International Journal of Mechanical Engineering and Technology (IJMET)
Volume 10, Issue 12, December 2019, pp. 559-577, Article ID: IJMET_10_12_052
Available online at />ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication

A REVIEW OF RENEWABLE POWER
GENERATION USING PIEZOELECTRIC
MATERIALS
Ojo E. Olufisayo and Professor Freddie Inambao*
Mechanical Engineering,
University of KwaZulu-Natal, Durban, South Africa
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*Corresponding Author Email:
ABSTRACT
Piezoelectric materials are crystals that can convert energy associated with
mechanical deformation into electrical energy. This material is identified with higher
energy density and stronger electromechanical coupling properties than other
contemporary technologies. Vibration, which is a cheap and common source of
mechanical energy, is often encountered in our environments with good energy levels.
Therefore, the energy generated by the vibration of piezoelectric materials provides
an ideal energy solution for portable and wireless devices. This paper reviews and
summarizes recently published journal articles on piezoelectricity and vibration
energy harvesting using piezoelectric materials, covering key areas such as
piezoelectricity, piezoelectric materials, harvesting kinetic (mechanical vibration)
energy, piezoelectricity power generators, piezoelectricity modelling, charge collector
/ energy storage equipment. The emphasis is on renewable power generation using
piezoelectric materials.
Keywords: Renewable energy harvesting, mechanical vibration, piezoelectric
materials.
Cite this Article: Ojo E. Olufisayo and Freddie Inambao, A Review of Renewable
Power Generation Using Piezoelectric Materials. International Journal of Mechanical

Engineering and Technology 10(12), 2020, pp. 559-577.
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1. INTRODUCTION
In recent years, renewable energy sources have become popular in attempting to reduce global
warming. These energies are an alternative form of energy resource which can reduce the
consumption of traditional energy resources like fossil fuels. Typical renewable energy
resources are wind, tidal waves, geothermal heat, solar, and rain [28]. Piezoelectricity was
first discovered by Pierre and Jacques Curie in the 1880s [29]. They found that in
asymmetrical crystals possessing a polar axis, the effect of compression parallel to the polar
axis was to polarize the crystal, resulting in the generation of a positive charge on one side of
the material and a negative charge on the other side of the material. One of the innovative
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ways to accomplish this is through harvesting unexploited and wasted energies in our
environment. The concept of renewable energy generation from human surroundings has
stimulated renewed interest in piezoelectricity. This renewable energy source is a selfsustainable system for generating electricity without depleting natural resources. In the past
two decades there has been a strong interest in converting mechanical energy from human
motion to electrical energy. This power can then be used to charge the battery in an electronic
device or directly power a small, low-power circuit.
The three main transduction methods for conversion of vibrations to electrical energy for
self-powering devices are electromagnetic, electrostatic, and piezoelectric harvesters.

2. PIEZOELECTRICITY
According to [1], piezoelectricity can be defined as the property of some dielectric materials

that have developed a polarization as a result of being subjected to mechanical strain
deformation. This mechanical property (tension) in their view was produced by the liking
force realignment of atoms and in so doing, producing polarization and thus an electric field.
If this is reversed and a piezoelectric material not being compressed is exposed to an
electric field, it will create a mechanical deformation in its lattice structure. Considering the
two applications, the reversal in the course of application of force, either by mechanical or
electrical means, will generate a reversal in the direction of the residual effect. [2] explains
that the piezoelectric effect on piezoelectric material occurs in two forms: the first is the direct
piezoelectric effect that describes the material‟s capacity to convert mechanical strain into
electrical charge; the second is the converse effect, which is the capacity of a material to
convert an applied electrical potential into mechanical strain energy (deformation). The direct
piezoelectric effect is responsible for the material‟s ability to function as a sensor and the
converse piezoelectric effect is responsible for its ability to function as an actuator. The
authors submitted that a material is deemed piezoelectric when it has this ability to convert
electrical energy into mechanical strain energy, and likewise to transform mechanical strain
energy into electrical charge. According to [3], renewable energy which is harnessed from a
natural source has evolved as the power source of the future due to diminishing fossil fuel and
nuclear power sector volatility. The authors state that renewable energy harvesting plants that
can generate power at the kW or mW level are classified as macro energy harvesting
technology. Micro energy harvesting technology is centered on mechanical vibration such as:
mechanical stress and strain; thermal energy from furnaces, heaters and friction sources;
sunlight or room light; the human body; chemical or biological sources which generate mW
or µW levels of power. Micro energy harvesting is is an alternative to conventional macro
renewable energy.
Wireline Sensor Network systems (WSNs) and Micro Electromechanical Systems
(MEMs) represent pressure electricity which is attributed to the properties of certain
crystalline materials such as quartz, rochelle salt, tourmaline and barium titanate which
develops electricity when subjected to pressure, a term known as the direct effect. These same
crystals can undergo deformation when subjected to an electric field, known as the converse
effect. The converse effect of these materials can be used as an actuator while the direct effect

can be used as a sensor or energy transducer.
[4] listed the benefits, advantages and applications of piezoelectricity as a proven and
viable form of renewable energy as being:
 Long lasting operability
 No chemical disposal
 Cost saving
 Safety

