Design of Piezoelectric Energy Harvesting Shoes

for Charging Phones

Shahriar Khan Muhit Kabir Sarneabat
Dept of EEE Chemical Engg Division
Indepencent University, Bangladesh Institute of Engineers, Bangladesh
Dhaka, Bangladesh Dhaka, Bangladesh
skhan@iub.edu.bd smkabir87@gmail.com
2021 IEEE International IOT, Electronics and Mechatronics Conference (IEMTRONICS) | 978-1-6654-4067-7/21/$31.00 ©2021 IEEE | DOI: 10.1109/IEMTRONICS52119.2021.9422636

Abstract—In spite of great advances in cell phone Soft-cushioned shoes are comfortable, and good for the
technology, the rapid discharge of phone batteries remains a feet and for general health [1, 2]. Much or most of the
widespread problem. One solution may be recharging phones energy of walking or running is dissipated in these shoe
with movements of the body, such as with energy harvesting soles. The absorption of impact means that the mechanical
shoes. Soft-soled impact-absorbing shoes are best for the feet energy of running is being largely converted to heat energy
and for general health. Energy harvesting from shoes have at the soles. At least some of this mechanical energy may
been of interest for decades, but the technology is still in the instead be converted to electrical energy for charging
research phase. Piezoelectric voltage generation at shoes is electronic devices such as the phone.
made more feasible with recent advances in flexible
piezoelectric materials. The characteristics of energy
conversion at shoes have been studied. Shoes connected B. Harvesting Energy at Shoes
through wires on the legs to power electronics can charge a Harvesting the energy of body movement for charging
phone at the waist. An extra battery source at the power mobile and wireless electronics has been proposed for long
electronics would be an inconvenience, and a simple full wave [3]. The build-up of static electricity at devices at shoe soles
rectifier with filter is proposed for initial testing. The resulting has been suggested for harvesting energy [4]. Another
large time constant implies the need for a large capacitor. The possibility is electromagnetic voltage generation at the shoe
design considerations in this paper may be the basis of further sole, such as with an armature and field [5, 6]. But the
research and experimentation, and possible commercial electromagnetics may be cumbersome and not durable
implementation. enough to be placed at a shoe sole.
Keywords—energy, phone, phone, harvesting, shoe, power Shoe outputs of many volts have been reported in the
electronics, piezoelectric, piezoelectricity, walking, rectifier, filter, literature. A relatively high one watt of power has been
capacitor, battery. reported from electromagnetic energy harvesters.

I. INTRODUCTION II. PIEZOELECTRICITY IN SHOES

In spite of recent rapid advances in smart phones, there Of all the proposed methods for energy harvesting in
has been relatively slow progress in battery technology, and shoes, piezoelectricity is the most common [7, 8, 9, 10]. One
rapid depletion of phone batteries remains a major problem. option is to use piezoelectric bending beams as shoe inserts
Over the last two decades, microprocessors for smart phones [11].
have shown exponential growth keeping up with Moore's
Law, but Lithium ion batteries have shown slower Piezoelectric ceramics for a 90 kg person showed 0.4% or
development. 1.43 mW of walking energy can be harvested [12].
Electric sockets for recharging may rarely be available in Experimental results from 2009, report a "6-layer heel
public areas, away from the home. The electric grid may be footwear harvesters have an average power output of 9
less available in remote locations and in less-developed mW/shoe at walking speed of 4.8 km/h." Another study
countries. Stranded hikers may be calling for help with the reports 55.6 μJ of energy, and peak power of 1.6 mW
last bit of charge on their cell phones. Foot soldiers in remote generated with each footstep.
locations may need additional sources for recharging their One study reported walking or running generating 10-20
portable electronics. μJoules per step [13].
Power banks for charging phones have limited backup Piezoelectricity at the shoe is further supported by recent
charge and require prolonged connection to a power socket. developments in flexible piezoelectric materials [14].
Solar powered phone chargers are likely to be bulky and not Piezoelectric powered shoes have been proposed to power a
portable. These point to the possibility of recharging phones GPS device [15].
with body movements, such as with energy harvesting shoes.
A. Power Electronics Interface
A. Well-Cushioned Shoes Good for the Feet
With a piezoelectric shoe energy harvester, what type of
From thousands (millions) of years of selective evolution, power electronics is to be used? The piezoelectricity has
the feet of Man have mostly evolved for walking on the soft special voltage and current outputs requiring specific design
grass of grasslands, or the soft decaying plant matter on the to interface with the phone [16, 17].
forest floor. The human foot is not well adapted to the hard,
smooth and uniform pavements of today. It has been proposed the power electronics and a battery
be kept inside the shoe sole [5]. But this would be
problematic as it would increase complexity of the shoe and

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would not be rugged and durable. Also, different phones may

