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Flow-Driven Triboelectric Generator for Directly Powering
a Wireless Sensor Node
Shuhua Wang, Xiaojing Mu, Ya Yang,* Chengliang Sun, Alex Yuandong Gu,
and Zhong Lin Wang*
Urbanization, major influx of people from rural areas to the
cities, marks one of the most significant global trends in the 21st
century.[1–3] This drastically increased logistic burden demands
smarter ways to manage complexity, increase efficiency, reduce
expenses, and improve quality of life, or “smart city.” As the
backbone of the implementation of the “smart city” concepts,
wireless sensor networks/nodes have attracted increasing attention in the past decade due to the continuous development of
intelligent cities.[4] The conventional wireless sensor nodes are
usually powered by an external power source such as a Li-ion
battery. This has forced all sensors to exercise painful trade-off
between costly Li-ion battery replacements and data transmission duty cycle. Therefore, a perpetual power source, such as an
energy harvester, is the main bottleneck preventing and the ideal
solution for large area adoption of many “smart city” concepts.
Wireless gas meter reading is an integral part of the “smart city”
concept due to the prevalence of natural gas in modern metropolitan life as well as demand ventilation control system that is
indispensable to people’s daily life. Scavenging fluid mechanical
energy in the environments such as metering system and ventilation control system shows its promising features prior other
energy scavenging solutions. Several methods to harvest flowdriven mechanical energy have been demonstrated,[5–8] where
most of them were based on the piezoelectric effect. However,
the main limitation is that the output of the piezoelectric generators was rather limited so that it is not powerful enough to
directly drive most of the wireless sensor nodes.
Currently, triboelectric generators (TEGs) have been invented
to scavenge mechanical energy from irregular impacts, rotations, and vibrations.[9–12] The mechanism is based on the
contact/separation between two triboelectric materials to drive
the electrons flow in the external circuit due to the coupling
between triboelectrification and electrostatic induction. In our
previous investigations, the TEG has been utilized to directly
S. Wang,[+] Prof. Y. Yang, Prof. Z. L. Wang
Beijing Institute of Nanoenergy and Nanosystems
Chinese Academy of Sciences
Beijing 100083, P. R. China
E-mail: [email protected]
Dr. X. Mu,[+] Dr. C. Sun, Prof. A. Y. Gu
Institute of Microelectronics
Agency for Science, Technology and Research (A*STAR)
Singapore
Prof. Z. L. Wang
School of Materials Science and Engineering
Georgia Institute of Technology
Atlanta, Georgia 30332, USA
E-mail: [email protected]
[+]S. Wang and X. Mu contributed equally to this work.
DOI: 10.1002/adma.201403944
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light up tens of commercial light-emitting diodes (LEDs) and
powering some motion sensor systems,[13–15] suggesting that
the TEG may also have the potential applications for the flowdriven mechanical energy harvesting to directly power the wireless sensor nodes. However, some crucial problems need to
be addressed for the TEG to scavenge flow-driven mechanical
energy in the gas tubes for driving wireless sensor nodes. First,
the TEG needs to work under the gas flow condition, which
requires the special design for the device structures and materials. Second, the cross-sectional dimensions of TEG should
be strictly controlled for the available space inside a gas tube
with the diameter of smaller than about 5 cm. Another problem
is that the produced energy from TEG needs to be enough to
directly power the wireless sensor nodes.
In this paper, we report a TEG that can scavenge flow-driven
mechanical energy for directly powering a wireless sensor node
for the first time. The mechanism of TEG is based on the flowdriven vibration of a Kapton film in an acrylic tube, which can
induce the periodic contact/separation between a triboelectric
polytetrafluoroethylene (PTFE) film and an Al electrode on the
Kapton film. As a result, the triboelectric charges can be transferred between two electrodes, resulting in the flow of electrons
in the external circuit as an alternating current. By systematically investigating the output of TEGs with different dimensions, the device with a size of 22 mm × 10 mm × 67 mm has
the best output performance, where it delivers an open-circuit
voltage up to 400 V, a short-circuit current of about 60 µA, and a
corresponding output power of about 3.7 mW under an external
load of 3 MΩ, which can directly light up tens of commercial
LEDs. The vortex shedding effect has been used to explain the
flow-driven vibration of Kapton film in the acrylic tube, where
the theoretical calculation is consistent with the experimental
result with the vibration frequency of about 155 Hz at the air
flow speed of about 7.6 m/s. The short-circuit current (Isc) of
device can be enhanced to about 0.15 mA by integrating ten
devices connected in parallel, which can be utilized to light up
ten spot lights to provide sufficient illumination for reading
printed text in complete darkness. By using a transformer and a
power management circuit, the TEG can produce a continuous
direct current (DC) source with a constant voltage of about
1.8 V for directly and sustainably powering a wireless sensor
node with an inductor signal frequency of about 436 MHz. This
work is an important progress toward the practical applications
of TEG for harvesting the flow-driven mechanical energy for
directly powering wireless sensor nodes.
