Chapter 9 ̶ Piezoelectric
Energy Harvesting
9.1 Introduction
In day to day life and in nature many
mechanical motions are present which are being wasted and are not utilized.
Theses mechanical motions can be trapped and energy can be harvested from them.
Human
body/motion -Breathing, blood flow/pressure, exhalation, walking, arm motion,
finger motion, jogging, talking; Transportation-Aircraft, automobile, train,
tires, tracks, peddles, brakes, turbine engine, vibration, noises;
Infrastructure-Bridges, roads, tunnels, farm, house structure, control-switch,
water/gas pipes, AC system; Industry-Motors, compressor, chillers, pumps, fans,
vibrations, cutting and dicing, noise; Environment-Wing, ocean current/wave,
acoustic wave are examples of such mechanical motions or vibrations. These vibrations
can be converted into electricity through following mechanisms:
i.
Electromagnetic effect
ii.
Electrostatic effect
iii.
Piezoelectric effect
Among these, piezoelectric effect produces
the maximum voltage in comparison to other two. In electrostatic effect an
initial voltage is required to produce the voltage through vibration. But in
Piezoelectric effect no external voltage is required. The word
‘‘piezoelectricity’’ is derived from the Greek ‘‘piezein’’, which means to
‘‘squeeze’’ or ‘‘press’’. Piezoelectric effect can be realized as direct effect
and converse effect. In direct effect, mechanical stress produced on
piezoelectric crystal generates electric charges, i.e. crystal gets polarized.
This process is the basic of piezoelectric generator. In converse effect electric
field produces mechanical movement and it is the basis of motor. For harvesting
energy through piezoelectric effect, it is used in direct mode.
9.2 Physics and
Characteristics of Piezoelectric Effect
Piezoelectric effects are more pronounced
in ferroelectric materials. Ferroelectric materials are the materials which
have permanent electric dipoles. These dipoles are grouped into domain type
structure. Thus each domain is spontaneously polarized even in absence of
external electric field. In presence of external electric field, all the
domains get oriented in the direction of external field, which results in high
value of polarization.
In case of piezoelectric crystal, two
charge distributions are symmetric in unstressed position creating a net zero
electric dipole moment. On application of stress two charges are displaced with
respect to the centre of symmetry such that a net dipole is not zero. Ferroelectric materials can be grouped into four subcategories
according to their crystal structures: perovskite group, pyrochlore group,
tungsten-bronze group, and bismuth layer structure group, among which the
perovskite group is the most important and thus the most widely studied.
Perovskite is usually expressed as ABO3. A typical ABO3
unit-cell structure is shown in Figure, taking PbTiO3 as an example. It
consists of a corner-linked network of oxygen octahedra, creating an octahedral
cage (B-site) and the interstices (A-sites). Ti4+ ions occupy the
B-site while Pb2+ ions occupy the A-site.
9.3
Piezoelectric Materials
Quartz, rochelle salt and tourmaline are
the familiar examples of piezoelectric crystals. Piezoelectric strains are very
small and the corresponding electric fields are very large. In quartz, a field
of 1000V/cm produces a strain of the order of 10-7. Conversely,
small strains can produce large electric field. Quartz is piezoelectric but not
ferroelectric.
A different class of ferroelectric
materials is ferroelectric ceramics. This includes lead zirconatetitanate (PbZr1-xTixO3,
or PZT) piezoelectric ceramics, transparent electro-optical lead lanthanum
zirconate titanate (Pb1-xLaxZr1-yTiyO3,
or PLZT) and lead magnesium niobate (PbMg1/3Nb2/3O3,
or PMN) relaxor ferroelectric ceramics, together with many other nonperovskite ferroelectric
ceramics. Among these, PZT has been demonstrated to possess best performances
as piezoelectric ceramics.
9.4 Mathematical
Description of Piezoelectricity
All crystals in ferroelectric states are
also piezoelectric: a stress Z applied to the crystal will change the electric
polarization. Similarly, an electric field E applied to the crystal will cause
the crystal to become strained. In schematic one-dimensional notation, the
piezoelectric equations are (in CGS system)
P
= Zd + Eχ ; e = Zs + Ed
Where,
P
= polarization
Z
= stress
d
= piezoelectric strain
E
= electric field
χ = dielectric susceptibility
e
= elastic strain
s
= elastic compliance constant
These
relations exhibit the development of polarization by an applied stress and the
development of elastic strain by an applied electric field. The general
definition of piezoelectric strain constants is
dik
= (∂ek/∂Ei)z
where,
i
= x, y, z
k
= xx, yy, zz, yz, zx, xy
9.5
Piezoelectric Parameters
Properties of piezoelectric materials are
generally characterized by k, d and g factors. k-factor: k-factors measure the strength of electromechanical
effect i.e. ability of transducer to convert energy from one form to another.