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 Maintenance free
 No charging points
 Inaccessible site operability
 Flexibility
 Applications otherwise impossible
Applications include:
 Environmental monitoring
 Habitat monitoring (light, temperature, humidity)
 Integrated biology
 Structural monitoring
 Interactive and control
 RFID, real time locator, TAGS
 Building, automation
 Transport tracking, car sensors

 Surveillance
 Pursuer-evader
 Intrusion detection
 Interactive museum exhibits
 Medical remote sensing
 Emergency medical response
 Monitoring, pacemaker, defibrillators
 Military and aerospace applications

3. HARVESTING KINECTIC (MECHANICAL) VIBRATION ENERGY
[5] defined piezoelectric energy harvesting as the conversion of energy absorbed by a
transducer from the environment to usable electric voltage that can be either used immediately
for actuation or otherwise stored in batteries for future or later usage. The world is
dynamically moving from the era of electrical equipment to electronic devices due to the
energy crisis; the crisis is leading to a wanton voluntary reduction in global electrical power
consumption, resulting in the evolution of micro and nano powered electronic circuits. [3]
established that piezoelectric energy harvesters are viable substitutions for the conventional
battery. The authors‟ explained that ultra-low power portable electronics and wireless sensors,
which hitherto have been using conventional batteries as their power sources, have proven to
be unreliable as the life of these batteries are limited and very short compared to the working
life of the devices themselves. Therefore, the replacement or recharging of these batteries is
cumbersome and sometimes absolutely impossible. Consequent upon this, many researches
have been instituted to study energy harvesting technology as a self-dependent power source
for portable devices or wireless sensor network systems. The authors‟ view on energy
conversion recognized that human beings have already been using energy harvesting
technology from time immemorial in the form of windmills, watermills, geothermal and solar
energy which has helped in the research and development of micro energy harvesting
technology.
According to [6], the most extensively deployed method of harvesting mechanical energy
is piezoelectric energy conversion which utilizes the piezoelectric effect to convert timedependent mechanical deformations into electricity. The authors listed other methods for

direct mechanical to electrical energy conversion including electromagnetic, electrostatic, and
electroactive polymer generators. Harvesting these wasted energies (thermal and mechanical)
would contribute to more efficient and sustainable energy consumption. This wasted

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mechanical energy which is a by-product of objects in motion usually exists in the form of
vibrations, shocks or strains. Sources of this wasted mechanical energy include fluid flow,
household appliances, industrial equipment, motor vehicles and structures such as buildings
and bridges.
According to [7], energy harvesting or scavenging is the process of seizing and
resourcefully utilizing wasted energy from naturally occurring energy sources, accumulating
and storing it for immediate or future use. Essentially, it is the conversion of ambient energy
that exists in our environment into electrical energy.
Harvesting kinetic energies is a sustainable method for producing electricity without
depleting valued natural resources. The main methods employed in kinetic energy harvesting
are piezoelectric, electromagnetic, electrostatic or by using magnetostrictive materials.
Attention has been focused on harvesting of walking energy as one of the easiest and most
reliable means of vibration energy harvesting, and then to explore and compare different
technologies used for converting same to electricity, and identifying the most effective
technology to be used in generating it. There are numerous types of harvester that can be
positioned on a user‟s body to harvest kinetic energy during walking. Furthermore, some
pavement slabs have been designed and produced for harvesting energy; these slabs with
inbuilt harvesters are more dependable than body-located technologies for piezoelectricity

harvesting, since they are independent of physiological parameters. Piezoelectric transduction
remains the most desirable and commonly accepted micro energy harvesting technology due
to its advantageous stance which includes simplicity, flexibility and availability despite
producing less output current than electromagnetic transduction.
Energy harvesting is one of the most favorable techniques that can be deployed to tackle
ever increasing global energy demand problems without adverse residual effects on the
environment. Piezoelectric energy harvesting at the moment typically connotes micro- to
milli-watts small power generation systems that have been developed as viable replacements
for battery stored power. It explores kinetic, thermal, solar or electromagnetic radiation
sources to generate movement mainly in the form of vibrations which are later converted into
electrical energy. An energy harvester basically has three main components; the microgenerator for converting ambient energy into electrical energy, the voltage booster to pump up
and regulate the generator voltage, and the storage element. Since vibration powered
generators are mainly resonant systems, maximum power is produced when the resonant
frequency of the generators equals the ambient vibration frequency of the piezoelectric
material. Available are different types of vibration energy that can be harvested which are
being studied by several researchers, including: human motion, ocean waves, harvesting strain
from beam elements in critical structures etc. Energy harvesting in the form of mechanical
loading generated from the ground in the shape of compressive forces while people move
across the floor, is a typical example of a sustainable method to generate electrical energy. [8]
pointed out that the performance of a piezoelectric energy harvester principally hinges on the
piezoelectric properties of the material used to construct the generators.
Usually, thin film piezoelectric materials display improved piezoelectric properties
compared to bulky piezoelectric materials. The usage of single crystals and nanomaterials
(nanowires) has enhanced the power density and energy conversion efficiency which has
resulted in improvement in the miniaturization of the device size while upholding a
reasonable power output. In spite of prodigious research efforts on these nanomaterials, there
is still a deficiency in the basic scientific understanding of and experimental research on
piezoelectric and flexoelectric effects in single crystalline nanowires. This gap in research at
this fundamental level compromises fidelity of the mathematical algorithms deployed in

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modeling and forecasting the piezoelectric potential mechanical to electrical energy
conversion efficiency and device material optimization.
A related task is research on the combination of piezoelectric and semiconducting effect
which brings about the so-called piezotronic effect. The scientific theory of the interaction
between electron distribution and semiconductor band structures still call for additional
research efforts. While single crystal materials are believed to offer a better piezoelectric
performance and offer enhanced power density compared to their bulk material counterparts,
the costs of these materials remain exorbitantly high and occasionally hyper-inhibitive. The
existing construction methods of these generators and the associated device incorporation
techniques at nano scale are not yet suitable for large scale processing, and research efforts in
this regard will substantively lessen fabrication costs and assist in transforming piezoelectric
energy harvesting from ordinary experimental curiosity into genuine engineered device
achievement to power wireless sensors.

