Energy Conversion and Management 254 (2022) 115272

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Energy Conversion and Management

journal homepage: www.elsevier.com/locate/enconman

A review of piezoelectric energy harvesting tiles: Available designs and

future perspective
Saurav Sharma a, b, Raj Kiran a, c, Puneet Azad d, Rahul Vaish a, *
a
School of Engineering, Indian Institute of Technology, Mandi, Himachal Pradesh 175075, India
b
Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands
c
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
d
Department of Electronics and Communication Engineering, Maharaja Surajmal Institute of Technology, New Delhi 110058, India

A R T I C L E I N F O A B S T R A C T

Keywords: Piezoelectric energy harvesting has played a vital role in powering several engineering devices and systems,
Piezoelectric energy harvesting where conventional power supply is either not possible or not desirable. Another perspective for piezoelectric
Piezoelectric tiles energy is its utilization as a non-conventional clean energy source, harnessing the ambient mechanical vibra­
Electric circuit
tions. With the increasing global population and developing infrastructure, the load from human footsteps can be
a source of significant amount of freely available mechanical vibration energy. The piezoelectric tiles are aimed
at harnessing this otherwise wasted energy with minimum interference to the regular activities. This article aims
to provide a comprehensive review of the technologies and methodologies that have been implemented in the
literature. A comprehensive discussion on the various designs and mechanisms utilized in piezoelectric energy
harvesting tiles is provided . Electrical circuits, which are crucial for successfully extracting the electrical energy
from piezoelectric harvesters in usable form, are also discussed in detail. The feasibility aspects, from economic
and energy perspectives, are also presented critically. Lastly, the challenges in the successful implementation of
the piezoelectric tiles and their possible solutions are presented.

1. Introduction Bi0.5Na0.5TiO3, Pb(Zr,Ti)O3, ZnO, and a few polymers, as documented in

the literature in various reviews and books [10-13]. In the context of
In the last two decades, the energy crisis has emerged as one of the piezoelectric energy harvesting, it has been explored for various kinds of
biggest global challenges, having a significant contribution from the mechanical energy harvesting from acoustics [14,15], water flow energy
explosion in the development of electronic devices [1]. Though we have [16], wind energy [17], human motion [18], railway tracks [19], and
high-capacity electrical energy generation sources such as hydro/ther­ highways [20]. Typically, energy outputs (from piezoelectric) are
mal power plants, renewable and sustainable energy sources are now alternating current and voltage where current is of the order of micro­
important to explore. The last decade witnessed many interesting energy amperes and voltage is in the range of few volts [21]. With appropriate
harvesting techniques such as triboelectric [2], electromagnetic [3,4], power conditioning circuits, the electrical output can be modulated.
piezoelectric [5], thermoelectric [6,7], and solar energy [8,9]. All these Recently, piezoelectric energy harvesting has been reported for water
suggested energy sources are unparalleled due to their unique features. cleaning applications using piezocatalytic phenomenon [22]. There is a
Among these, piezoelectric materials are very well known for their wide scope of water splitting for hydrogen generation using piezoelectric
range of sensing and actuation applications. It is because of their func­ energy harvesting [23]. All these findings indicate that there is an un­
tional solid-state coupling between electrical and mechanical forces. tapped potential of piezoelectric energy harvesting techniques and de­
There are hundreds of new piezoelectric materials explored to fulfill vices. Piezoelectric energy harvesting devices such as shoes, tiles,
society’s thrust. These materials typically belong to the ferroelectric pavements, roads, small energy devices of wireless sensor networks, and
family. Mostly these materials are insulator ceramics. Broadly these are pacemakers have been reported in the past to utilize this potential to
categorized based on their structures, compositions, and phases. Some of some extent.
these materials belong to the family of BaTiO3, K0.5Na0.5NbO3, Recently, research focusing on the energy harvesting from human

* Corresponding author.
E-mail address: rahul@iitmandi.ac.in (R. Vaish).

https://doi.org/10.1016/j.enconman.2022.115272
Received 11 September 2021; Received in revised form 30 December 2021; Accepted 17 January 2022
Available online 25 January 2022
0196-8904/© 2022 Elsevier Ltd. All rights reserved.
S. Sharma et al. Energy Conversion and Management 254 (2022) 115272