They are defined as the square root of the ratio of energy output in electrical
form to the total mechanical energy input (direct effect), or the square root
of the ratio of the energy available in mechanical form to the total electrical
energy input (converse effect). Since the conversion of energy from one form to
another is always incomplete therefore k is always less than unity. k- value is
also known as figure of merit for piezoelectric materials. Higher k – values
are preferred. For BaTiO3 k–value is 0.35, whereas for PLZT it is 0.72.
d-factor: The
d coefficients are called piezoelectric coefficients, having magnitudes of X 1012 C N-1 (or pC N-1)
for the direct effect and 10-12 m V-1 (or pm V-1)
for the converse effect, respectively. Subscript is used to describe the
relative direction of inputs and outputs. For example, d31 means
that this piezoelectric coefficient relates to the generation of polarization
(direct effect) in the electrodes perpendicular to the vertical direction (3)
and to the stress mechanically applied in the lateral direction (1), while d33
indicates the polarization generated in the vertical direction (3) when the
stress is applied in the same direction. There are also other similar symbols.
g factors: these are called open-circuit coefficients, another parameters used to
evaluate piezoelectric ceramics for their ability to generate large amounts of
voltage per unit of input stress. The g constant is related to d constant: g =
d/KЄ0 (K is relative dielectric constant and Є0 is the
dielectric constant of free space that equals to unit). High-g-constant
piezoelectric ceramics are usually ferroelectrically hard materials whose
polarizations are not readily switched and thus they possess lower K values.
9.6 Models of Piezoelectric
Energy Harvesting Devices
Piezoelectric
materials can produce electrical charges when they are subject to external
mechanical loads. Figure shows working principle of a piece of piezoelectric
material. The magnitude and direction of the electrical current are determined
by the magnitude and direction of the external mechanical stress/strain applied
to the materials. There have been various modes of vibration that can be used
to construct piezoelectric harvesting devices.
Among the various piezoelectric structures for energy harvesters, the
cantilevered beams with one or two piezoelectric ceramic thin sheets, which are
named unimorph and bimorph, respectively, are the simplest ones. The harvester beam is positioned onto a
vibrating host, where the dynamic strain induced in the piezoceramic layer(s)
results in an alternating voltage output across their electrodes. When a
harmonic base motion is applied to the structure, an alternating voltage output
is produced.
9.7 Piezoelectric Energy
Harvesting applications
The
voltage produced by piezoelectric effect is not very large. But it is
comparable to the voltage provided by a cell/battery. In the applications,
where power is provided by battery, may be replaced by piezoelectric energy.
The battery is needed to be replaced after certain time. This can be avoided by
Piezoelectric Energy Harvesting. Typically, growth in the field of portable and wireless electronics demand
battery as a power source. Sensors installed in remote places also need battery
power. All these may utilize Piezoelectric Energy around their location.
One application is mounting of piezoelectric materials in shoes.
9.8 Human Power
Human movements have
kinetic energy, which can be tapped to produce power. Except from vocalization,
most of the human activities have low frequency. It is only up to few hertz in
most cases. To utilize such a low frequency, ‘Frequency up-conversion method is
used. The actual human motion triggers another motion. Here human motion actuates an array of
piezoelectric bi-morph beams. With an initial excitation, the transducers start
to vibrate at natural frequency. By using this approach, the operational frequency
range can be widened while the electromechanical coupling is significantly
increased. At a frequency of 2 Hz and an
acceleration of 2.7 m s-2, the harvester has a maximal power output
of 2.1 mW. This type of harvesters can be used for promising medical
applications, for example, as power suppliers for wearable and implantable
sensors for heart rate, blood glucose level, blood pressure and oxygen
saturation. These may be wearable devices. Piezoelectric materials can be
mounted into shoes. Energy may be derived from joint motions also. In the
following few cases have been discussed to harness human power.
i.
A piezoelectric material mounted in shoes.
ii.
Treadmill exercise in gym. The power of
many people can be transferred to generator to produce electricity.
iii.
Stationary bikes in a gym harvest energy
during workouts. Pedalling turns a generator, producing electricity.
iv.
Dancing floor in a club. Such dancing
floor has piezoelectric material beneath it. When people dance mechanical
pressure is developed on piezoelectric material to produce electricity.
v.
Wearing devices may take power of joint
motions.
vi.
A bag fitted with spring may utilize
energy of hip motion.
vii.
Research is going on to prepare cloths
of nanofibres containing nano size generators to produce electricity.
viii.
This may be very useful for military
people who carry battery with them to power their gadgets. Their own power may
produce electricity.