Figure 1a illustrates a schematic diagram of the fabricated
device that consists of two PTFE films on the Al electrodes at
the top and bottom of a acrylic tube, and a Kapton film with
the Al electrodes deposited on its surfaces. One side of the
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Figure 1. a) Schematic diagram of TEG. b) Photograph of a fabricated TEG. c) The cross-sectional photograph of a fabricated TEG. d) Sketches that
illustrate the electricity generation process in a full cycle.
Kapton film was fixed on an acrylic sheet at the middle of the
end surface of the tube, where the other side of the Kapton
film is freestanding. The air flow driven vibration of Kapton
film can induce the contact/separation between the PTFE and
the Al electrodes, where both TEGs can work in the acrylic
tube. In this study, the PTFE and the Al film were used as the
triboelectric materials. Many other triboelectric materials can
also be used to fabricate the TEG.[16] As shown in Figure 1b,c,
a typical as-fabricated device has the inner dimensions
22 mm × 10 mm × 57 mm, where the thickness of the acrylic
tube is about 2 mm.
Adv. Mater. 2015, 27, 240–248
The mechanism of the device is schematically depicted in
Figure 1d. At the original state before the contact of the PTFE
film and the Al electrode, there is no electric output for both
the TEG 1 and TEG 2. Once the Al electrode is in physical contact with the top PTFE film, electrons are transferred from Al
into PTFE since PTFE is much more triboelectrically negative
than Al according to the triboelectric series.[17] However, no
electron flow occurs in the external circuit since the produced
triboelectric charges with opposite polarities are fully balanced.
The produced negative triboelectric charges in this process can
be preserved on the PTFE film surface due to the nature of the
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insulator.[18] The positive triboelectric charges on the conductive Al electrode can flow through an external load to the Al
electrode on the PTFE film, when these triboelectric charges
cannot be compensated due to the separation between the top
PTFE film and the Al electrode, as illustrated in Figure 1d (3).
This electrostatic induction process can give the output voltage/
current signals for the TEG 1 until the negative triboelectric
charges on the top PTFE are fully screened from the transferred triboelectric charges on the Al electrode, as displayed in
Figure 1d (4). At this state, the contact between the Al electrode
and the bottom PTFE film can produce the negative and positive triboelectric charges on the bottom PTFE film and the Al
electrode, respectively. When the Kapton film was moved back
from the bottom to the top of the device, the electrons flow
from the bottom Al electrode to the middle Al electrode on
the Kapton film for the TEG 2, while the electrons flow from
the middle Al electrode to the top Al electrode for the TEG
1, as depicted in Figure 1d (5). Therefore, when the air flow
vibrates the Kapton film up and down in the acrylic tube, the
electrons can be driven to flow back and forth between the two
Al electrodes in both TEG 1 and TEG 2 due to the electrostatic
induction, producing an alternating current in the external
circuit.
