Piezoelectric Energy Harvesting

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 (PbZr_1-xTi_xO₃, or PZT) piezoelectric ceramics, transparent electro-optical lead lanthanum zirconate titanate (Pb_1-xLa_xZr_1-yTi_yO₃, or PLZT) and lead magnesium niobate (PbMg_1/3Nb_2/3O₃, 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

d_ik = (∂e_k/∂E_i)_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.