4. PIEZOELECTRIC MATERIALS
According to [2], piezoelectric materials can play a pivotal role as mechanisms for energy
harvesting, as they possess the ability to absorb energy from the environment and convert it to
electrical energy that can then be deployed to drive electronic devices directly or indirectly.
Piezoelectric materials belong to a wider class of materials called ferroelectrics. One of the
essential behaviors of a ferroelectric material is that its molecular structure is arranged in such
a manner that the material exhibits a local charge separation, a situation known as an electric
dipole. All the way through the material configuration, the electric dipoles are arranged

randomly and upon subjecting the material to heat energy above a particular point (the Curie
temperature) and subject to an extreme electric field, the electric dipoles will re-orientate
themselves relative to the applied electric field. This process is known as poling. As soon as
the material is cooled, the dipoles will maintain their new orientation and the material at that
moment is said to be poled. Subsequent to the completion of the poling process, the material
will then exhibit the piezoelectric effect. [9] classified piezoelectric materials used in
piezoelectricity energy generation into four basic groups namely: ceramics, composites,
polymers and monocrystals. Most experimental research conducted has found ceramics to be
most commonly used, followed by composites, polymers and monocrystals.
Ceramic are the most commonly used materials in piezoelectric generator design. PZT
cerama is a form of ceramic used in generators. PZT is a mixed crystal of titanate and lead
zirconate with the general formula (x)PbTiO3-(1-x)PbZrO3. The properties of piezoelectric
ceramic PZT can be manipulated by modification of the percentage content of individual
compounds comprising mixed crystal, x = (0 – 1). This enables the production of ceramics
PZT with a diverse set of material constants. Many PZT variations are used in generators with
no single ceramic PZT dedicated to generator design e.g. PZT-5H, PZT-5A, PZT-PIC255,
PZT-APC 841, PZT-APC 850, PZT-PPK11.
Composites are materials made of piezoelectric materials of diverse shapes, polymer film,
layers of adhesive and properly formed electrodes. On parts of the composite layers that are
fastened together by bolts are electrodes that properly fitted. There are two types of
composites that can be applied in generator design: PFC (piezoelectric fiber composite) and
MFC (macro fiber composite). PFC is a composite consisting of circular piezoceramic fibers
located in the layer of adhesive and on bolt-on parts of the polyimide film and electrodes.
MFC manufactured by Smart Materials Corp. are primarily made of rectangular piezoceramic
bars, distinguished by adhesive layers, polyimide film and an electrode on the bolt-on part.
Polymer materials are chemical substances comprising numerous component parts,
prominent among polymers is polyvinylidene fluoride (PVDF). PVDF is a semi-crystal,

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comprising a maximum of 50 % to 60 % of the crystal phase. Piezoelectric properties of
PVDF were discovered in the 1960s. PVDF is generally applied as a foil so that it can be
easily formed and shaped, compared to ceramics which are not.
Monocrystal were invented a few years ago and are presently the most auspicious
piezoelectric material in the piezoelectric generator field of research, as it is the most effective
energy conversion material available. There are two types of monocrystal materials that can
be used for generators: PZN-PT (Pb(Zn1/3Nb2/3)O3-PbTiO3) and PMN-PT ((Mg1/3
Nb2/3)O3-PbTiO3). It has been generally observed that PZN-PT and PMN-PT monocrystals
have the highest efficiency when converting energy. Composites or PVDF polymers are
mostly used in research projects with ceramic generators. Ceramic materials have been found
to be very effective in energy conversion, which is duly reflected in the high rates of
electromechanical coupling coefficient. However, they can be very fragile and are susceptible
to more wear as a result of fatigue compared to composites and polymers.
[10] investigated piezoelectric ceramics with a microstructure texture experimentally
prepared by tape casting of slurries containing a template SrTiO3 (STO) under external
mechanical stress. They established that STO-added specimens indicated excellent power
compared to the STO-free specimen when a large stress was applied to the specimen.
[11] analyzed aluminum nitride (AIN) as a piezoelectric material for piezoelectric energy
harvesters because of their high resulting voltage level. They reported a maximum output
power of 60 µW for an unpackaged device at an acceleration of 2.0 g and at a resonance
frequency of 572 Hz.
[12] analyzed a piezocomposite composed of layers of carbon/epoxy, PZT ceramic and
glass/epoxy to harvest energy. They reported that piezocomposites have the potential to
harvest energy subjected to vibration after numerical and experimental validation.