movements and motions using the piezoelectric tiles, floors, and pave­ zincite (ZnO) with a non-centrosymmetric hexagonal wurtzite-type
ments has gained momentum and various studies have been conducted crystal structure [48,49]. Despite this, however, the largest piezoelec­
[24-36]. In general, the piezoelectric-based tiles can be perceived as tric properties have been observed in perovskite crystals, where single
regular ceramic and granite tiles fitted with piezoelectric materials crystals can display piezoelectric coefficients over three orders of
which facilitate harvesting the waste energy resulting from human magnitude larger than quartz that was originally demonstrated by the
movements. These tiles can be planted over a larger area, thus serving as Curie brothers [50]. Due to the significant variability and tunability of
macro-power sources. Although a plethora of review articles discussing the transduction properties as well as the capability to integrate piezo­
the current and future directions of piezoelectric energy harvesting are electric materials in various processing methods, e.g., polycrystalline,
existing [37-42], they fail to present the to-date design and imple­ single crystal, and films, piezoelectricity has become a vital enabling
mentation of piezoelectric tiles and associated challenges in greater technology for a number of sectors, such as transportation, medical,
detail. This motivated the authors to consolidate the state-of-the-art military, energy, and consumer goods.
mechanical designs, outputs, potential applications, and challenges Mathematically, the piezoelectric effect can be described as the
associated with the piezoelectric tiles and pavements. A total of 120 electromechanically coupled linear relationship between mechanical (e.
publications have been reviewed in furnishing this comprehensive re­ g., stress and strain) and electrical (e.g., electric field and electric
view on piezoelectric energy harvesting tiles. Piezoelectric energy har­ displacement) field quantities. The strain charge form of piezoelectric
vesting through tiles has gained much popularity after 2014, and the constitutive law is given as,
number of publications has seen an increasing trend since then, as can be
S = sT + dE, (1)
seen in Fig. 1, with the maximum number of publications in the year
2019.
D = dT + ∊T E. (2)
The present review paper has been organized as follows: Section 2
provides an overview of the basics of the piezoelectric effect primarily in Alternatively, in stress charge form,
terms of governing equations and primary modes of operation. Section 3
T = CS − eE, (3)
is devoted to the various design types of piezoelectric energy harvesting
tiles and fabricated prototypes. Section 4 details the electrical circuits
D = eS + ∊S E, (4)
interfaced with the piezoelectric tiles for power storage and optimiza­
tion. Several case studies on the installation of the piezoelectric energy where, S, T, E, and D are mechanical strain, stress, electric field, and
harvesting tiles and their feasibility assessment are presented in Section electric displacement, respectively. s, C, d, and e are the mechanical
5. Section 6 propounds the future prospects, several challenges, and compliance, stiffness, and piezoelectric coefficients’ tensors in strain-
feasible solutions associated with the implementation of the technology. charge and strain-charge forms, respectively. While ∊T , and ∊S are the
Finally, the concluding remarks are presented in Section 7. dielectric permittivity matrices at constant stress and constant strain,
respectively.
2. Fundamentals of piezoelectricity The crystal anisotropy is reflected in the non-zero coefficients,
resulting in variations of the piezoelectric tensor for different crystal­
2.1. Piezoelectric effect lographic point groups [51]. For centrosymmetric crystals, namely
materials with an inversion center, as well as point group 432, all
The direct piezoelectric effect, first demonstrated by the Curie piezoelectric coefficients vanish. For non-ferroelectric and non-
brothers in 1880 on quartz (SiO2) single crystals [43], is the ability of a ferroelastic crystals, such as zinc oxide (ZnO), aluminum nitride (AlN),
material to convert a mechanical load into an electrical response, such as and silicon oxide (SiO2), the crystal symmetry plays an important role,
an electric field, electric displacement, or polarization. Following the as the appropriate crystallographic orientation is required to observe the
initial discovery, the Curie brothers also demonstrated the indirect piezoelectric effect and also results in the elimination of the piezoelec­
piezoelectricity, i.e., an applied electric field could also induce a me­ tric effect for randomly oriented polycrystalline materials. In contrast,
chanical response of certain crystals. Although initially believed to be ferroelectric materials, which possess a spontaneous polarization that
only present in a few special crystals, piezoelectricity has been discov­ can be oriented by an external electric field, can display a piezoelectric
ered in a large number of ceramic materials and compositions, in response in crystallographically oriented configurations, such as single
addition to other material classes, such as polymers [44], bone [45], crystals, textured polycrystalline materials, and oriented films, as well as
wood [46], and viruses [47]. Notably, the piezoelectric effect is not randomly oriented polycrystalline materials. This simplifies the
limited to materials with a spontaneous polarization but can also be manufacturing of sensors and actuators, both by reducing costs and
found in nonpolar materials that lack a center of symmetry, such as improving integration into devices. Importantly, however, the mecha­
nisms responsible for the reorientation of the spontaneous polarization
can significantly affect the piezoelectric properties.

2.2. Extrinsic contribution to piezoelectricity

The piezoelectric response comprises both intrinsic and extrinsic

contributions. The intrinsic contribution is understood to be due to field-
induced lattice effects, i.e., reversible polarization extension and rota­
tion of the unit cell during application of an external field, which results
in net changes to both the polarization as well as the unit cell strain. In
contrast, extrinsic contributions are contributions that do not originate
from the lattice, which typically is suggested to be largely due to domain
wall and phase boundary nucleation and growth under applied external
mechanical [52] and electric fields [53,54]. Other hysteretic processes,
such as mobile defects, however, can similarly influence the piezoelec­
Fig. 1. The number of publications per year since 2014 on piezoelectric energy tric response. For example, Schader et al. observed a significant increase
harvesting tiles. Data from ‘scopus.com’ using the keywords ‘piezoelectric’ and in the frequency dispersion of the direct piezoelectric coefficient in co-
‘tile’. Only the publications related to energy harvesting tiles are selected. doped hard Pb(Zr,Ti)O3 with increasing temperature, which was

