AN EXPERIMENTAL STUDY ON THE ENERGY HARVESTING SYSTEM
USING PIEZOELECTRIC ELEMENTS(PVDF)
Jae-yun Lee1, Sanghwan Kim1, Kwangsoo Kim1, Jongdae Kim2 and Bumkyoo Choi1
1
Mechanical Engineering, Sogang University, Korea
2
Electronics and Telecommunications Research Institute, Korea
Abstract: This paper presents optimal environments and specimens for an energy harvesting system using
resonator. An important thing is the correlation of the resonant frequency of diaphragm and piezoelectric elements
in experiments. Experimental results indicate a maximum peak to peak voltage of 46.2V and power of 1.84µW.
Based on the experimental results, when piezoelectric materials (PVDF) are arranged regularly and resonant
frequencies of a diaphragm and piezoelectric materials correspond to driving energy source, it will be expected to
improve the efficiency.
Keywords: energy harvesting, piezoelectric, PVDF, resonator, resonant frequency
INTRODUCTION
changed by height of neck, cross section of neck and
volume of cavity. The dimensions of the resonator are
25 mm of height of neck, 314 mm2 of cross section
and 55600 mm3 of the volume of cavity. And then the
resonator has 820 Hz of resonant frequency by
equation (1). Duct and resonator are manufactured of
acrylic.
Measurement devices are NI (National
Instrument)-PXI which can generate signal or
frequency and control the operation, HAS 4014 (NF
Corporation) that can amplify the generated signal and
frequency from NI-PXI and microphones (PCB) of
1/4″ pressure type that can measure the status in duct
and resonator. The other is oscilloscope (Agilent) that
can display a status.
For the practical application of wireless sensor
network based on USN (Ubiquitous Sensor Network),
it is important for powering sensor systems to be free
wiring or batteries with the limit of life because of a
number of sensors. Energy harvesting technology can
provide sensor node with the solutions to replace
batteries. Sources of energy for energy harvesting
system are thermal, acoustic, vibration, impact and so
on. Especially piezoelectric, electromagnetic and
electrostatic phenomena are applicable to the
technologies to convert mechanical energy to
electrical energy.
Many studies and industrial approach progress on
the piezoelectric elements to change vibration to
electrical energy. Many papers are published by lots
of the laboratories of universities and research centers.
Existing papers are mainly focused on using vibration.
However, the alternative source easily to meet is
acoustic. Current researches on acoustics are focused
on reducing or absorbing noise, so the acoustic results
as energy source are rare. In this study, the energy
harvesting technology with acoustic source as the
alternative adopts a resonator to effectively transmit
an external energy to piezoelectric elements as energy
conversion components.
f
r
=
v
2!
A
VL
(1)
fr = resonator frequency[Hz]
v = speed of air[m/s]
L= height of neck[mm]
A= cross section of neck[mm2]
V= volume of cavity[mm3]
EXPERIMENTS
Experimental setup
Experimental setup is divided into three parts
which are driving source, duct and resonator. Driving
source uses a speaker of 85 dB of sound pressure level
and 20 W of maximum power. The length of the duct
is 1 m, the distance of which the plane wave from the
speaker can be constantly and stably transmitted to the
resonator. A resonant frequency of the resonator is
0-9743611-5-1/PMEMS2009/$20©2009TRF
Fig. 1: Photograph of experimental setup and
measurement device
324
PowerMEMS 2009, Washington DC, USA, December 1-4, 2009
Experimental method
To drive proper frequency band using speaker,
NI-PXI has control of generating sine wave and sinesweep wave. The frequency generated from NI-PXI
moves to speaker through the amplifier. The speaker
generates the amplified frequency. The frequency
which is plane wave moves toward resonator through
1m of duct. The plane wave in the resonator drives a
force to piezoelectric materials. And then
piezoelectric materials generate a voltage. The test
method of microphones is “standard test method for
impedance and absorption of acoustical materials
using a tube, two microphone and a digital frequency
analysis system” by ASTM (American Society for
Testing Method) E1050. To display and check the
phenomenon, a speaker generates each frequency. The
phenomenon of moving plane wave in the duct and
the resonator is checked by microphones and a
oscilloscope. Also, theoretical resonant frequency by
equation (1) is compared with experimental data. If
thickness of the diaphragm, shape and the number of
PVDF (Polyvinylidene Fluoride) are changed, then
generated voltage will be checked.
Fig. 2 shows the circular PVDF and the cantilever
of PVDF on the brass diaphragm and the stainless
diaphragm.
