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Abstract— In this paper a detailed study on the
piezoelectric energy harvesting of rainfall is presented.
Different features have been taken into account in order to
define the limits in this energy harvesting. Only commercial
transducers have been considered: a lead zirconate titanate
and polyvinylidene difluoride transducer.
Keywords—energy harvesting, piezoelectric.
I. INTRODUCTION
Today, renewable energy sources are widely promoted
worldwide [1-10]. Among renewable energy sources, fuel
cell use has been deeply investigated for a wide variety of
research areas, from handheld devices to household
appliances [11-15]. Due to the combined heat-power
generation option, fuel cells are the most promising
source of energy for residential use, often coupled with
other renewable sources as photovoltaic arrays. In recent
years, together with a rapidly growing interest in
renewable energy sources and their reliable working [16-
19], much attention has been given to the possibility of
generating energy without the use of conventional
thermal power or nuclear plants, in order to meet also the
growing demand for energy in developing countries. A
discussion that it is undertaken relates to convert, by
means of piezoelectric plates, the kinetic energy
possessed by the drops of rainwater into electrical energy
[20-21]. For this application, since a limited amount of
environmental energy is drawn, the power conversion
efficiency of the whole harvesting system is the key
target, even overcoming the conventional performance
indicators [22-23]. In the literature, several solutions to
improve the power conversion efficiency are proposed
[24-28].The works reported in the literature agree that the
single drop of water hitting the piezoelectric plates
generates voltages less than the tens of volts. The
objective of this study is to demonstrate whether similar
values can certainly be used for feeding single electronic
devices, by means of storage systems, or connected in
series and parallel may be desirable for the supply of
power equipment. Accurate system modeling is key to a
successful conclusion of the overall design process. In the
literature, the importance of accurate system modeling as
well as the chance of a unique co-simulation environment
to match several heterogeneous models is widely
addressed [29-30]. For this purpose accurate
electromechanical model, as well as accurate uncertainty
analysis of measured data is required.
Another aspect regarding the production of electricity
from rain is related to the protection of the soil. In times
of sowing, the soil is protected by sheeting to protect the
seeds. Sheets must be used to damp the energy of rainfall
and prevent the disintegration and the collapse of the
rocks which can be turned in hazardous landslides. In the
first case the protective sheeting have seasonal character,
not compatible with the installation of photovoltaic
systems; in the second case the need to protect soil in
steep points cannot provide ideal conditions for the
deployment of solar panels. The question is if a
piezoelectric sheet can provide a solution to these
problems.
II. PIEZOELECTRIC MATERIALS AND MODEL
In the experiments two types of piezoelectric materials
have been taken into consideration: piezoelectric ceramic
Lead Zirconate Titanate (PZT) and polyvinylidene
difluoride (PVDF). This choice was made to try to
provide a comprehensive overview of the possibility to
extract energy from precipitation. Indeed, the presence of
lead in PZT transducer places him among the materials
shall not be used to avoid contaminating the environment.
Piezoelectricity is a property present in many materials,
which, subjected to mechanical stress, develop electrical
charges on their surface (direct piezoelectric effect) and,
vice versa, subjected to an electric field, are deformed
mechanically (effect inverse piezoelectric). The ability of
piezoelectric materials to convert electrical energy into
mechanical and vice versa depends on their crystalline
structure. The necessary condition occurs because the
piezoelectric effect is the absence of a center of symmetry
in the crystal, which is responsible for charge separation
between positive and negative ions and the formation of
the Weiss domains, i.e. groups of dipoles with parallel
orientation. Applying an electric field to a piezoelectric
material, the Weiss domains are aligned in proportion to
the field. Consequently, the size of the material change,
by increasing or decreasing if the direction of the Weiss
domains is the same as or opposite to the electric field.
To describe in simplistic terms the direct effect it can be
said that by applying an external force to a piezoelectric
material the modification of the positions of the ions in
the crystal lattice induces a separation of charge that
Harvesting rainfall energy by means of
piezoelectric transducer
F. Viola, P. Romano, R. Miceli, G. Acciari
DEIM – Dipartimento di Energia, Ingegneria dell’Informazione e Modelli Matematici
Università degli Studi di Palermo - Viale delle Scienze, Palermo (Italy)
634978-1-4673-4430-2/13/$31.00 ©2013 IEEE
produces an electric dipole with a single axis of
symmetry. It is beyond the scope of this study provide an
exhaustive description of the phenomenon and of the
changes that have been developed to optimize the
electrical performance and mechanical specifications,
interesting discussion can be found in [31].