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S. Sharma et al. Energy Conversion and Management 254 (2022) 115272

suggested to be due to enhanced thermal mobility of defect associated directions of the electric field and stress/strain with respect to the di­
[55]. Importantly, extrinsic contributions to the overall piezoelectric rection of polarization. Assuming the material to be poled in 3rd di­
response can be significant, where it has been suggested that up to 20% rection, different modes of operations can be identified as dij, where j is
of the piezoelectric response in single-crystal 0.67PMN-0.33PT origi­ the applied strain/stress component, and i is the direction of the
nates from the formation of a multidomain state [56]. Other ferroelec­ resulting electric field. The most common modes of operations for
tric materials, such as BaTiO3 and Pb(Zr,Ti)O3 display irreversible practical applications are d31 (transverse mode), d33 (longitudinal
extrinsic contributions on the order of approximately 40% [57] and 30% mode), and d15 (shear mode). These are schematically shown in Fig. 2
[52], respectively, above the lattice piezoelectric effect and reversible (A)-(C). The transverse mode is activated in the bending of beams and
displacement of domain walls. The extrinsic contributions, however, has been widely used in cantilever-based energy harvesting setups. The
increase with increasing applied maximum field amplitude, as higher longitudinal mode requires the compression of the piezoelectric material
external fields can induce additional hysteretic processes. In addition to in the direction of the electric field and, thus, generally appears in thick
applied fields, the mobility of domain wall and phase boundaries and, piezoelectric samples such as cylindrical ones. Shear mode of piezo­
therefore the magnitude of the extrinsic contributions, depend signifi­ electricity is the least used mode for energy harvesting applications
cantly on a number of factors, such as crystal phase [52,58], crystallo­ among the three.
graphic orientation of applied fields [59], dopants [60], temperature In addition to individual operating modes, the utilization of a com­
[59,60], applied bias stress [55], and microstructure [52]. These effects bination of different modes has also been utilized by researchers for
result in a piezoelectric response that is sensitive to both the amplitude achieving enhanced piezoelectric output. In this direction, the optimi­
of the applied external field as well as the application frequency [61]. zation of poling direction of piezoelectric materials has been devised as a
way of activating multiple modes of operation simultaneously [62-64].
2.3. Modes of operation The relative rotation of the poling orientation with respect to the strain
tensor components results in combined electrical output from longitu­
The piezoelectric effect results from the electromechanical coupling dinal, transverse, and shear modes. The varying electric field direction
of strain/stress tensor and electric field vector. The nature and magni­ in the material was also studied to harness the maximum potential of
tude of the resulting piezoelectric effect are determined by the relative different modes by having d15 mode active at the mid of the beam where

Fig. 2. Different modes of operation for piezoelectric energy harvesting; conventional modes, (A) transverse (d31) mode, (B) longitudinal (d33) mode, (C) shear (d15)
mode; (D) graded mode for having transverse mode at top and bottom of the beam, where axial stresses are maximum and shear mode at the center, where shear
stress is maximum [64], (E) schematic of the metamaterial designed to realize all the shear normal and shear-strain modes, by programming the polarization and
applied electric field in subunits [65], (a) d11 (d22) mode, (b) d13 (d23) mode, (c) d12 (d21) mode, (d) d14 (d16) mode, (e) d16 (d26) mode, (f) d34 (d35) mode, and (g)
d36 mode.

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S. Sharma et al. Energy Conversion and Management 254 (2022) 115272

shear stress is maximum and d31 mode at the top and bottom where axial
stress is maximum. By employing the graded mode, as shown in Fig. 2
(D), a sensing voltage 15 times that in shear mode was observed, while
actuation strain was observed to be 75 times that in shear mode. Yang
et al. [65] designed metamaterials by introducing artificial anisotropy at
the microscopic level to achieve all non-zero coefficients in the piezo­
electric material matrix, in contrast to only five non-zero components in
naturally occurring piezoelectric materials (Fig. 2(E)). This strategy was
proposed to enhance the energy harvesting potential of piezoelectric
ceramics several times due to all the possible modes acting
simultaneously.

3. Designs of piezoelectric energy harvesting tiles

Due to increasing growth in population and enhanced energy de­

mands for comfort and building services, the energy consumption of
enclosed spaces has increased manifold in the past two decades [66].
However, a surge in the population and the time spent by the people
indoors provides with scope to harness the mechanical energy due to
human motion in the building sector. Piezoelectric energy harvesting
tiles are a viable option for vibration energy harvesting from occupants’
movements inside a closed space. An estimated energy generation of 1.1
MW h/ year has been shown in an education building, with a possibility
of improvement up to 9.9 MW h/year [67]. Various designs have been
employed by researchers to incorporate piezoelectric elements inside
floor tiles for mechanical to electrical energy conversion. These designs,
based on the type of piezoelectric element used, can broadly be classified
into three main categories, namely, (i) cantilever type, (ii) curved type,
and (iii) array/stacked type. These are discussed in detail in the
following subsections.

