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A high-efficient breeze energy harvester utilizing a full-packaged triboelectric nanogenerator based on flow-induced vibration
Journal Pre-proof A high-efficient breeze energy harvester utilizing a full-packaged triboelectric nanogenerator based on flow-induced vibration Qixuan Zeng, Yan Wu, Qian Tang, Wenlin Liu, Jun Wu, Ying Zhang, Guoying Yin, Huake Yang, Songlei Yuan, Dujuan Tan, Chenguo Hu, Xue Wang PII:
S2211-2855(20)30081-1
DOI:
https://doi.org/10.1016/j.nanoen.2020.104524
Reference:
NANOEN 104524
To appear in:
Nano Energy
Received Date: 11 December 2019 Revised Date:
7 January 2020
Accepted Date: 20 January 2020
Please cite this article as: Q. Zeng, Y. Wu, Q. Tang, W. Liu, J. Wu, Y. Zhang, G. Yin, H. Yang, S. Yuan, D. Tan, C. Hu, X. Wang, A high-efficient breeze energy harvester utilizing a full-packaged triboelectric nanogenerator based on flow-induced vibration, Nano Energy, https://doi.org/10.1016/ j.nanoen.2020.104524. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.
BRIEFS A full-packaged triboelectric nanogenerator based on flow-induce vibration effect can convert low-speed air flow energy into stable electrical output without additional process, even in rainy days. TOC
A high-efficient breeze energy harvester utilizing a full-packaged triboelectric nanogenerator based on flow-induced vibration Qixuan Zeng †, Yan Wu †, Qian Tang, Wenlin Liu, Jun Wu, Ying Zhang, Guoying Yin, Huake Yang, Songlei Yuan, Dujuan Tan, Chenguo Hu, Xue Wang*
Department of Applied Physics, State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing, 400044, P. R. China. †
These authors contributed equally to this work.
*Corresponding author: [email protected]
Keywords: flow-induced vibration, triboelectric nanogenerator, breeze energy harvesting, full-packaged
Abstract Wind energy is an important and promising renewable energy for the sustainable development of society. However, considering the limitations of device structures, collection of low-speed wind energy by triboelectric nanogenerators (TENGs) still faces some challenges such as huge energy loss, destructive friction wear and restrictions of onset wind speed. To solve these problems, we have developed a novel-designed TENG based on flow-induced vibration (FIV) effect (FIV-TENG). Distinguishing from previous wind-driven TENGs, the TENG components of this
device are packaged in a bluff body and connected with a cantilever beam. This unique design not only separates the TENG units from the wind-driven part to free from environmental disruptions, but also avoids the great rotation resistance and friction wear in the ordinary designed TENG-based wind energy harvesters. Benefiting from the novel configuration, this FIV-TENG can be easily triggered by wind and delivers an excellent electrical output. The output performance of the as-fabricated device is systematically investigated under wind speeds ranging from light breeze to moderate gale, and a theoretical model is constructed to further understand the working mechanism and oscillating behaviors of the FIV-TENG. Additionally, structural parameters of the device have been optimized to achieve an optimal energy production, and the stability when working in harsh environment is also investigated. The creative device structure realizes the superior robustness and reliability, and also provides an efficient approach to realizing practical wind energy harvesting and applications.
1. Introduction Wind is deemed to be one of the most promising renewable energy resources with advantages of wide distribution, clean nature and environmental friendliness [1, 2]. Wind energy is converted to electrical energy by the wide use of conventional electromagnetic generators (EMG) on the basis of electromagnetic induction principle and turbine structure. Although it is an efficient and reliable power generation technology, EMG is often criticized by its disadvantages of bulky volume, massive
weight, high cost, and low efficiency at low wind speeds [2, 3]. Therefore, new technology for harvesting widely-distributed low-frequency breeze energy with reduced device volume and weight, lower cost and increased energy production is highly desired and still poses challenges. As an emerging energy-conversion technology, triboelectric nanogenerator (TENG) exhibits great potential as a wind energy harvester due to its easy fabrication, lightweight, good scalability and low cost [4-8]. With regard to these advantages, some TENGs designed to scavenge wind energy have been reported lately, which usually adopt rotational configurations based on freestanding mode or flutter-driven structures utilizing contact-separation mode [9-12]. These TENGs with ordinary designs demonstrate ideal performances to harvest miniature wind energy. However, until now, they are not widely used in application domains, due to some obstacles such as severe requirement on durability of friction materials, and restrictions of cut-in wind speed and working environment [5, 13-16]. Moreover, according to the literature, the global average wind speed is 3.28 m/s at the height near the ground [10, 17, 18]. Therefore, it is of great significance to develop feasible TENGs for collecting breeze energy in that wind-speed range with desirable output performance, long-term stability. Flow-induced vibration (FIV) effect is a kind of widespread physical phenomenon of fluid-structure interaction. Elastically mounted, rigid, bluff bodies that are long in one direction perpendicular to a free stream are susceptible to this phenomenon [19, 20]. FIV is always treated as a negative effect in the engineering field because large