The piezoelectric transducer can be considered as a
charge generator or a voltage generator. When the
piezoelectric film is subjected to a pressure, inside
charges are generated which give rise to an electric field.
The electrodes that are located close to the surface, are
affected by this field and accumulate on their faces a
quantity of charge proportional to pressure. At this point
the role of the transducer may be interpreted differently
depending on the type of load that is connected at its
ends: if the load has an input impedance very low, the
charges that accumulate on the electrodes are poured
entirely on it, similarly to a charge generator; if the
piezoelectric material is connected to a high-impedance
load the charges remain confined on the faces of the
sensor thus keeping the electric field unchanged, as in a
voltage generator. A suitable equivalent electrical model
can be that of a voltage source in series with a capacitor
or the equivalent Norton’s one. Also a resistance that
connects the two ends of the active component can be
used to refine the model, so introducing the electrical
loss. In order to define the behavior of the
electromechanical transducer also the mechanical part has
to be modeled, Fig.1.
R
m
stress C
e
L
m
C
m
n
mechanical electrical
V
+
Fig. 1. Equivalent electro-mechanical scheme.
In the mechanical part the inductor Lm represents the
equivalent mass and the inertia of the piezoelectric
generator, Rm represents the mechanical losses, Cm
represents the mechanical stiffness, stress generator is
caused by mechanical vibration, n is the transformation
ratio of the transformer equivalent, element that relates
the physical quantities with those electrical [32]. Ce
represents the capacitance of the piezoelectric element
and V is the voltage across the piezoelectric transducer.
Electric and mechanical parameters depend on the shape
of the piezoelectric transducer. By applying pressure or
torsion on the material, is created inside an electric field
due to the polarization of the material. The application of
force brings the internal lattice structure of the
piezoelectric element to deform, causing the separation of
the centers of molecular gravity, and therefore to the
generation of small dipoles, which global effect is taken
into account by modeling the transformer. A sheet of
piezoelectric material has some limitations in the
mechanical-electrical transduction for low-frequency
signals, since it fails to generate pulses at high pressure
when the sheet is very large. If the device is placed in the
cavity saturated with air, such as headphones or hearing
aids, then the yield is much higher and the low frequency
response of the piezoelectric is improved. The presented
model depends also on the state of locking of the
piezoelectric film: if it is bound by both ends, or only
one. In order to improve the response of the model such
choice has to be made: PZT can be easily locked by one
or two ends, due to their rigid structure, only short PVDF
can be locked by only one end, due to their flexible
structure.
Different studies in the literature show encouraging
results with regard to the generation of electricity from
water droplets. The piezoelectric transducers can reach
tens of volts, but this result does not yet allow to attribute
to them the character of power generators. The water
drops continuity in the same place is very variable: there
may be intervals of seconds (small rainfall) or fractions of
seconds (downpour). The previous expressed voltage is a
peak-peak voltage, not a continuous voltage, so an
equivalent average voltage has to be defined. For a power
system the equivalent average current can be obtained by
using a bridge rectifier and a smoothing capacity; for the
theoretical model initially this approach has been not
considered.
To evaluate the power output of a piezoelectric
transducer is necessary to define a range of possible
stresses. The single drop of water can have a diameter
that varies between 0.2 to 6 mm. Considering a cruise
speed on impact of approximately 2 m/s for the small
drop and 9 m/s for the largest, it is possible to estimate
the energy input: Emin=3.1J, Emax=0.063J. Also consi-
dering the interval of two seconds to have a successive
drop, the power is: Pmin=1.5W , Pmax=0. 031W.
The energy input is low, so no comparison can be made
with a traditional photovoltaic system.