3.1. Cantilever-based designs

Cantilever type piezoelectric energy harvesters are one of the most

commonly used types for most applications, including energy harvesting
tiles. Mechanical load from footsteps on the tile is transferred to the
cantilever beam with embedded piezoelectric material. The bending of
the cantilever beam results in axial stress on the piezoelectric material,
which yields an electrical output. Both bimorph and unimorph beams
can be used for this purpose. The superiority of the cantilever type
piezoelectric harvester has been shown by comparing its output with a
single-electrode mode triboelectric energy harvester with the same di­
mensions [68]. The power density of the piezoelectric harvester was
found to be 5773.35 μW/cm3 as compared to 752.34 μW/cm3 of the Fig. 3. The design of piezoelectric tile developed by Hwang et al. [69], (A) the
triboelectric harvester. prototype of piezoelectric energy harvesting tile, (B) schematic illustration of
the tile, and (C) top view of the piezo installed layer.
In one of the initial studies on piezoelectric energy harvesting tiles,
Hwang et al. [69] designed a three-layered piezoelectric energy har­
vesting tile with four cantilever type piezoelectric modules at the middle was conducted to examine the dynamic behavior of the cantilever beam
plate, as shown in Fig. 3. A 150 mm × 150 mm tile was used as the upper constructed using the campsite. Experimental studies were also con­
plate, on which the mechanical load from footsteps is applied directly. ducted by applying a dynamic loading similar to that experienced by a
The bottom plate supports the tile through four springs of length 40 mm. subway train floor tile. Another cantilever type harvester was utilized by
The piezoelectric modules contain PZT-PZNM patches with dimensions Kim et al. [71] in a piezoelectric energy harvesting tile for controlling
47 mm × 32 mm × 0.2 mm, placed on stainless steel plates of di­ electrical appliances. A novel piezoelectric material system 0.72Pb
mensions 67 mm × 37 mm × 0.2 mm as host structure. The vibration (Zr0.47Ti0.53)O3-0.28Pb[(Zn0.45Ni0.55)1/3Nb2/3]O3 + x mol% CuO
frequency of the piezoelectric module was made to match with the tile (PZNxC) was designed for achieving the enhanced energy harvesting
by altering the proof mass for enhancing the vibratory response. Circuit performance. A peak voltage of 42 V at 11 µA was obtained for a human
optimization was also performed through impedance matching, and at weight of 80 kg.
15 kΩ of load resistance, the highest RMS output power of 770 µW with a The use of a permanent magnet at the location of the proof mass is
peak of 55 mW. Theoretical analysis was carried out using a lumped also a concept widely utilized in literature for enhancing the output of
parameter model, and the tile was manufactured based on the theoret­ piezoelectric energy conversion. Panthongsy et al. [72] employed a
ical analysis. The tile was found to be capable of lighting a sixty-chip frequency up-converting mechanism in a piezoelectric energy harvest­
LED lamp through a single time-stepping by a 68 kg person. El-Etriby ing tile to convert the low-frequency input vibrations to high-frequency
et al. [70] analyzed a fiber composite with PZT-5A particles vibrations of piezoelectric transducers. A set of 24 unimorph cantilever
embedded in an epoxy polymer matrix for applications in energy har­ beams were mounted at a supporter, and a stainless-steel proof mass was
vesting tile. The effective electro-elastic coefficients were calculated placed at the free ends of the beams to increase the amplitude of vi­
using a transformation field analysis (TFA), and finite element analysis brations, as shown in Fig. 4(a). Additionally, a permanent magnet is also

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S. Sharma et al. Energy Conversion and Management 254 (2022) 115272

Fig. 4. Schematic demonstrating the working mechanism of the piezoelectric energy harvester with frequency up-conversion (A) waiting for load; (B) loaded, (C)
unloaded states of energy harvesting floor tile, and (D) overall structure of the energy harvesting tile. [72].

mounted on the free end to attract the iron bar placed at the top layer. different widths, 20, 30, and 40 mm of energy harvester, were used, and
The top and bottom layers of the piezoelectric tile are separated by it was found that 40 mm piezoelectric energy harvester provided the
mechanical springs and the rest of the setup is placed in between. The maximum power output of 1.01 mW. A small LED display board with 30
working of the tile is sequentially demonstrated in Fig. 4(b). Initially, LEDs was turned on using an undervoltage-lockout (UVLO) module.
when there is no load on the tile, the air gap between the iron bar and
permanent magnet prevents any attraction force. Upon application of 3.2. Curved piezo element-based designs
force on the top of the plate, the air gap decreases, and the magnet is
attached to the iron bar. In the unloading state, the restoring force of the Simple cantilever-based piezoelectric energy harvesters usually
springs becomes larger, and the magnet is separated rapidly from the require a fixture for constraining one end of the beam, and a stopper is
iron bar. This sets the unimorph piezoelectric beams in vibrating mo­ usually installed to prevent excessive deformation and damage to the
tion. The energy conversion of the fabricated tile was analyzed by using piezoelectric material. Curved piezoelectric elements have been
both the finite element method (FEM) and experimental studies. The explored as energy harvesters inside smart tiles, which have a compact
energy per step derived from the tile was 0.93 mJ, and it was able to size and higher power output than the cantilever harvesters [78]. Due to
power an accelerometer, a capacitive strain gauge, and a smoke detector the curved shape, these can easily be placed inside the tile in simply
for 1.60 s, 70.15 s, and 8503.03 s, respectively. A similar type of supported boundary conditions, without requiring additional fixtures at
arrangement was employed by Isarakorn et al. [73,74] for designing a the ends. In this direction, an analytical and experimental investigation
double stage energy harvesting floor tile. Jabbar et al. [75] designed a on a unimorph curved piezoelectric transducer (THUNDER transducer,
six cantilever piezoelectric energy harvesters-based tile with a perma­ developed by NASA Langley research center) for energy harvesting
nent magnet and the free end of each cantilever, as shown in Fig. 5. The through a smart paver tile was conducted [79]. The piezoelectric
dimensions of the designed tile were 200 mm × 1500 mm × 500 mm, harvester was placed on the center of a hexagonal tile, as shown in Fig. 6
and the beams were vibrated at their resonance frequency through (a). A prestressed contact was provided between the harvester and the
magnet interaction. Krishnasamy et al. introduced a two-phased trian­ top glass plate to ensure an immediate response to even a small loading
gular bimorph cantilever energy harvester for designing a piezoelectric applied on the tile. A maximum of approximately 85 µW power was
energy harvesting tile for harvesting electrical energy twice per footstep generated by a 7 N force with a single curved piezoelectric transducer. A
[76]. FEM simulations of the tile were conducted using COMSOL Mul­ curved tile transducer was fabricated using ceramic grinding and pol­
tiphysics software. While a pedestrian floor energy harvesting (PFEH) ishing techniques for use in roadway tile applications [78,80]. Analysis
tile was designed by Jhun et al. [77], to accommodate variable widths of of the tile transducer under different boundary conditions, excitation
harvesters were designed for operating Internet of Things (IoT). Three frequencies, and external loads was carried out before the packaging of

Fig. 5. The six cantilever piezoelectric energy harvester tile (A) top view of the size cantilever harvesters, and (B) schematic illustration of the tile with labelled
parts[75].