transverse vibrations may lead to fatigue failure of related facilities [20-22]. On the contrary, the power of FIV contains vast amount of kinetic energy which can be converted into electricity. Although some related research of FIV for scavenging energy from air flows and water flows have been reported [23, 24], relevant studies on FIV driving TENGs for flow energy harvesting are still very rare. In this study, to realize a high-efficient and practical wind energy harvester, we have developed a novel TENG driven by FIV effect (named as FIV-TENG). In this structure, two identically designed TENG units were mounted face-to-face at the end of a steel cantilever beam and sealed in a unique trapezoid cylinder shell. This specific design can amplify the system vibration, enhances the TENG electrical production at a relatively low wind speed and protects the device against the disturbance of external environment. A comprehensive study of the impacts of structural parameters on the output performance of the FIV-TENG was performed experimentally, and the electrical output of the as-fabricated device was systematically studied under wind speeds ranging from light breeze to moderate gale range. At a low wind speed of 2.9 m/s, open-circuit voltage (VOC)，short-circuit current (ISC) and transfer charge (QSC) of one TENG unit could reach around 270 V, 7.6 µA and 98 nC, respectively. The maximum output power of FIV-TENG could reach 0.9 and 1.3 mW at a load resistance of 44 MΩ when wind speed was 2.9 and 7.8 m/s, respectively, implying the FIV-TENG could demonstrate a superior output performance than the previous wind energy harvesters based on TENGs [13, 25, 26]. Furthermore, the stability of FIV-TENG working in a harsh environment such as rainy condition was also
investigated. Electrical energy converted by the FIV-TENG could be stored or directly used for powering the distributed sensors or smart devices, and could also be utilized for self-powered wind speed monitoring [27]. This work demonstrates an efficient and reliable strategy for capturing breeze energy, and marks a step toward practical application of TENG for wind energy harvesting.
2.Results and discussion
2.1. Structural design and working principle of the FIV-TENG
Fig. 1a illustrates the design inspiration of the FIV-TENG. It is a common phenomenon in nature that trees will sway when a breeze blows over, which implies treelike structure can capture the energy of breeze motion. In order to demonstrate the feasibility of this model, we have designed an FIV-TENG and its holistic structure is schematically showed in Fig. 1b. In general, the main frame of the device is made by a spring steel sheet with a sealed trapezoid polyethylene terephthalate (PET) cylinder mounted on the end, and digital pictures of the whole device with different view angles are displayed in the insets of Fig. 1b. In this structure, the spring steel sheet acts as a tree trunk to supply an elastic restoring force to maintain the system equilibrium. Meanwhile, the sealed trapezoid shell serves like the external contour of tree and plays a dual role: One is to effectively capture the wind energy by means of FIV effect, and the other is to isolate the interference of the external environment, such as humidity change, dust contamination and so on. Moreover, the energy
converting components of this system are two same-designed TENGs, which are symmetrically mounted at the end of the spring steel sheet, acting as the tree branches. The manufacturing process of the TENG components can be summarized as follows: Firstly, aluminum (Al) foils adhering to both sides of the middle acrylic sheet act as the contact electrodes of TENGs. Then, polytetrafluoroethylene (PTFE) film, Al foil and PET film were sequentially attached to each other layer-by-layer to form a sandwich-structured film, and fixed on the inner surfaces of the two side acrylic substrates. Since the width of composite film (PTFE/Al/PET) was slightly wider than the distance between mounting holes of the acrylic sheet (as shown in Fig. S1a), it could shape an arced structure here (see the real shot diagrams in Fig. S1b and S1c), which was beneficial to store potential energy and further facilitated the contact-separation activity of the electrodes. Additional details of the manufacturing processes are presented in the Experimental section. Furthermore, an idea of array assembling of FIV-TENGs is shown in Fig. 1c. As demonstrated in Fig. 1d, with the further scale-up of such array system, the network of FIV-TENGs can generate huge power output, so that it can serve as a robust power supply [28]. FIV effect is a double-edged sword, which can cause destructive vibrations as well as supplying valuable kinetic energy. Herein, we aim to study and develop a simple and practical device for converting wind energy into electricity based on the FIV effect. According to the fluid-structure interaction theory, when a blunt body placed in a fluid flow field, large-scale vortices can create an unsteady fluid loading on the body surface and further induce periodic lift forces to act on the cylinder, which can
eventually cause vibrations to the system [19, 29]. Here, we utilize COMSOL platform to simulate the vortex distribution surrounding the FIV-TENG system when airflow speed is 2.9 m/s, and results are depicted in Fig. 2a. The interval time between Fig. 2a-I and Fig. 2a-II is 0.5 s. It can be seen, as the flow field shifts, vortex velocity surrounding the trapezoid body will change and induce a changing fluid force loading on the body surface. Fig. 2b shows the detailed motion status and calculated pressure change process of the FIV-TENG system. At the very beginning, the device is at its equilibrium position (Fig. 2b-III, state 1). Then, when the wind blows across the trapezoid body, the pressures loading on both bevel sides (Fig. 2b-I, point A and B) will achieve amplitude value alternatively (as shown in Fig. 2b-II). Due to the flexibility of the spring steel sheet and the cantilevered structure of this system, the device will take a vibrational activity. A complete vibration cycle occurs when the body moves from one extreme position (Fig. 2b-III, state 2) to the other extreme (Fig. 2b-III, state 3), and moves back again. To further verify the motion mechanism, distributions of the pressure field are also calculated and results are illustrated in Fig. S2a, and the pressure field of FIV-TENG array is also simulated as shown in Fig. S2b, which is consistent with the trend of a single FIV-TENG. Apparently, pressure loading on both bevel sides of the cylinder alternatively reaches the maximum value with time change. As long as wind blows, the FIV-TENG will demonstrate a reciprocating oscillation, and the TENG components sealed in the cylinder will keep taking the contact-separation motion so as to generate a continuous electrical output.