5. PIEZOELECTRICTY POWER GENERATOR
[2] defined a power generator or harvesting as the process of obtaining the energy
surrounding a system and transforming it into functional electrical energy. Modern
development in wireless technology and low-power electronics such as
microelectromechanical systems have brought about renewed efforts in piezoelectricity
research. This development has led to the usage of piezoelectric materials which exploit the
ambient vibrations of the surrounding system as a tool for power harvesting. Piezoelectric
materials are made of crystalline structures that enable them to convert mechanical strain
energy into electrical charge, and also convert subjected electrical potential to mechanical
strain. This property offers these materials the capacity to absorb mechanical energy from
their surroundings, usually in the form of ambient vibration, and convert it to electrical energy
that can power other devices. While the use of piezoelectric material is the foremost method
of generating/harvesting this energy, other methods exist, one of which is using conservative
methods of electromagnetic transduction.
[13] conducted an investigation relating to the rudiments of a generator that converts
mechanical energy to electrical energy utilizing a piezoelectric vibrator and a steel ball. The
effect of the various characteristics of the piezoelectric vibrator was also investigated. In
simulating the generation mechanism, an electrical equivalent model was introduced. The
essential modes of bending vibration for two models were calculated: Model A (the
transducer with the steel ball) and Model B (the transducer only). The admittance
characteristics of each model were measured and it was confirmed that the peak frequencies
of the system equals the vibration modes. It was also confirmed that the calculated waveform
of the output voltage was similar to the measured one, therefore confirming the model to have
provided an accurate simulation of the output voltage. An efficiency curve of the model was
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A Review of Renewable Power Generation Using Piezoelectric Materials

drawn for different input mechanical energies, and it was determined that as the potential
energy of the ball increased, the maximum efficiency decreased. A larger chunk of the applied
energy was reverted back to the steel ball in the form of kinetic energy thereby triggering
bouncing off of the plate. The conclusion thereafter was that the energy generated would be
large if the steel ball had vibrated with the piezoelectric plate instead of bouncing off after the
impact. This scenario was simulated and it was resolved that a maximum efficiency of 52%
could be achieved. The characteristic effects of the piezoelectric vibrator were studied and it
was concluded that the system efficiency increased as the mechanical quality factor and the
electromechanical coupling coefficient increased while the dielectric loss decreased.
[14] analyzed the efficiency of stacked piezoelectric material in terms of electric power
generation. An analytical model was considered and it was recommended that the basic
challenge of generating electrical energy from piezoelectric material is that most of the
generated energy in the system is held or stored and subsequently returned to the excitation
source that originally triggered the charge to be generated. This occurrence may be
specifically problematic when a piezoceramic is positioned parallel to a capacitor in series
with the applied load. It was therefore recommended that the maximum efficiency of power
generation can be realized by reducing the total amount of energy stored in the piezoelectric
material. The efficiency of this model was investigated across a spectrum of frequencies and
resistive values, and it was concluded that at frequencies above 100 Hz, the efficiency of the
stack actuator was insignificant and that the maximum efficiency occurred at 5 Hz. This
obtained frequency is far lower when compared to the first mechanical and electromechanical
resonances of the stack, which occurred at approximate frequencies of 40 kHz and 60 kHz,
respectively. It was also recorded that the frequency of the maximum efficiency was low due
to the relative energetic structure of the stack. Also recorded was that the stack efficiency
intensely depended on the excitation frequency, while the load resistance had a low effect on
it.
According to [15] the piezoelectric transducer, which is a key component of a generator,

is designed to generate electric voltage in response to thermal, electrical, mechanical and
electromagnetic input. The authors concentrated on energy generation for low-powered
circuits with a PZT energy harvester. Nano and micro watts of power can be produced from
PZT harvesters by applying mechanical, thermal, electrical, light and fluid input. Mechanical
is considered to be the most efficient input compared to other input options, because it is
readily available and can be tapped easily from the environment. The conversion of this
mechanical energy from waste vibrations into electrical energy can be achieved through
electromagnetic, piezoelectric, or electrostatic transduction mechanisms. The piezoelectric
transduction generator is the most effective and efficient mechanism for microelectronics,
wireless sensors, and nano electronics because it can be easily fabricated and one is able to
harvest or generate energy at variable frequencies. This theory was first discovered by Pierre
and Jacques Curie in 1880 as having a direct effect, that is conversion of mechanical energy to
electrical energy, as expressed in Equation (1) and a converse effect i.e., the conversion of
electrical energy to mechanical energy, as expressed in Equation (2).
Di = e0ij Ej + ddim Ơm

(1)

Ek = dcjk Ej + SEkm Ơm
(2)
where Di is the dielectric displacement vector, Ek is the strain vector, Ej is the applied
electric field vector, Ơm is the stress vector, ddim and dcjk are piezoelectric coefficients for
direct and converse effects of piezoelectricity respectively, e0ij is the dielectric permittivity at
constant stress, and SEkm is the elastic compliance matrix at constant electric field.