Figure 2a depicts the measured Isc of the TEG 1 under the air
flow speed of about 7.6 m/s, where the output signals were rectified by using a bridge rectification circuit. The rectified Isc of
TEG 1 is about 50 µA, while the measured rectified Isc of TEG 2
is about 30 µA, as displayed in Figure 2b. To obtain the larger
electric output, the rectified output signals were connected in
parallel, as illustrated in Figure 2c. It can be found that there
is no obvious enhancement of the current as compared with
that for the TEG 1, which is associated with the asynchronism for the output signals of the two TEGs. As depicted in
Figure 2d, we measured the charging performance of a 10 µF
capacitor by utilizing the TEG 1, the TEG 2, and the TEG 1//
TEG 2, where the TEG 1//TEG 2 has a much better charging
performance than the individual TEG 1 or TEG 2. The result
indicates that the parallel connection of two rectified TEGs is
an effective method to obtain the better output performance of
the device. Figure 2e presents the rectified open-circuit voltage
of the device, showing that the voltage is about 300 V. Usually,
the effective output power of the TEG depends on the match
with the loading resistance. Once an external load is connected
with the TEG, the output current drops as the load resistance
increases, as displayed in Figure 2f. The output power is equivalent to the Joule heating of the load resistor, which can be calculated as I2R, where I is the output current
and R is the loading resistance. As shown
in Figure 2f, the output power of the TEG
increases in the resistance region from 1 kΩ
to 3 MΩ and then decreases under the larger
loading resistance (>3 MΩ). The maximum
value of the output power reaches 2.35 mW
at a loading resistance of 3 MΩ.
To obtain the largest output power of
the TEGs, the output performances of the
devices with the different dimensions were
systematically investigated. Figure 3a illustrates the height dependence of the Voc and
Isc of the device, showing that both Voc and
Isc are closely related to the height and reach
the maximum value at the height of 10 mm.
Figure 3b displays the corresponding output
current at the loading resistance of 3 MΩ and
the output power of the device, indicating
that the largest power of about 2.35 mW can
be obtained by choosing the device height of
10 mm. The detailed output performances
of the devices with the different heights are
presented in Figures S1–S3 (Supporting
Information). The length of the Kapton film
is also a major factor that determines electric
output of the TEGs. As depicted in Figure 3c,
both the Voc and Isc increase with increasing
the length of the Kapton film, where the
length of the device is about 57 mm.
Figure 2. a) Rectified short-circuit current of the TEG 1. b) Rectified short-circuit current of the
Figure 3d shows the corresponding output
TEG 2. c) Rectified short-circuit current of the TEG 1 and TEG 2 connected in parallel. d) The current and the output power under the difmeasured voltage of a 10 µF capacitor charged by the rectified TEG 1, TEG 2, and the TEG 1
ferent conditions for the length of Kapton
and TEG 2 connected in parallel. e) Rectified open-circuit voltage of the TEG 1 and TEG 2 confilm, indicating that the device has the largest
nected in parallel. f) Dependence of the output current and the corresponding power on the
output power when the Kapton film has the
external loading resistance. The acrylic tube of the TEGs has the inner dimensions of 57 mm
× 22 mm × 10 mm.
same length with the device. The data for the
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Figure 3. a) Dependence of rectified open-circuit voltage and short-circuit current of the device on the device height. b) Dependence of the output
current under the load of 3 MΩ and the corresponding power on the device height, where the length of the device is 57 mm. c) Dependence of
rectified open-circuit voltage and short-circuit current of the device on the length of the Kapton film, where the inner height of the device is 10 mm.
d) Dependence of the measured output current under the load of 3 MΩ and the corresponding power on the length of the Kapton film. e) Dependence
of rectified open-circuit voltage and short-circuit current of the device on the device length. f) Dependence of the output current under the load of
3 MΩ and the corresponding power on the device length, where the inner height of the device is 10 mm. The working frequencies of the g) TEG 1 and
h) TEG 2 under the different flow rates, where the length of the device is 67 mm.
devices with the different lengths of Kapton films are displayed
in Figure S4 (Supporting Information). The length of the device
can also largely influence the output power of the TEGs. As
revealed in Figure 3e, the Voc of the device has the largest value
of about 400 V at the device length of 67 mm, where the corresponding Isc is about 60 µA. By analysis of the output current and the corresponding power under the different lengths
of the devices, the largest output power of the device is about
3.68 mW at the device length of about 67 mm, as shown in
Figure 3f. The data for the devices with different lengths can
be found in Figures S5–S8 (Supporting Information). The
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results indicate that all the three feature sizes of the device
play a key role in achieving the highest output power of TEGs.