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Fig. 6. (A-D) Schematic of curved piezoelectric smart paver tile [79], and (E) packaging design of a curved piezoelectric transducers’ based tile [78].

tile, as shown in Fig. 6(b). Based on the studies, simply supported plate was used to distribute the footstep force across the piezoelectric
boundary conditions with concentrated load and a maximum allowable matrix. These tiles were installed on stairs to power emergency lighting
displacement of 0.614 mm were found to be the optimum parameters for and were able to generate up to 17.7 mJ of energy per activation, suf­
the tile packaging. ficient to turn on the light for 10.6 s. Moreover, preloading and area
reduction was performed to optimize the performance of the piezo­
electric harvesters. A similar type of piezoelectric energy harvester was
3.3. Piezoelectric diaphragm-based designs used to harvest the electrical energy by transferring the human walking
force to piezoelectric diaphragms using load transfer columns [86].
A piezoelectric diaphragm is made up of a thin piezoelectric disc Additionally, a solar panel was also integrated with the tile to ensure
placed on a metal sheet (usually copper) with a diameter larger than the uninterrupted energy generation even when there is no load on the tile.
piezoelectric disc. The diameter of the metal sheet is larger than the Several other studies have been conducted using a similar type of
piezoelectric disc and a silver electrode is applied on the top of the disc, concept of using an array of piezoelectric diaphragms inside a floor tile
as shown in Fig. 7(a). When the diaphragm experiences a stretch or a [81,83,85,87-93].
shrink in its plane, a voltage difference is generated in the thickness Another way to enhance the net output of a piezoelectric energy
direction. Piezoelectric diaphragms are the most easily available type of harvesting tile is by employing stacks of PZT diaphragms under the top
piezoelectric energy harvesters and can easily be installed at any plane plate, as shown in Fig. 7(b) [84]. Nine stacks of piezoelectric diaphragms
surface using an adhesive. Due to this reason, a large number of research were used, each comprising of five piezoelectric diaphragms separated
articles have focused on this type of element in piezoelectric tiles [81- by a ring to allow the deformation. An analytical model was developed
89]. An 8 × 7 matrix of piezoelectric diaphragms of 50 mm outer for calculating the optimal output voltage of the harvester subjected to a
diameter, connected through the conductive tape, was used as an energy pulse excitation force. The experimental results were found to be in good
harvester in piezoelectric tile, as shown in Fig. 7(A) [82]. A 1 mm gap agreement with the analytical results. The optimal output voltage was
was left below piezoelectric elements to allow the deflection, and a rigid

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Fig. 7. Piezoelectric energy harvester tile based on (A) an array of piezoelectric diaphragms [82], and (B) a stacked piezoelectric diaphragms [84].

between 1/3 and 1/2 of the open-circuit voltage. [94] developed a floor tile with cymbal type PEH and termed it as STEP
Tech (Smart Tile Energy Productions Technology), as shown in Fig. 8. A
truss-like structure is designed to transfer the load of footstep such that
3.4. Miscellaneous designs the piezoelectric element is in tensile state of stress. An analytical so­
lution was developed based on the truss analysis by the method of joints
Apart from the most widely used three designs discussed above, there and was validated with the finite element analysis. For experimental
are other miscellaneous designs that can be a viable option for a analysis, a PZT element laminated with brass was designed as a cymbal
piezoelectric energy harvesting tile. One of the sophisticated designs of transducer, and five of such transducers were installed inside an
the PEH for tile applications is a cymbal transducer. In this arrangement, enclosure. The optimum number of transducers was decided based on
a mechanism is designed such as the compressive load from the tile is various factors, such as the weights of an average person, maximum
converted to a tensile load on the piezoelectric element. Sharpes et al.

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Fig. 8. Detailed illustration of the tile enclosure, (A) the cymbal energy harvester in loaded and unloaded states, (B) inside view of the fabricated enclosure for testing
under human foot-steps, and (C) final tile enclosure with all the components internal to the tile [94].