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A typical PEH is made up of three key components:
 The piezoelectric patch. This is responsible for the converting of environmental input
(e.g. vibration, fluid structure interaction (FSI), biomechanical, etc.) into alternating
current.
 The storage unit. This is usually a super capacitor or a battery which is responsible for
the storage of the charge generated by the PEH;
 The modulating circuit. This is responsible for conversion of AC into DC.
The storage unit may be ignored in order to use generated energy directly from the PEH.
[16] analyzed further that the power generation ability of piezoelectric energy harvesters
(PEH) does not only relate to the piezoelectric material properties, the vibration magnitude
and the succeeding conditioning circuit, but also to the fixation modes and the prescribed
adjustment methods. A commercially inclined piezoelectric ceramic plate (PCP) in simply
supported beam fixation mode and cantilever beam fixation mode were both evaluated using
finite element simulations and comprehensive experiments. The two methods of adjusting the
natural frequency of PCP were evaluated and compared, and as a result, some procedures
were suggested for the application of PCPs according to the simulation and experimental
results which revealed that:


The simply supported beam fixation mode is appropriate for environments where the
exciting frequency exceeds 50 Hz, while the cantilever beam fixation mode is suitable
for a condition where the exciting frequency is below 50 Hz.



The maximum generation power of a PCP generated in simply supported beam
fixation mode is higher than that in cantilever beam fixation mode.



Regulating the mass block weight affixed to the PCP can alter the natural frequency of
the PCP more efficiently than the length-width ratio can do.
[8] explains that in their research ambient mechanical vibrations were harvested and
transformed to beneficial electrical energy which could then either be stored in a storage
element or be delivered directly to the load. Energy storage is a crucial component of the
energy harvesting system because it is a conduit of stability between the energy source and
the load that offers a continuous energy flow from an otherwise variable environmental
source. The power interface circuits align the harvested energy to permit the charging of low
capacitor batteries or supercapacitors and also provide compatibility with the load
requirements. For a sensor node that is fully powered by ambient energy, the generated mean
power (Pg) must be higher than or equal to the mean consumed power (Pc):
Pg ≥ Pc

(3)

Where Pg is the generated mean power and Pc is the mean consumed power.
As earlier pointed out, the power used by a wireless sensor node is usually a few tens to
hundreds of milliwatts. When this is compared to the power output of MEMS piezoelectric
energy harvesters with a range of a microwatt to tens of microwatts, it is apparent that energy
harvesting will not be able to continuously power the sensor node. This therefore led to the
basic question of in what way can the power consumption of the wireless sensor node be
decreased so that the energy harvesting would be able to handle the supply requirements? The
answer to this concern is practically realized by what is called „duty cycling‟, a phenomenon
which permits the sensor to function in a spasmodic regime instead of a continuous form. In
this approach, wireless sensor nodes are fundamentally designed to function in an extremely
low duty cycle (D), with average power consumption in an active mode (P), and low power
consumption while in sleep (or idle) mode, (P active). This alternating operation of the sensor

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node has been conditioned in a way that the monitoring procedure of the wireless sensor
network is not compromised. The average power used up by the sensor node can be calculated
by:
Pc = Psleep + DPactive

(4)

Where Pc is the average power used, Psleep is the low power consumption in sleep mode, D is
the low duty cycle and Pactive is the average power consumption in active mode.
From Eqs (3) and (4) it can be deduced that if the duty-cycle (D) is diminished, and the
sensor node is put to the sleep mode for most of the time and again stimulated to perform
sensing and communication when required, this will lead to a pronounced decrease in the
average power consumption of the wireless sensor node. Thus, careful choice of a suitable
duty cycle is required in designing of the power management algorithms.
Vibrations from ambient energy sources are variable in nature, and there are occasions
and circumstances when Pg < Pc. In order to deal with this, a collector or storage element
such as a supercapacitor or a thin film battery is required. For any subjective protracted period
of time, T, a long-term storage (Estorage) element must be designed to satisfy the condition of:
Estorage ≥ max ∫(Pc – Pg)dt

(5)

Where Estorage is the long term storage element, dt is the change in time, Pg is the generated
mean power and Pc is the mean consumed power.
The authors described piezoelectric micropower generator and nanogenerator design to be
a multidisciplinary area with problems emanating from basic physics, material science,
mechanical engineering, and electrical engineering. This multidisciplinary methodology and
model is considered the most reliable means of designing piezoelectric energy harvesting
devices, but much work still needs to be done to advance the power output of piezoelectric
generators to equal the requirements of wireless sensor devices. This task can be solved by
selecting piezoelectric material with the best piezoelectric properties, device geometries, and
power electronics to stabilize the power output.
[3] in their work noted that the enhanced technique of vibrational energy harvesting with
piezoelectric materials has led to the need to develop a scavenging energy device. Generally,
harvesting vibrational piezoelectric energy depends on the induced power from mechanical
vibrations with varying amplitude, leading to induced output voltage with alternating current
(AC) from the piezoelectric elements. Many piezoelectric harvesters designed earlier have
shown that the power produced from such a device must be rectified. Diverse rectifiers have
been studied and recommended, including vacuum tube diodes, mercury arc valves, siliconbased switches and solid state diodes. The simplest approach to rectifying alternating input is
to connect the piezoelectric harvester with a P-N junction diode, but this can only work in half
input wave. In order to achieve full wave rectification of a vibrating piezoelectric device, a
bridge-type rectifying circuit with 4 diodes is required. In the search to improve the power
harvesting circuit efficiency, many efforts have been recorded to modify the rectifying circuit.
Engaging a buck-boost DC-DC converter with intelligence to track the power generator‟s
dependence with the acceleration and vibration frequency of a piezoelectric device, a lofty
efficiency of 84% was recorded. Furthermore, to improve the conversion efficiency of the
bridge-type rectifying circuit, the synchronized charge extraction method with inductor was
introduced, thereby leading to an increase in the harvested power by a factor of 4. [17]
analyzed the actual energy flow that is behind numerous energy conversion practices like
parallel synchronized switch harvesting on inductors (SSHIs) and series SSHI for
piezoelectric vibration energy scavenging and introduced the pyroelectric effect which

extracts energy due to temperature variation. [18] suggested energy production using a
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mechanically excited unimorph piezoelectric membrane transducer under dynamic conditions
and predicted a new SSHI to improve the harvested power by the piezoelectric transducer of
up to 1.7 mW, which would be adequate to supply a large range of low consumption sensors.