We also measured the working frequency of the device under
the different air flow rates by using the time period between
two adjacent output voltage peaks, as presented in Figure S9
(Supporting Information). A linearly rising relationship can be
derived from Figure 3g,h between the working frequency and
the air flow rate, where the working frequency of TEG is about
155 Hz at the air flow rate of about 7.6 m/s.
The working mechanism of the TEG is based on the flowdriven vibration of Kapton film in the acrylic tube to induce the
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periodic contact/separation between the PTFE film and the Al
electrode on the Kapton film. It is of critical importance to understand the mechanism of the flow-driven vibration of Kapton film
in this study. Karman vortex shedding is a well-known phenomenon in fluid dynamics research field.[19–21] It is an oscillating
flow that takes place when a fluid such as air or water flows
past a bluff body at certain velocities, depending on the size and
shape of the bluff body. In this study, as illustrated in Figure 4a,
vortices are created at the back of the buff body and detached
periodically from either side the body, which is due to that the
generated low-pressure vortices always tend to move toward the
low pressure zone on the downstream side of the bluff body.
When a nonrigid mounted structure is attached on the back of
the bluff body, the structure can begin to vibrate with harmonic
oscillations under the air flow condition if the frequency of vortex
shedding matches the resonance frequency of this structure.
Figure 4b presents the model of the fabricated device, where
one side of the Kapton film is fixed on the bluff body, whereas
one side is free. The air flow is guided in the acrylic tube from
one side, split by the bluff body and forces to interact with the
Kapton film, in turn to vibrate the free side of the Kapton film
up and down to beat the walls of the acrylic tube. As a result, the
vibration of the Kapton film can result in the
output of the TEGs. To confirm that the vibration of the Kapton film is due to the vortex
shedding effect, we calculated the frequency of
Karman vortex shedding, where the Karman
vortex has been shed periodically in the wake
of the bluff body. In the calculation, the frequency of Karman vortex shedding can be estimated by the Strouhal number (St), which is
a dimensionless number to describe the oscillating flow mechanism. The St number can be
expressed as
St = f sD / V
Figure 4. a) Schematic diagram of the vortex shedding effect. b) Schematic diagram of one
device dimension. c) The theoretical simulation of the relative deformations and the vibration
frequencies of the Al/Kapton/Al composite film with the first six vibration modes. d) The comparation between the 4th vibration mode in the theoretical simulation and the real deformation
of the Al/Kapton /Al composite film obtained by using a high-speed camera.
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where f s is the vortex shedding frequency,
D is the height of the edge perpendicular to
the streamline for rectangular cross section
of the bluff body, and V is the inflow fluid
speed.[22] Some previous literatures have
discussed the relations between Strouhal
numbers and vortex shedding as well as
the occurrence conditions of the Karman
vortex,[22–24] which pointed out that the vortex
shedding occurs when the breadth to the
height (B/D) ration is in the range of 0.5–4.
Strouhal numbers under different breadth
to the height (B/D) ratios for rectangular cylinders were also investigated in detailed as
well.[23,24] In this study, the calculated breadth
to the height (B/D) ratio is 2.5 by using the
height D of 2 mm and breadth B of 5 mm,
which corresponds to the Strouhal number
of 0.04 from previous experiment works.[25]
By substituting the Strouhal number and the
inflow fluid speed V of 7.6 m/s, the vortex
shedding frequency of about 152 Hz can be
obtained, which is consistent with the measured working frequency of the TEGs (about
155 Hz) in Figure 3g,h.
To understand which vibration mode of
the Al/Kapton/Al composite film has the
main contribution to the output of the TEGs,
the constraint mode analysis was conducted
in the finite element method software CoventorWare, where the Kapton film has a width
of 22 mm, a length of 62 mm, a thickness of
50 µm, a Young’s module of 2.5 GPa, and the
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Al film has a thickness of 300 nm. Figure 4c illustrates the mode
shapes and the corresponding frequencies of the first six modes
under the wind speed of 7.6 m/s, where the 4th vibration mode
of the Al/Kapton/Al composite film has the frequency of about
155.27 Hz, which is consistent with the measured working frequency of the TEGs (about 155 Hz). Moreover, Figure 4d displays that the vibration shape for the simulated 4th-order mode
agrees well with the real deformation of the Al/Kapton/Al composite film obtained by using a high-speed camera, which can
also be seen in Movie 1 (Supporting Information).