Fig. 3: FRF at 100 Hz to 1400 Hz (brass diaphragm
thickness 200µm)
Fig. 4: Phase degree function at 100 Hz to 1400 Hz
(brass diaphragm thickness 200µm)
Modal analysis of PVDF’s specimen
Fig. 2: Photograph of specimen by shape, number and
quality
(a)
RESULT AND DISCUSSION
Relation of duct, resonator and diaphragm
Fig. 3 presents FRF (Frequency Response
Function) at sine-sweep 100 Hz to 1400 Hz and shows
the peak value near 800 Hz. Another unexpected peak
value occurs near 500 Hz. According to mode analysis
of a brass diaphragm, unexpected peak value is the
resonant frequency of a brass diaphragm. By changing
thickness of PVDF or material of diaphragm,
thickness of stainless diaphragm is found to 50 µm in
order to adjust resonant frequency of PVDF with
diaphragm.
(b)
(c)
(d)
Fig. 5: FEM analysis by ANSYS
(a) 1st mode (b) 2nd mode (c) 3rd mode (d) 4th mode
The result of the modal analysis of PVDF
specimen by using FEM (Finite Element Method)
program ANSYS is shown in Fig. 5. The 1st mode is
325
dominant in the vibration analysis. Therefore,
resonant frequency of diaphragm is matched to that of
resonator. As shown in Fig. 5 (a), the frequency of 1st
mode has nearly around 400 Hz. This value is almost
same as the resonant frequency of the stainless
diaphragm. As a result, we can predict that PVDF
attached on diaphragm will be mostly deformed at 400
Hz.
Generated voltage
Table 1 shows the generated voltage by shape of
PVDF. The cantilever shaped PVDF generates more
voltage than circular shaped one on the almost every
frequency band. A generated voltage on every
frequency shows the similar tendency of
independence of PVDF’s shape.
Fig. 6 shows the generated voltage with the
various thickness of PVDF. There are many regions
which generate twice larger voltage when the
thickness of PVDF is doubled. But there are some
regions which generate lower voltage. The resonant
frequency band of cantilever is shifted by the
thickness. This means the resonant frequency band by
thickness should be checked before choosing the
driving frequency band.
Fig. 7 shows the generated voltage by the number
of piezoelectric element. Contrary to the thickness, the
generated voltage is doubled regardless of the change
of the frequency band, when two PVDF are used. And
Fig. 8 shows the generated voltage at 400Hz that are
measured by oscilloscope when two PVDFs are
attached on the diaphragm.
Fig. 6: Generated voltage graph by thickness of
PVDF at 100 Hz to 1400 Hz (stainless diaphragm
thickness 50 µm, PVDF thickness 110 µm)
Table 1: Generated voltage by shape of PVDF
Brass t : 200µm
cir. PVDF t : 52 µm
Frequency
[Hz]
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
Voltage
[Vp-p]
0.122
0.18
0.132
0.187
0.613
0.344
0.314
0.262
0.253
0.178
0.137
0.156
0.121
0.116
Fig. 7: Generated voltage graph by the number of
PVDF at 100 Hz to 1400 Hz (stainless diaphragm
thickness 50 µm, PVDF thickness 110 µm)
Brass t : 200µm
rec. PVDF t : 52 µm
Frequency
[Hz]
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
Voltage
[Vp-p]
0.869
5.070
0.488
4.370
6.350
0.900
1.561
2.158
3.890
1.381
0.599
2.406
0.195
0.226
Fig. 8: Generated voltage at 400 Hz measured
oscilloscope (stainless diaphragm thickness 50 µm,
PVDF thickness 110 µm)
326
ACKNOWLEDGMENTS
Capacitor
This experiment uses a capacitor of 2.7V and
4.7µF. During charging the capacitor, the driving
frequency is 400 Hz of frequency band that generates
a high voltage. A result indicates 1.84µW by equation
(2).
1
P = CV 2
2
Financial support for this paper was provided by
Electronics and Telecommunications Research
Institute.
REFERENCES
[1]
(2)
[2]
[3]
[4]
Fig. 9: Progress of charging a capacitor
CONCLUSION
[5]
The resonator has been used to eliminate the noise
and the wind from surrounding environment, but
almost it has been not used to generate energy.
Therefore, this paper tries to use waste energy source
using resonator. The voltage in the resonant frequency
band of resonator and diaphragm has been measured
and charged. We check up the changing a voltage
according to the number of piezoelectric materials
(PVDF) and the shape.
As a result, the cantilever type of PVDF generates
higher voltage than the circular type of one. The more
the number of PVDF arrays on diaphragm, the more
the voltage generates in PVDF. A maximum voltage
is 46 Vp-p, when resonant frequency is 400 Hz and the
number of PVDF is two. If array of PVDF is changed,
the voltage can be more generated.
Thus, if the resonant frequency of the resonator,
the diaphragm and the piezoelectric materials are
match, it would be the ideal environment. For the
future work, the improvement of the circuits for
storing generated electric energy on piezoelectric
elements is executed for an applicable energy
converting system.
327
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