6. PIEZOELECTRICITY MODELLING
According to [2], the mechanical and electrical behavior of a piezoelectric material can be
modeled by two linearized constitutive equations. The direct effect and the converse effect
may be modeled by the subsequent matrix equations (IEEE Standard on Piezoelectricity,
ANSI Standard 176-1987):
Direct piezoelectric effect:
{D} = [e]T {S} + [αs] {E}

(6)

{T} = [CE] {S} – [e] {E}

(7)

Converse piezoelectric effect

In Eqs (6) and (7), {D} is the electric displacement vector, {T} is the stress vector, [e] is the

dielectric permittivity matrix, [CE] is the matrix of elastic coefficients at constant electric field
strength, {S} is the strain vector, [S] is the dielectric matrix at constant mechanical strain, and
{E} is the electric field vector.
After the poling of the material, an electric field may be introduced to prompt expansion
or contraction of the material, and this electric field may be introduced at any point along the
surface of the material, thereby resulting in a potentially variable stress and strain generation.
However, the piezoelectric properties must include a sign convention to expedite this
capability to apply electric potential in three directions.
Piezoelectric material can be universally categorized for two cases; the first is the stack
configuration which operates in the -33 mode and the second is the bender that operates in the
-13 mode. The sign convention concludes that the poling direction is always in the “3”
direction, and with this assumption, the two modes of operation can be understood. In the -33
mode, the electric field is applied in the “3” direction and the material is strained in the poling
or “3” direction. In the -31 mode, the electric field is applied in the “3” direction and the
material is strained in “1” direction or perpendicular to the poling direction. These two modes
of operation are essential when describing the electromechanical coupling coefficient that
occurs in two forms: the first is the actuation term d, and the second is the sensor term g. The
term g refers to the sensing coefficient for a bending element poled in the “3” direction and
strained along “1”.
According to [3], coupled electro-mechanical behavior of piezoelectric materials can be
modelled by these two linearized constitutive equations:
Direct piezoelectric effect
D = e E1 + de Ơm

(8)

Ek = dcuc E1 + SEƠm

(9)

Converse piezoelectric effect

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Table 1 Piezoelectric characteristics [3]

Evaluating the mathematical modelling of vibrational energy harvestings with
piezoceramics using several types of vibration devices and single crystal piezoelectric
materials confirms that:
Cantilever type vibration energy harvesting is made of typically modest structure and can
yield a large deformation under vibration. [19] suggested a theoretical model using a beam
element and carried out an experiment to harvest power from PZT material. The authors
revealed that a simple beam bending can deliver the self-power source of the strain energy
sensor. Flynn and Sander ascertained some fundamental limitations regarding PZT (Lead
Zirconate Titanate) material and specified that the mechanical stress limit is the effective
constriction in typical PZT materials. They recounted that a mechanical stress-limited work
cycle was 330 W/cm3 at 100 kHz for PZT-5H. [20] offered several vibrational energy
harvesting devices. Firstly, they showed small level vibrations happening in household and
the environments as a reliable power source and studied both capacitive MEMS and
piezoelectric converters in this respect. The replicated results revealed that power harvesting
by piezoelectric conversion is considerably higher, and a two-layer cantilever piezoelectric
generator was later enhanced and authenticated by theoretical analysis (Fig. 1). They also
modelled a small cantilever-based device deploying piezoelectric materials that can harvest
power from low-level ambient vibration sources, and arrived at a new design concept to
improve the power harvesting capability. The concept used axially compressed piezoelectric

bimorph to decrease resonance frequency up to 24%.

Figure 1 A two layer Bender mounted as a Cantilever [3]

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A cymbal type structure by design can generate huge in-plane strain under the influence of a
transverse external force, which undoubtedly is valuable to the micro energy harvesting. [21]
confirmed that a piezoelectric energy harvester exhibited encouraging results under prestressed cyclic situations and authenticated the experimental results with predictable element
analysis. [22] presented two ring-typed piezoelectric stacks, one duo of bow-shaped elastic
plates, and one shaft that pre-compresses them (Fig. 2). They stated that flex-compressive
mode piezoelectric transducers possess the capacity to produce more electric voltage output
and power output when compared to conventional flex-tensional mode.

Figure 2 Conventional Piezoelectric Harvester [3]

A stack type piezoelectric transducer can generate an enormous electrical energy as it utilises
d33 mode piezoelectric materials and enjoys a great capacitance due to multi-stacking of
piezoelectric material layers. [23] recommended a stochastic methodology exploiting stack
configuration instead of cantilever beam harmonic excitation at resonance and investigated
two cases, one with inductor in the electrical circuit and the other without inductor. [24]
advocated for synchronized switch samping (SSD) in vibrational piezoelectric energy
harvesting (Figure 3). They ascertained that SSD improves the electrically converted energy
occasioned by the piezoelectric mechanical loading cycle. This stack type can be feeble under

mechanical shocks.