energy harvested, and sensitivity of the tile. A peak voltage of 25 V was and the cost and environmental aspects of these tiles were discussed.
stored in a capacitor with 1 µF (energy can be inferred from the rela­ Another compressive element-based piezoelectric energy harvesting
tionship E = 12 capacitance × Voltage2) for five cymbal transducers in element, in the form of a disc, was used in a flooring tile to harvest
parallel under 410 N simulated step. The tile was tested in a real-time energy and collect data for footsteps [97]. The floor tile was developed
application and was demonstrated to transmit a signal to remotely as a part of a smart city concept in Malaysia for reducing non-renewable
control a lamp. A compressive piezoelectric harvester was analytically energy consumption.
investigated for piezoelectric tiles in stairways [95]. The characteristics Force amplification mechanisms are often used in piezoelectric en­
of two types of commercially available tiles, Navy Type III and Navy ergy harvesters, where the piezoelectric element is surface bonded to a
Type V, were used to formulate the coupled electromechanical model. It substrate and is subjected to tensile stress. In the conventional force
was found that tiles with a highly nonlinear behavior substantially amplification mechanisms, a large amount of force is absorbed by the
outperform the linear tiles. Also, the Navy type V tile was found to be substrate, which reduces the tensile stress in the piezoelectric element.
better than the Navy Type III. Another compression-based piezoelectric This issue was overcome by employing a force amplification mechanism
energy harvesting tile was developed for utilizing the potential of human for achieving the bending of piezoelectric beams through a double-layer
footsteps on a foot overbridge [96]. Fig. 9(a) shows the cylindrical squeezing mechanism [98]. Two pre-curved aluminum beams, sepa­
compressive element used in the proposed design, and Fig. 9(b) shows rated unevenly along the horizontal axis, were bonded with the piezo­
the assembled arrangement of such multiple elements to form the energy electric clamped–clamped beams, as shown in Fig. 10. A rubber pillar
harvesting tile. The tile was analyzed analytically using an equivalent was placed in between the two beams of every couple of beams, to
system approach. The equivalent electrical and mechanical systems for a enable the bending of the beams during foot strike. Although the peak-
single piezoelectric element are shown in Fig. 9(c). An analysis of the to-peak voltage increased with the increasing stroke of the upper plate,
force waveform experienced by the tile due to footstep was performed, the stroke was limited to 5 mm. This was due to the reason that an
excessively large stroke can give a sense of falling and is impractical for
floor tiles. A voltage magnitude of 49 V (peak-to-peak) was achieved at a
5 mm stroke. The maximum power output of one beam was 134.2 µW for
a step frequency of 1.84 Hz, while the total power output of 40 such
beams under a single tile was 5.368 mW.
A dual-axial underfloor piezoelectric energy harvester was devel­
oped by Wu and Xu, using piezoelectric stacks [99]. The novelty of the
energy tile was that it could utilize both the vertical and horizontal
forces for energy conversion. The vertical force comes from the direct
compression from the weight of the person stepping onto the tile,
whereas the horizontal force is imparted to the tile by the lateral friction
force between the tile and the human foot. The tile consisted of several
conical transmission mechanisms (CTMs), two two-stage force ampli­
fiers (TSFAs), and two piezoelectric stacks. The conical groove ends of
the CTMs were connected to the upper plate and the position limiters to
avoid any damage to the energy harvester. The conical ends of the CTMs
were also connected to the input ends of the TFAs, and the piezoelectric
stack was embedded into the TSFA, generating electrical power. The
optimization for maximizing the power output of the tile was performed
by first identifying the influencing parameter and then performing
optimization using a genetic algorithm. The finite element simulations
were performed using ANSYS Workbench. The average power output of
the piezoelectric energy harvester was 1.25 mW and 0.85 mW under 5
Hz input force frequency with a magnitude of 100 N in vertical and
horizontal directions, respectively. On the other hand, the harvested
energy per footstep of a person weighing 62 kg was 7 mJ.

Fig. 9. (A) Schematic of a single piezoelectric transducer and assembled 4. Electrical circuits: Types and optimization
numerous piezo tiles, and (B) mechanical and electrical system design of the
piezo tile [96]. The piezoelectric tiles are engineered to produce electrical energy by

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Fig. 10. Schematic of the energy harvesting tile using double layer mechanism for force amplification, (A) the front view, and (B) the isometric view.

converting the applied pressure to electrical signals using suitable provided sufficient voltage to transmit a signal via a wireless sensor
transducers. These signals are converted into desired signals using rec­ (Fig. 11 (A)). The transmitted signal is received via a receiver, and a
tifiers or power optimization circuits and stored in rechargeable batte­ relay is operated to switch on the lamp, as shown in Fig. 11 (B).
ries or used directly to run sensors or low-power electronic devices. The simplest form of a circuit to convert AC signals obtained from a
Thus, electronic circuits have special significance for the utilization of piezoelectric tile energy harvester into DC signals consists of a bridge
power generated by piezoelectric tiles and can be categorized into rectifier and a load tested as shown in Fig. 12(A) [77,90]. The harvester
different applications such as running sensors, and energy harvesting. with variable harvested widths and loads was further used to provide
The power obtained from a piezoelectric tile due to kinetic energy from enough power to run 30 LEDs using an undervoltage-lockout (UVLO)
human steps can be utilized into useful electricity for running various module with a turn-on voltage of 6 V under 1 kΩ load impedance [77]. If
kinds of sensors installed for various applications. One such application the voltage drops below the detection voltage of UVLO (4 V here), the
of piezoelectric floor tile consisting of PZN0.5C supplied energy to a internal circuit reaches a standby state. It is claimed that the power
wireless sensor node (both transmitter and receiver) to run an electrical provided by the harvester is sufficient to run IoT sensors.
appliance by controlling the application switching in real-time [71]. A The use of a simple bridge rectifier in most circuits is enhanced by
rectifier, capacitor, and voltage regulator optimized the power and using multiple rectifiers in parallel, to sum up, the current and in series

Fig. 11. (A) Circuit diagram of a wireless switch system operated by the floor tile. (B) Photograph of a wireless switch system synchronized with an air conditioner, a
table lamp, and an air purifier, showing the capability for use in a real-time system [71].