The removable power, however, is affected by several
factors. The drop, while centering fully the piezoelectric
film, is not able to transfer maximum energy as it is
subject to the phenomenon of splashing: the collision is
not complete since the impact surface are separated some
small drops. It must therefore associate an efficiency of a
collision. In the same way we should introduce a
performance of the electrical-mechanical system. The
drop stresses the piezoelectric according to the 31 mode
and not all the energy is converted into charges on the
plates of the transducer. Finally an electrical performance
coefficient is to be introduced to take into account the
losses of the rectifying bridge. The output power is given
by:
Pout = Școllision·Șpiezo·Șrect Pmax.
The output power is certainly reduced, then the objective
is to maximize it.
The transducers on which the experiments were
conducted are Mide Volture V22B and V22BL and the
MEAS LDT1-028K. Volture the sensors were mounted in
a suitable support, caged to extremes, due to their
fragility. The sensor Meas was mounted as a cantilever in
a suitable support.
635
Fig. 2. Lead zirconate titanate (PZT) transducers: V22BL in the top and
V22B in the bottom of the picture.
Fig. 3 Meas LDT1-028k polyvinylidene difluoride (PVDF) transducer.
Fig. 2 and 3 show the piezoelectric transducers. The
PVDF Meas is customable, the PZT Volture has a rigid
structure and no modification can be introduced.
The Meas piezoelectric has been deeply studied and
here only a part of the study is reported [33]. The value of
inductance present in the electromechanical model is:
,
21
mkkL
m
where m is the mass of the raindrop, k1 and k2 are
geometrical coefficient, given by:
bI
LLb
keb
2
2
1
eb
b
LLb
L
k
23
2
3
2
,
with I(b) is the moment of inertia relative to a
homogeneous mass; other quantities are reported in Fig.4.
A more detailed study on the model will be published
soon, but some important reflections can be reported. In
the first place it is possible to observe that the reduction
of the thickness b determines a decrease of the damping
factor (it affects also the resistance). The magnitude of
this reduction, however, is subjected to a particular
constraint, a lower limit determined by the fact that it is
necessary to fulfill the requirement of sub-damping
model, otherwise, the similar system is no longer able to
represent the physical one. Secondly, it can be stated that
the turns ratio n increases with decreasing the thickness
of the single layer PVDF and in general is influenced by
the characteristics of the piezoelectric material. It is an
important factor, since it relates the physical tension, of
which it is made similar to that of electricity in the
primary circuit, with secondary voltage which represents
the voltage on the load of the piezoelectric generator.
By employing the electromechanical model a first
investigation on the shape of the transducer is made:
considering a constant rain drop source of stress (5mm),
the outgoing power is maximized if w=3.3mm and Le=
25-30mm. This result has been tested on PVDF
transducers that have been shaped according to these
directives.
L
tot
w
L
b
L
e
b
Fig. 4 Geometrical features of the PVDF transducer.
III. EXPERIMENTAL STUDY
The piezoelectric transducers were exposed to rain and
were measured the voltage values generated by the
impact of the droplets. Roughly some typical generated
waveforms are definable: transducers bound to the both
ends have generated waveforms more regular, in which a
first pulse largest is followed by a second smaller and of
opposite sign, Fig. 5. Sometimes the second pulse has
been followed by a third and sometimes was not present.
In some cases we have obtained single negative peaks,
this physical behavior has required a bit more attention
since the physical stress did not change, but it happened
to the tension. It was found that the negative peaks of
tension occurred in correspondence with a state of
piezoelectric plate already burdened with a water film, as
a result of impact the compression status ranged,
generating a negative peak of voltage.
The voltage levels were maintained with maximum
peaks of 6 V during the different tests.
The piezoelectric sensor Meas has been used in
cantilever configuration. An output voltage is represented
in Fig. 6. The output voltage has an oscillating behavior,
due to the presence of an underdamped system. The
energy of the drop of water is absorbed and then released
to the electrical system in a longer interval than that used
by the Volture transducers.
To characterize the behavior of the transducers were
carried out some measures by placing a resistive load
connected to the electrodes. Ten measurements for each
load were considered useful. The output parameter of the
experiments is given by the power exchanged during the
impact. This power value is not straightforward to be
assessed. It is chosen to bring the voltage pulses, which
assume waveforms, similar to those of Fig. 5 and 6,
equivalent to continuous values, evaluating the areas
subtended by the pulse shapes.