Figure 3 Model of a Vibrating Structure including a Piezoelectric Element [3]

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A shell type structure can produce a higher strain than a flat plate, and when properly applied
can improve the efficiency of piezoelectric energy harvesting. [25] used a curved
piezoceramic material to increase the charge due to mechanical strain (Figure 4) and enhanced
the analytical mode by deploying shell theory and linear piezoelectric constitutive equations
to form a charge generation expression. Yoon examined a ring-shaped PZT5A element
opened to gunfire shock in an experiment using a pneumatic shock machine and found
reliance of the piezoelectric constant on load-rate, the shocking-aging of piezoelectric effect,
and the reliance of energy-transfer efficiency on the change in normalized impulse. [26]
investigated a circular piezoelectric shell of polarized ceramic under a torsional force to
harvest electric output, and the projected structure was able to harvest electrical energy from
torsional vibration.

Figure 4 Curved PZT Unimorph excited in d31 Mode by a normal Distributed Force [3]

[16] experimented with PCPs affixed with a mass block in simply supported beam and
cantilever beam fixation mode using a vibration test rig as shown in Figure 5.

Figure 5 Vibration Test Rig [16]

The electrical energy produced by a PCP typically cannot support or carry any loads (such
as a microcontroller unit) owing to its extreme voltage and ultra-low current. The approach of
deploying a load resistance to use up the electric power or storing the generated energy with a
storage like capacitor are the two most common methods used to estimate the amount of
electricity. A super capacitor was considered and adopted for the storage of the electric energy
produced by the PCP due to its great energy density, stable performance, fast-charging speed
and high efficiency. The alternating current (AC) as a matter of necessity needs to be
stabilized by rectifying, filtering and regularizing prior to charging the super capacitor (Figure
6).

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(a)

(b)
Figure 6 (a) Circuit schematic for rectifying (b) Rectifying and filtering board, capacitor and filtering
AC super board [16]

[16] set up the vibration test rig in the simply supported beam fixation mode to sweep the
frequency working mode to achieve the full voltage and its equivalent exciting frequency for
each PCP. In line with the simulation results, the exciting frequency value was the original
frequency value of the PCP when the produced voltage attains its peak. However, to get the
maximum power of each PCP, the natural frequency (or exciting frequency that will assist the

voltage to attain its peak) was set to excite each PCP for 2 min and the exciting force
amplitude was set to 1 mm. With the aid of a rectifying and filtering board and a super
capacitor board, the maximum power generated by every PCP with different mass block
weights was documented. Settings in cantilever beam fixation mode were maintained with the
above settings, maximum voltage and its corresponding exciting frequency and maximum
generation power were all obtained and recorded. Manufacture limitations and experimental
errors were the main reasons for the lower voltage obtained in the experiments.
Comparing the results of the above described simulation and experimental statistics, the
natural frequency of the PCP in simply supported beam fixation mode remained higher; and
the natural frequencies of PCPs affixed with mass block range were 200 Hz to 400 Hz in
simulations and 50 Hz to 120 Hz in experiments. Likewise, the deformations of PCPs in
simply supported beam fixation mode were comparatively small, thereby making the
maximum voltages generated by PCPs to be relatively low. Conversely, the maximum
generated power from every PCP in simply supported beam fixation mode was higher than
that in cantilever beam fixation mode. However, experiments in simply supported beam
fixation mode indicated that the maximum generated power and maximum voltage were both
very sensitive to the precise excitation frequency, because a slight deviation could lead to an
ultra-low power generation compared to the maximum generated power. As regards the
cantilever beam fixation mode, the natural frequencies of PCPs are comparatively low. Most
of the natural frequencies of PCPs with diverse length-width ratios were not more than 50 Hz
for both simulations and experiments. Nonetheless, the distortions of PCPs were moderately
large causing a great escalation in the produced voltage (the maximum voltages produced by
PCPs can be as high as 138 V in simulations and a peak voltage of 40 V in experiments).
Nevertheless, care should be taken on the strength of the PCP as high deformation may

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damage it. When compared to PCPs in simply supported beam fixation mode, the maximum
power generated by PCPs in cantilever beam fixation mode was considerably lower (most of
the maximum generated power in cantilever beam fixation mode were not more than 0.01
mW), but the majority of maximum generated powers in simply supported beam fixation
mode exceed 1 mW.
In addition, comparing the maximum generated power in simply supported beam and
cantilever beam fixation mode, the following conclusions were made:
1)
The natural frequency of the PCP in simply supported beam mode is higher than that
in cantilever beam fixation mode.
2)
The maximum generated power of the PCP in simply supported beam mode is
considerably higher than that in cantilever beam fixation mode.
3)
The natural frequency defines the maximum generated power of a PCP.
4)
In the course of the experiments, it was observed that the maximum power generated
and maximum voltage in simply supported beam fixation mode were extremely sensitive to
the exact excitation frequency as a small deviation in excitation frequency can cause it to
generate power with sharp deviation from the maximum power, but the excitation frequency
range in cantilever beam fixation mode was wider.