To increase the output of the TEGs further, more devices were
connected in parallel after the output signals were rectified.[25]
As presented in Figure 5a, the integrated two devices can deliver
the Isc of about 70 µA. The fabricated devices exhibit the stable
output performances. Since the measured working frequency
of the TEG is about 155 Hz under the air flow rate of 7.6 m/s,
the stable output current in 60 s clearly displays little drops
after 9000 cycles of operation, as displayed in Figures 2c and 5a.
The output current can be increased to 150 µA by integrating
ten devices, as displayed in Figure 5b. Figure 5c exhibits the
dependence of the output current on the device number. It can
be found that the output current increases with increasing the
device number, where the corresponding data is presented in
Figure S10 (Supporting Information). As shown in Figure 5d,
the charging performance can be effectively enhanced by
increasing the integrated devices. Moreover, the integrated
Figure 5. Rectified short-circuit current of a) two and b) ten TEGs connected in parallel. c) Rectified short-circuit current under the different TEGs connected in parallel. d) The measured voltage of a 10 µF capacitor charged by different TEGs connected in parallel. e) Photograph of ten spot lights that
can be directly powered by the ten rectified TEGs connected in parallel. f) Photograph of the printed text on a paper illuminated by the ten spot lights
in complete darkness by using the ten rectified TEGs connected in parallel.
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devices can be also utilized as a power source for powering
some electronics without using a storage unit. As illustrated in
Figure 5e, the integrated ten devices were directly connected to
regular ten light bulbs, which can be simultaneously lighted up
(Movie 2, Supporting Information). As visualized in Figure 5f,
the obtained sufficient illumination can be used to read printed
text in complete darkness (Movie 3, Supporting Information).
In fact, the produced electricity energy of one device is enough
for directly powering the ten light bulbs by scavenging the
human mouth blowing-induced air flow through the device
(Movie 4, Supporting Information). Moreover, 83 commercial
LEDs can be simultaneously lighted up by using one device
under the air flow speed of about 7.6 m/s (Movie 5, Supporting
Information).
Before the TEGs can be utilized for directly powering the
wireless sensor node, there are two issues that needs to be
addressed. First, the TEGs usually have the high output voltages but relatively output current. As a result, the large output
impedance can largely affect the applicability of the TEGs as
a power source for powering the wireless sensor nodes that
require the low input voltage and relatively high input current.
Second, the output signals of the TEGs exhibit the features
of pulse peaks, resulting in the fluctuation of the output. As
shown in Figure 6a, a transformer can be used to reduce the
Figure 6. a) Schematic diagram of a TEG to power a wireless sensor node by using a transformer and a power management circuit. The measured
b) open-circuit voltage and c) short-circuit current at the transformer output port. d) Photograph of the self-powered system including a TEG in an
acrylic tube with the inner diameter of about 31 mm, a transformer, a power management circuit, and a wireless sensor node. e) Output voltage of
the system reaches a constant value of 1.8 V in about 4.9 s. Energy spectra of the received RF signals in f) cable wire transmission and g) wireless
transmission.
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output voltage and increase the output current of the TEGs,
and a power management circuit can deliver a DC output at a
constant voltage.[26] After that, the power-supplying system was
connected with the wireless sensor node, which can be powered if the produced electricity energy by the TEGs is enough.
Figure 6b displays that the measured Voc of a TEG with the
transformer was decreased to about 12 V, while the Isc of the
TEG with the transformer was increased to about 0.9 mA, as
shown in Figure 6c. Figure 6d presents an optical image of
the self-powered wireless sensor system including a power
management circuit, a wireless sensor node, and the TEGs in a
transparent gas tube with the inner diameter of about 31 mm.