Force (Newtons)
require different power electronics. Weight of person tep
Ups
A piezoelectric cantilever device was implemented and n ste
p
tested together with its electronics [18]. This required a Dow Area = Work done
microprocessor, which would be difficult without an extra = Energy released
battery. A Buck converter has been proposed [19], but this
too has the requirement of having an independent, separate, Deformation of the sole (mm)
battery supply. Fig 1. If the shoe acts like a perfect spring, the energy absorbed during down
step would be completely released during the up step. Harvested energy will
This paper proposes keeping the power electronics away be zero.
from the shoe at the waist (or pocket) level, and to not
require any separate intermediate battery. The mechanical energy during downstep and during
upstep will be about half the force multiplied by distance:
A separate independent battery may be an added We consider a person of 70 kg, deforming a sole by 1 cm at
inconvenience and has been avoided in the design. The every step.
simple full-wave diode rectifier with filter configuration is
proposed here for initial testing and for proof-of-principle. Work during Downstep = Work during Upstep
1
III. CHARGING AND BATTERY SPECIFICATIONS = Weight x Deformation of sole
2
The charging requirements and the battery specifications Work done = 0.5 x 70 kg x 9.8 x 0.1 = 3.43 Joules
of phones will determine the design of power electronics
interface with the piezoelectric shoe. In comparison to the spring above, a rubber band, shows
some mechanical hysteresis (figure below). The energy lost
Lithium Ion batteries for phones are mostly rated at 3.8 to hysteresis, or converted to heat, is the difference in the
V, and may start working at 3.6 - 3.7 V. Other charger energy used for stretching and the energy released from
voltages are in the region of 5 V, contraction.
Two safety precautions include that a battery with less
ion
than 1.8 V should never be recharged. Secondly, the charger Force (Newtons) nt r
ac t
voltage should not exceed 4.25 V. A battery below 3.0 V g Co
hin
needs a trickle current, before its voltage rises to 3.4 V. r etc
St Work done on rubber
= Energy of stretching
A phone with a 4 inch screen may consume 0.75 w - Energy of contraction
power from a battery of 5-6 watt hours capacity. Under Length of rubber band (cm)
practical usage, a battery may last for no more than 5-6 hours
of usage. Fig 2. The stretching and contraction of a rubber band shows energy lost to
hysteresis (converted to heat) as the work done during stretching minus the
Mobile phone batteries range from about 700 mA-hrs to work released during contraction.
1200 mA-hrs, whereas smart phone batteries are in the range The energy absorbed or released equals the area under
of 1000 - 1300 mA-hrs. The internal resistance of the battery the curve above.
may be about 0.15 ohms. After 300-400 charging-
discharging cycles, the internal resistance may double to 0.3 Energy absorbed or released = ʃF(x) x dx
ohms.
If the material of the sole has a hysteresis effect such as
above, the work done during downstep will be greater than
IV. ENERGY ABSORBING CHARACTERISTICS the energy during upstep. The shoe sole may recover slowly
The energy absorbed by the soles will be the sum of the from deformation, meaning the force during downstep would
energy absorbed by the rubber, and the energy converted by be greater than during upstep.
the piezoelectric material. The energy absorbed by the rubber The energy of hysteresis would be the area within the
will depend on the material and its energy absorbing curve (figure below).
characteristics.
Weight of person
If the shoe sole acts like a perfect spring, and no energy is
Force (Newtons)

converted to piezoelectricity, the energy absorbed during the p ed

n s te sorb
down step would be completely released during the upstep Dow rg y ab
(figure below). Ene tep
Up s

Deformation of the sole (mm)

Fig 3. The area in the hysteresis curve will be the work done in the shoe sole.

If very little energy from the sole is returned during

upstep, the total energy converted in the shoe is almost half
of the energy in the rectangle (figure below).
1
Energy available = x Weight x Deformation of sole
2

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 To tronic
ctr er

Ele
ics
Ele Pow
Force (Newtons)
p

Po
on
nste

c
Dow

we s
To

r
p
ste
Up
Deformation of the sole (mm)
Piezoelectric Layers
in Soles
Fig. 4 . If due the hysteresis, the sole exerts little force during upstep, the
energy converted would be 0.5 x weight x deformation of sole.
Fig 6. Similar piezoelectric layers may be distributed in both soles of shoes,
In case the hysteresis effect is very strong during minimizing chance of breakage. Connections may be in series, so as to
downstep and upstep (from shoe material and energy maximize voltage.
harvesting), the energy conversion per cycle will be the close Identical piezoelectric devices may be placed in both
to the full area of the rectangle below shoes for maximizing energy harvesting.
Weight of person Series or parallel connection of the above piezoelectric
p layers in the soles will be largely decided by the needs of the
Force (Newtons)

nste power electronics. However, a series connection appears

w
Do best, as the voltage would be maximized.

p
ste
A. Pressure and Generated Voltage
Up

Deformation of the sole (mm)

Considering the stepping patterns and speeds of walking,
the forces on the soles (heels) and the resulting deformation
Fig 5 . In the extreme case, the force-deformation curve (hysteresis effect) may be as follows.
will take up almost the full rectangular area (maximum energy harvesting).
shoe (Newtons) Left shoe
Force on each

Assuming maximum conversion of the energy, the sole Right shoe

Weight
energy available with each stepping cycle would be the area sole
of the rectangle above:
W = MgH = 70 kg x 9.8 m/s2 x 0.01 m
W = 6.86 ~ 7 Joules time (seconds)
~ 0.5 sec