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Fig. 12. (A) Circuit of an individual PZT cantilever [77]; (B) Circuit of electrically connected PZT cantilevers, rectifiers, and load resistors [72].

to sum up the voltage. In a study, a permanent magnet is used in place of selected for the deployment of tiles, as shown in Fig. 13. The three
a proof mass for enhancing the output of piezoelectric energy conver­ hotspots, i.e., (a) the main entrance of the building, (b) the cafeteria, and
sion, and a frequency up-converting mechanism is employed to convert (c) the books collection area, were estimated to have a cumulative daily
the low-frequency input vibrations to high-frequency vibrations of crossing of 26,188 people. Finally, the power generation potential of the
piezoelectric transducers in piezoelectric tiles [72]. Due to the low installation was calculated as a function of the number of people, elec­
output current obtained from PZT cantilever, the rectifier outputs of tricity generated per step, number of tiles, and the enhancement rate due
each are connected in parallel, and an optimal load resistor was used for to the plucked method used to improve the energy harvesting efficiency.
evaluation, as shown in Fig. 12(B). A similar parallel combination of The economic and environmental benefits obtained by installing 1820
rectifiers is used in piezoelectric tile consisting of multiple PZT elements tiles, covering an area of 491.5 m2 (3.1% of the total area) in the library
[84]. building, were assessed, as shown in Table 1. The total amount of energy
In another design [75], a six-cantilever piezoelectric energy harvested was estimated to be 1.1 MWh/year, which could be increased
harvesters-based tile using a permanent magnet is used to design a to 9.9 MWh/year by integrating the plucked method.
sustainable power supply for driving low power electronics. The circuit The feasibility of piezoelectric energy harvesting floor adoption at
includes a DC-DC converter for impedance matching, transformer, Kuala Lumpur International Airport (KLIA) was carried out by Chew
switch, gate drive, and start-up circuits for self-powering and self-start of et al. [100]. The piezoelectric energy harvesting solution was proposed
the circuit. to complement already existing renewable energy systems, i.e., solar
energy. The study inspected the potential factors that play an essential
5. Available products and feasibility aspects role in adopting the piezoelectric tiles at KLIA. A qualitative study was
conducted based on a decision-making model for new technology
Apart from the significant amount of academic research, piezoelec­ adoption, known as TEMIF. Under this approach, the technical, envi­
tric energy harvesting-based floor tiles are also being commercialized as ronmental, managerial, intuitional, and financial factors are considered
a power source for low-power devices and sensors for interior spaces. while evaluating the potential of fostering a new technology. Strategies
The feasibility of the piezoelectric-based energy harvesting tiles in to speed up the rate of adoption were also discussed briefly.
terms of cost and power generation has been assessed in various oper­ In another study, conducted by Elhalwagy et al., the feasibility of
ation environments in the past. These studies are usually conducted in
public places with high pedestrian mobility. One such study was con­
Table 1
ducted for the central hub building (a library) at the Macquarie Uni­
The estimated economic and environmental impact of the tiles’ installation [67].
versity in Sydney, where the floor area with the highest pedestrian
Enhanced Economic Benefits Environmental Benefits
mobility was first identified, and then the piezoelectric tiles deployment
annual
was proposed in the 3.1% most active floor area of the building [67]. For generation
this purpose, a detailed description of the five-story library, in terms of (kWh/year)
traffic, mobility, the number of occupants, and working days in a year,
Energy Running Emission Greenhouse gas
was laid out. The building had a total floor area of 16,000 m2, with a Price (AU costs saving factor mitigation
capacity of 3,000 seats for students and 150 working staff, and 353 $/KWh)* (AU$/year) (kgCO2-e/ (kgCO2-e/year)
working days throughout the year. The next step was to identify the KWh)**
locations of high traffic areas to maximize energy harvesting efficiency. 9888 0.055 543.84 1.07 10580.16
This step becomes essential due to the high cost ($3850/tile) of the tiles
* Average annual electricity prices in New South Wales, Australia.
selected for the study (commercially available Pavegen tiles). Based on **
Greenhouse gas emission factor for New South Wales, Australia from Na­
the functional groups of students and staff, three hotspot areas were tional Greenhouse Accounts (NGA) factors workbook.

Fig. 13. Top view of library ground floor and the pathways followed by the students and staff [67].

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piezoelectric tile installation was assessed by discussing the available Table 2