7. CHARGE COLLECTOR / ENERGY STORAGE EQUIPMENT
According to [5], numerous researches have been conducted on the usage of PEHs as selfpowered energy sources rather than for storage of generated electric voltages in batteries
which is identified with a shortcoming of voltage drop. As a result of recent technological
developments, these harvesters are suitable for usage in micro electromechanical systems
(MEMS), smart structures, structural health monitoring, and in wireless sensors for suborbital

missions
Many researchers have worked on super-capacitors as a means to store electrical energy
rather than in conventional storage devices (i.e., electrical batteries or electrochemical
capacitors), and it has been established to have an overbearing advantage of less maintenance,
easy charging, and optimum efficiency compared to conventional batteries. PEH adoption has
become popular because batteries traditionally have less operational life when compared to
the circuit they are powering. In many circumstances, changing or maintenance of these
batteries may become impossible, and at times the usage of a battery may raise maintenance
concerns when they are used in harsh environments (i.e., high-altitudes, cold or hot climates,
icy or snowy regions) as extreme conditions can damage battery life.
The following researches have adopted the use of circuitry to either store the energy
generated by the piezoelectric material or to build circuits that permit the generated energy to
be removed from the piezoelectric in an efficient manner that will allow more power to be
produced.
[13] investigated the characteristics of energy storage by a piezo-generator using a bridge
rectifier and capacitor. As previously highlighted in their earlier research, the piezo-generator
comprised a steel ball and a piezoelectric vibrator, and with the technology of introducing a
bridge rectifier and capacitor, they were able to decipher the energy storage characteristics
both theoretically and experimentally. To simulate these generation and storage mechanisms
they engaged a corresponding circuit model, where the input mechanical energy was
transformed into an initial electrical energy. By changing the parameters of the circuit they
simulated the separation of the vibrator and the ball. Upon investigating the storage
characteristics for the first effect they discovered that as the capacitance increased the
electrical charge also increased due to an increased duration of oscillation.

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It was also discovered that for each value of capacitance, as the initial voltage increased
the stored electric charge diminished, and the efficiency increased. Again, considering the
overall storage characteristics for multiple impacts, they confirmed that for every value of
capacitance, the first effect gave the largest electric charge. The general storage characteristics
of the system were monitored when the initial voltage was altered; as the initial voltage
increased, the electric charge decreased for each value of capacitance, while the efficiency
increased.
Their model accomplished a maximum efficiency of 35%, three times better than that of a
solar cell.
[19] used a polyvinylidene fluoride (PVDF) piezofilm sensor coupled to a simplysupported plexiglas beam with an aspect ratio of 0.11 to produce an electrical signal. The aim
of this power harvesting experiment was to produce adequate energy from the strain induced
on the piezofilm by the bending beam to power a telemetry circuit. The energy produced from
the PVDF patch was accrued and stored in a capacitor. A switch was introduced into the
circuitry to facilitate the capacitor in charging to a programmed value of 1.1 V, at this point,
the switch would open and the capacitor would release via the transmitter. After the capacitor
had discharged to a value of 0.8 V, the switch would close again and the capacitor would be
prompted to recharge and repeat the whole process. The process of the power harvesting
system was instituted to offer the much-needed energy to power the circuitry and transfer a
signal holding information relating to the strain of the beam at a distance of 2 m.
[27] developed a lumped element model (LEM) using an equivalent circuit model to
define the power produced from the forced vibration of a cantilever beam coupled with a
piezoelectric element. It was discovered that the LEM produced results in line with those
generated with a finite element model from excitation frequencies ranging from DC through
the first resonance of the beam. A parallel result was discovered during a second model
substantiation using experimental results. The aim of this research was to utilize a flyback
converter to improve the efficiency of the power transmitted from the piezoelectric patch to a
power storage medium. The use of a flyback converter permits the circuit impedance to be

harmonized with the impedance of the piezoelectric device. It was discovered that when using
the flyback converter a peak power efficiency of 20% was achieved.

Figure 7 (a) Full Wave-Bridge type rectifying circuit for vibrational Piezoelectric Harvester (b)
Synchronous Charge extraction circuit with an Inductor L and a Switch S26 [3]

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According to [3], Ayers carried out an experiment on PZT ceramics to harvest electrical
energy and establish the principal equations for piezoelectric. The energy storage of both the
capacitor and rechargeable batteries was also probed and conclusions were prepared for future
feasibility and efficiency of the battery recharging (Figure 7). [22] examined leakage
resistances of these energy storage devices which are the predominant factor that governs the
charging or discharging phenomena. A quick test method was proposed to experimentally
examine the charge/discharge efficiencies of the energy storage devices using super capacitors
which were more appropriate and recognized than rechargeable batteries.
A rectifier-free piezoelectric energy harvesting circuit was proposed that is simple and
scalable and can reach 71% of conversion efficiency. Recently, another proposal for ultra-low
input piezoelectric voltage was made. This is a two-stage concept which includes a passive
and an active diode, resulting in effective rectification of tens of mV with very high efficiency
over 90%. Other methods using a bias flip rectifier with an inductor were presented in the
range of µW, which is 4X the power extraction of a conventional full bridge rectifier.

Figure 8 Rectifier-free Piezoelectric Energy Harvesting Circuit [3]

8. CONLUSION
This paper reviewed vibration energy as a potentially cheap and reliable source of
piezoelectric renewable energy. Also reviewed are properties of piezoelectric materials and
various available and effective harvesting techniques used in extracting this energy from the
environment.
The paper summarized several methods and processes involved in piezoelectricity with
the goal of maximizing the harvested energy and the development and application of a
piezoelectric energy harvester.

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