Assuming each step taking 0.5 seconds, the power Fig 7. Force (in Newtons) upon each shoe during a simple walking motion
available during walking would be: The generated piezoelectric voltages may be proportional
W 7J to the force, at the right and left shoe (with no output
P= = = 14 watt current).
T 0.5 sec
Higher energy and power may be obtained from greater Left shoe
voltage (V)

Right shoe
Generated
Pressure,

sole sole
depth of the shoe sole and with faster walking or running.
The energy from jogging or running may be of the order of 3.8 V
five times (estimated) greater than for walking.
There are few practical methods of conversion of this ~ 0.5 sec time (seconds)
mechanical energy to electrical energy. Piezoelectric
materials can hardly be made to deform by the above one cm Fig 8. Pressure and voltage generated in the left and right shoes (without
from the movement of stepping. flow of current).

A low efficiency of 10 % would give 1.4 watt in When the walking changes to running, the foot kicks up
conversion of walking to electrical energy. the body. The body will be in the air, and there will be a
momentary increase in force at the sole (figure below). The
body may stay suspended for about less than a second,
V. FORCE AND VOLTAGE FROM PIEZOELECTRICITY
during which the force will be zero. As the other foot lands
The option of piezoelectricity for shoes is supported by on the ground, there will be an sharp force of impact.
recent developments in flexible piezoelectric materials.
All forces may generate proportional voltage at the
Layers of piezoelectric material may be placed in the piezoelectric material.
heels and the rest of the soles, lowering the chance of
breakage (figure below). The work done will depend largely
on the speed of walking or running.

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 Left shoe Landing impact The alternating current from the piezoelectric material

shoe (Newtons)
Right shoe

Force on each
sole sole can be passed through a full-wave rectifier and then filtered
Push-off to give a mostly DC waveform.

Rectified current
Weight
i(t) i(t) i(t)
time (seconds) i(t)

Fig 9. Force upon each shoe for slow running or jogging. Spikes are seen v(t) v(t)

(mA)
during the landing impact and during the push-off.

~ 0.5 sec time (seconds)

VI. OVERALL STRUCTURE
In deciding the overall structure, the power electronics Fig. 13. After passing through a full-wave rectifier, the pulsed current will be
may be placed at the waist or at the shoe. Placing at the waist in the same direction at intervals of about 0.5 second.
close to the phone appears best at this time, as the shoe may
Passing through a full wave rectifier and a filter will give
be too compact, and present a source of additional wear and
tear. Also, different phones may require different power a DC voltage and current with a small ripple, suitable for
charging a phone. The two shoes may be placed in series,
electronic circuits.
effectively adding the voltages (figure below).

R +
Vout

Phone C

_
Power Trousers
electronics
Wir nectio
con

Fig. 12. The piezoelectric materials of the shoe can be connected in series
ed

with a full-wave rectifier and a filter.

wire

Rectified current
n

Plug i(t) i(t) i(t)

(mA)

Fig. 10. Wires connect from the piezoelectric soles to the power electronics
and then to the phone. A rugged wired connections avoid breakage from
running.

Behind the heal of the shoe, there may be a rugged plug, time constant = τ = RC
connecting the wire to the power electronics. The wires and Fig. 14. DC output with ripple after passing through a filter. The time
connections attached to the legs must also be rugged, in order constant may be too large (compared to conventional electronics), meaning
the capacitor may need to be large.
to withstand walking or running over long distances.
The diode rectifier/s may be compact enough to be
VII. POWER ELECTRONICS placed at the shoe or at the plug. However, the filter may
require a large capacitance, which may be too bulky to place
A switched mode converter in the power electronics will in or near the shoes.
require an additional power source or battery, which would
be inconvenient.
A. Filter Design
For harvesting energy from the generated voltage, a The time constant of the filter is the length of intersection
current flow is required. The discharging current from the on the time axis of a tangent from the curve (figure above),
piezoelectric material may show the pulsed waveform below. and will equal the product of the resistance R and the
capacitance C.
Voltage, current (A)

Left sole,
current τ = RC seconds
In order to keep the ripple value low, the time constant τ
Generated

v(t) v(t)
may have to be several times larger than the stepping time.
With stepping time of the order of 0.5 second, the time
Time (sec) constant may be
Right sole, τ = RC = 2 - 4 seconds.
current
Fig. 11. For piezoelectric shoes, the current may be positive during the This time constant of 2 - 4 sec is much larger than the
downstep, and negative during the upstep. order of milliseconds encountered in conventional power

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electronics. This implies that either R or C or both must be As different phones have different charging voltage and
unusually large. Having a large R may be undesirable as it current requirements, different power electronic circuits may
would dissipate too much energy. A large capacitor C may be required for different phones.
be too large and bulky. The electrolytic capacitor may be the
In spite of the clear challenges, the design considerations
best option, as it may have a large value (at the cost of low
voltage withstanding capability). in this paper may be a basis for further research,
development and experimentation.
A capacitor of the order of 1 μfarad, would imply a
resistance of the order of 2 - 4 Mohms, which would still be IX. REFERENCES
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