636
Figure 5 Three signals acquired: the first presents the typical behavior, a large positive pulse followed by a second smaller; the second signal acquired
differs from the first one because of two negative pulses; the third signal acquired presents only positive pulse. The waveform of the signals is often
conditioned by the presence of the water film on the transducers.
Fig. 6 Waveform of the voltage given by a PVDF transducer in
cantilever configuration. Oscillations are due to the particular structure.
In Fig. 7 the power extracted from the single drop of
water, in the case of PZT transducers, for to the loads of
10, 22, 47, 100, 220, 470 kȍ is represented. The two
transducers have values of comparable powers, this is due
to the fact that both have equal amplitude of piezoelectric
material, while it varies the length of the material which
constitutes the shell.
In Fig. 8 the power extracted from the single drop of
water, in the case of PVDF transducers, for to the loads of
10, 15, 22, 27, 33, 39, 47, 56, 68, 82, 100, 150, 180, 470 kȍ
is represented. Two cases are studied a single PVDF
transducer and two set in parallel. Contrary to what could
be expected maximum values of power are attributable to
the case of single piezoelectric and not to the two parallel.
Fig. 7 Power extracted from single drop of water using the PZT
transducers.
An explanation can be given to this behavior: the two
drops of water do not affect the transducers
simultaneously, so generating oscillating pulses in delay
or with opposing phases, resulting in a reduction of
voltage at the terminals. The shift to higher load resistors
in the position of the maximum, in the case of the two
PVDF in parallel, is attributable to an adaptation of the
system to double capacity for constant pulsation of
oscillation. In order to improve the power extractable
from a system of two transducers in parallel, a bridge
rectifier could be used, even if part of the signal
generated by the drop can be absorbed by diodes of the
bridge (0.6V).
IV. DISCUSSIO N
If the energy possessed by raindrops is taken into
account it is clear that it is not possible to compare the
production of electricity between piezoelectric and
photovoltaic systems. To make the comparison unfair is
also the cost factor, piezoelectric systems for energy
harvesting are experimental and their cost exceeds the
equivalent photovoltaic for same harvesting area, this is
especially felt in the case of PZT transducers.
An interesting comparison concerns the materials used:
PVDF and PZT can be used for the production of energy,
PVDF has a lower cost then PZT, PVDF is not toxic,
while PZT is toxic, and PVDF makes available to the
electrodes higher power.
3,00E-07
5,00E-07
7,00E-07
9,00E-07
1,10E-06
1,30E-06
10 22 47 100 220 470
Power (W)
Load (kɏ)
Volture V22B
Volture V22BL
0,00E+00
5,00E-07
1,00E-06
1,50E-06
2,00E-06
2,50E-06
3,00E-06
3,50E-06
4,00E-06
4,50E-06
5,00E-06
Power (W)
Load (kɏ)
single PVDF
two PVDF
Fig. 8 Power extracted from single drop of water using a single
and a double PVDF transducers.
637
By comparing the energy and the minimum and
maximum power possessed by the drops of water,
contained in the second paragraph, with those made
available to the terminals of the transducers, it is clear
that there is a low conversion efficiency, due to the
phenomenon of splashing and also to mechanical losses
of the system. A more detailed study will investigate the
possibility of improving the performance of the system
by acting on the mechanical characteristics of PVDF
transducers. Furthermore, particular attention will be paid
to partial discharge analysis on this transducer [34-35]
A future development will study in detail the use of
PVDF transducers for the supply of electronic devices
that require power for short intervals and then switch to
sleep mode. The feeding of these devices can easily be
obtained by interposing an electronic circuit that
implements a current pump between the PVDF transducer
and the electronic device.
V. CONCLUSIONS
This paper reports a detailed study on the
piezoelectric energy harvesting of rainfall and its limits.
Two types of commercial piezoelectric transducers were
considered: Lead Zirconate Titanate (PZT) and
polyvinylidene difluoride (PVDF). A comparison of
power output available at terminals of the transducer has
been performed by considering two different PZT
transducers, a single and a double PVDF transducers; this
comparison has shown that the single transducer PVDF
generates a greater amount of power.
ACKNOWLEDGEMENT
This publication was partially supported by PON
PON01_00700-2010 “Ambition Power” Italian Research
Programme. This work has been developed in the
L.E.PR.E. laboratory of DEIM.
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