As illustrated in Figure 6e, the power management circuit can
deliver a DC output at a constant voltage of 1.8 V after the TEG
starts to work in about 4.9 s. The transfer efficiency from the
high-voltage output of TEG to the stable low-voltage power
supply should be smaller than 50% due to the energy loss in
the transformer and the power management circuit. Under the
continuous uniform DC power provided by the TEG, the wireless sensor node (a homemade wireless temperature sensor
with the working frequency of about 436 Hz) can be directly
powered and sustainably work without the use of the Li-ion
batteries. Once the TEG delivers the power to the wireless
sensor, it can send out radio frequency (RF) signal with specific
frequency, where a spectrum analyzer was employed to receive
this signal and demonstrate its energy level. Figure 6f shows
that the received signal through a cable wire transmission has a
relative intensity of about 70 dBm at the emitting frequency of
436 MHz, which can also be observed in Movie 6 (Supporting
Information). As depicted in Figure 6g, the received signal
through wireless transmission has the frequency peak of about
436 MHz and the relative intensity of about 45 dBm as compared with the baseline of the environmental noise (–100 dBm),
which can also be seen in Movie 7 (Supporting Information).
Obviously, the signal frequency is the same but the energy level
of the signal in wireless transmission line drops more than
20 dBm as compared with that in cable wire transmission
due to the energy loss in the wireless emitting process. These
results indicate that the wireless sensor node can sustainably
work by using the TEG-based power-supplying system.
In summary, we have demonstrated a newly designed TEG
that can harvest the flow-driven mechanical energy for directly
and sustainably powering a wireless sensor node. The mechanism of TEG is based on the flow-driven vibration of a Kapton
film to induce the periodic contact/separation between a PTFE
film and an Al electrode due to the vortex shedding effect. The
output performances of the TEGs with the different dimensions
have been systematically investigated, indicating that the largest
output power of one TEG is about 3.7 mW under an external
load of 3 MΩ. The output of the devices can be enhanced further by integrating more TEGs in parallel, where the produced
energy can be utilized to directly light up ten spot lights to provide sufficient illumination for reading printed text in complete
darkness. By using a transformer and a power management circuit, the TEG can produce a continuous DC power source for
directly and sustainably powering a wireless sensor node with
the working frequency of about 436 MHz. This work may push
forward a significant step toward the practical applications
of TEG for scavenging the flow-driven mechanical energy to
directly power the wireless sensor nodes.
Experimental Section
Fabrication of the TEG: The TEG consists of a PTFE film and two
Al electrodes. An acrylic tube was fabricated by using a laser cutting
machine to obtain four acrylic sheets. A small acrylic sheet as the bluff
body with dimensions of 22 mm × 5 mm × 2 mm was fixed at the end
of the acrylic tube. Two PTFE films with the Al electrodes were attached
onto the top and bottom of the acrylic tube, respectively. A Kapton film
with the Al electrodes on the two sides was used as the vibration film
in the tube. The periodic contact/separation between the PTFE and
Al can be realized by the air flow driven vibration of the Kapton film
in the acrylic tube, resulting in the output voltage/current signals of
TEGs. There are two TEGs in one tube, where they can scavenge the
mechanical energy at the same time.
Measurement of the Fabricated Devices: The output current signals of
the TEG were measured by a low-noise current preamplifier (Stanford
Research SR570). The output voltage signals of the TEG were connected
with an electrostatic voltmeter. The vibration frequencies of the Kapton
film in the device can be obtained by the analysis of the measured output
signals by using a digital phosphor oscilloscope. The produced wireless
signals by the homemade wireless sensor were measured by using a
spectrum analyzer. A commercial power management circuit board was
utilized to produce the constant DC output. A temperature sensor was
used in the wireless sensor node and the power consumption of the
whole system including the RF transmission is smaller than 100 µW.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
Thanks for the support from the NSFC (NO. 51472055) and the
“Thousands Talents” program for pioneer researcher and his innovation
team, China. Patents have been filed based on the research presented
here. Prof. Y. Yang, Prof. Z. L. Wang, and S. Wang designed and
fabricated the TEGs. Prof. Y. Yang and S. Wang measured the output
performance of the TEG and prepared the manuscript by the analysis
of the experimental data. Prof. A. Y. Gu and Dr. X. Mu designed and
fabricated the wireless sensor nodes. Dr. X. Mu did the finite element
simulation via ABAQUS. Dr. C. Sun did the fluid theoretical study about
the vortex shedding. All the authors revised the manuscript.
Received: August 27, 2014
Revised: October 6, 2014
Published online: November 6, 2014
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