piezoelectric technology and then conducting the case studies in two Energy harvesting floor tiles available commercially.
different operation environments [66]. For the assessment of feasibility, S. Company Geometry Power Life Energy
the power required and the number of steps per tile were used as the No. span generation
inputs, while the number of tiles needed and the cost feasibility were the technology
outputs of the analysis. A private space (low pedestrian density) and a 1 Pavegen Triangular 5 W continuous 20 Electromagnetic
public space (high pedestrian density) were selected for the case studies [104] with each power for years
for the sake of variety in area and density of the tile coverage. Ramses side 500 footsteps
mm
subway station was considered as the high pedestrian public space, 2 Waynergy 40 cm × 40 10 W per step 20 Electromagnetic
which has a daily footfall of around 150 thousand persons and a power [105,106] cm years
requirement of 10 MW. The low pedestrian private space was a resi­ 3 Energy 70 cm × 70 Up to 35 W 15 Electromagnetic
dential apartment with five persons-family, with a power requirement of floors (The cm × 20 cm sustained years
Dancer) output from
10 kW. The feasibility of different available commercial tiles was carried
[107] each module;
out for both the spaces, in terms of cost and energy generation. Other 5–20 W per
case studies include the installation of piezoelectric tiles in a dance club person
in Rotterdam, Netherlands, as the world’s first sustainable dance club; 4 Energy Up to 315 W 20 Combination of
installation at the Tokyo train station’s north exit; and the tiles installed floors (The peak power. years Solar and
Gamer) This energy is Piezoelectric
at Ponte 25 De Abril bridge in Lisboa, Portugal [101]. The tiles were able [108] sent back to the
to provide for the dance club’s energy consumption by 30%, 65 % of the grid.
bridge’s power consumption, and enough to power the station facilities 5 Energy 60 cm × 60 Up to 315 W 20 Combination of
such as automatic gates and electroluminescence display. Sports ground floors (The cm peak power by years Solar and
Walker) 9 tiles. This Piezoelectric
of Ahmadi township, developed by the Kuwait Oil Company, in Kuwait
[109] energy is sent
was considered as one of the sources of renewable energy using energy back to the
harvesting tiles [102]. This installation was a part of the redevelopment grid.
project designed for the sports ground to support Kuwait’s 2030 energy 6 Powerleap 24 in. × 24 about 1kWh 20 Piezoelectric
vision of reducing the per capita energy consumption and increasing [110,111] in. per hour from years
100 square
energy supply from renewables to 15% of the total consumption. The
meters with
tiles were employed at jogging pathways and the game fields, which about
yielded a weekly energy output of 189 kWh and 20.9 kWh, respectively, 3,000–5,000
with a total weekly energy generation of 209.9 kWh. The piezoelectric people each
hour
tiles were recommended as a part of the hybrid renewable energy gen­
eration system consisting of piezoelectric, solar, and wind energy har­
vesters. A similar type of study was conducted for a children’s outdoor occupancy for a particular location, in order to optimize the usage of
play area at El-Shams club in Cairo, Egypt, to evaluate the feasibility of other appliances.
piezoelectric energy harvesting through occupants’ movement [103]. 2. Instead of relying on only the transverse mode of operation, mech­
anisms can be devised so as to make the piezoelectric energy
6. Challenges and future outlook harvester work in multiple modes. The transmission of compressive
load in the form of simultaneous compression, shear, and bending on
Piezoelectric-based energy harvesting tiles can be a viable renewable the piezoelectric material would be ideal for this purpose.
energy source, harnessing kinetic energy from human footsteps, with 3. The power density of piezoelectric materials can be enhanced to
minimum interference with regular activities. The low power density as some extent by altering poling direction. This technique can simul­
compared to other candidates in this category, such as solar and wind- taneously activate multiple modes of piezoelectricity, which are
flow-based energy harvesters, is sometimes cited as a shortcoming of dormant otherwise. Two methods have been suggested for achieving
the piezoelectric tiles. For outdoor areas, integration of piezoelectric inclined or tuned poling, i.e., modified electrode configurations [63]
tiles within solar modules has been implemented in the past to and cutting piezoelectric material at an angle from a normally
circumvent this issue. However, for indoor applications, piezoelectric aligned piezoelectric material [112].
tiles do not face any challenge from these energy harvesting techniques. 4. Material based solutions such as piezoelectric – polymer composites
The introduction of electromagnetic generator-based floor tiles in the [113], and auxetic structures [114,115] can be implemented for
open market has posed the biggest challenge to the commercialization of improved flexibility and power output.
piezoelectric tiles. The leading technology in this domain is the Pavegen
tiles, created by Laurence Kembell-Cook in 2009, with an aim to reduce 7. Conclusions
the environmental issues related to energy generation [104], with the
key mechanism being electromagnetic conversion. Since then, few other Piezoelectric energy harvesting tiles are a plausible way to support
similar products have also been developed. Table 2 enlists such clean and environment-friendly energy generation and reduce the reli­
commercialized tiles developed based on piezoelectric and electro­ ance on fossil fuels for powering indoor appliances. This review dealt
magnetic energy harvesters. with various aspects of piezoelectric energy harvesting tiles, from ma­
Other challenges facing the implementation of piezoelectric tiles are terial aspects to the feasibility of implementation. The different modes of
the brittle nature of most popular piezoelectric materials, relatively low piezoelectric energy harvesting are first discussed briefly, as it is crucial
power density compared to other alternatives, and the high cost of to understand the mechanism of mechanical to electrical energy con­
installation. The probable solutions to these challenges are, version for the design of piezoelectric energy harvesting tiles. The design
and mechanisms utilized in the literature for piezoelectric energy har­
1. Integration of solar, triboelectric, and electromagnetic modules with vesting are discussed under four categories. The beam bending and
piezoelectric modules in energy harvesting tiles. Apart from piezoelectric diaphragm are the most commonly used designs in the
providing energy output, piezoelectric module can be useful in data tiles, mainly due to their ease of implementation and availability. Other
collection in commercial and industrial indoor spaces, such as the designs, such as curved, cymbal type, stack type, and force amplification
number of visitors, mobility and movement of visitors, duration of

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mechanisms, are also used for enhancing the effectiveness of piezo­ [19] Gao MY, Wang P, Cao Y, Chen R, Liu C. A rail-borne piezoelectric transducer for
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Declaration of Competing Interest 2021.
[29] Yingyong P, Thainiramit P, Jayasvasti S, Thanach-Issarasak N, Isarakorn D.
Evaluation of harvesting energy from pedestrians using piezoelectric floor tile
The authors declare that they have no known competing financial energy harvester. Sensors Actuators A Phys 2021;331:113035. https://doi.org/
interests or personal relationships that could have appeared to influence 10.1016/j.sna.2021.113035.
the work reported in this paper. [30] Song GJ, Kim K-B, Cho JY, Woo MS, Ahn JH, Eom JH, et al. Performance of a
speed bump piezoelectric energy harvester for an automatic cellphone charging
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