The working principle of FIV-TENG is based on a rotational contact-separation mode, and its theoretical foundation is the coupling effect of electrification and electrostatic induction [30, 31]. The detailed charge generation and transfer process are schematically depicted in Fig. 2c. Since the two TENG units have the same structure and consequently demonstrate a same power generation process, we take one TENG to expound. When the device begins to capture wind energy and demonstrates vibration, a contact-separation activity will take place between the PTFE film and the top Al electrode. According to the triboelectric series [32, 33]. After the first contact (state I), the PTFE surface would be negatively charged, and the Al film would have an equal amount of positive charges [34, 35]. Subsequently, the two electrodes are separated at a certain angle by the external force (state II), and a potential difference will form between them. Therefore, the induced electrons will flow through the external circuit from the top Al electrode to the bottom Al electrode. When the angle between the two electrodes reaches the maximum (state III), the positive triboelectric charges on the bottom electrode will be completely neutralized by the induced electrons. Then, the external force drives two electrodes to gradually move back towards each other (state IV), and a current in the opposite direction would be produced. To demonstrate the working mechanism more clearly, the corresponding distribution of electric potential under the open-circuit condition is simulated via the finite element method [36], and results are illustrated in Fig. 2d.
2.2 Structural optimization of the FIV-TENG
It is interesting that the architecture of the electrode shows a significant impact on the output performance of TENGs in this FIV-TENG system. We firstly fabricated a simple device with a flat electrode configuration, where the PTFE/Al/PET composite film tightly adhered to the side acrylic substrate (as shown in the inset of Fig. 3a). To characterize the performance of the device, the electrical output was tested. As can be seen in Fig. 3a, the QSC gradually increased from 32 to 52 nC at the very beginning. However, after 180 seconds, the value reduced drastically to around 5.5 nC, which revealed that the contact-separation activity between two electrodes was no longer produced. The key reason for this phenomenon was that the moment of inertia could not overcome the strong electrostatic attraction between the Al electrode and PTFE film. To solve this problem, we further designed an arched structure, in which the PTFE/Al/PET composite film was mounted on the acrylic substrate with a certain radian (see the inset of Fig. 3b). Due to the flexibility of the composite film, the arched structure could present elastic deformation when it was squeezed, which could store elastic potential energy to promote the separation between Al electrode and PTFE film. Moreover, the arched structure, subjected to the deformation, could increase the effective contact area between two friction materials, so as to induce an improved output performance. It is interesting to know whether the variation of arch curvature could influence the system output performance. Therefore, we compared four arched structures with different curvatures to define an optimized design. As shown in the inset of Fig. 3c, for simplifying structure as well as facilitating analysis, the curvature was assumed to
be equal to the maximum distance from the acrylic sheet to the vertex of the laminated film (H), which varied from 0.5 mm, 1 mm, 1.5 mm to 2 mm, respectively. According to their electrical outputs displayed in Fig. 3b and c, the FIV-TENG with the H of 1 mm showed the best output performance compared with others. This result could be attributed to the change of the effective contact area between Al electrode and PTFE film. It has been recognized that the degree of materials’ mechanical deformation defines the device effective contact area and finally affect the output performance [37, 38]. When the curvature of the PTFE/Al/PET film was relatively small, system inertia could supply adequate external force to squeeze the composite film, and a larger contact area between two friction materials would be achieved at a larger curvature. However, when the film curvature was large enough, the force provided by the device oscillation could not overcome the huge elastic potential energy, resulting a less sufficient contact between the two electrodes. In order to further explore the intrinsic factors that influence the output performance of FIV-TENG, the detailed kinetic model of the system was investigated. In this work, the TENG components demonstrate a rotating contact-separation motion, and the larger rotational kinetic energy can help to induce a larger elastic deformation [39], which can finally increase the effective contact area between the two friction materials. Rotational kinetic energy Erotational and moment of inertia I can be separately expressed as:
=
=
(1) (2)
where
represents the angular velocity of the electrode. According to the above
equations, a larger effective contact area of FIV-TENG can be achieved by increasing and . Obviously, the value of radius
is proportional to rotator mass
and rotation
. Here, three acrylic boards with different weights (1 g, 2 g, and 3 g) served
as the additional objects adhering to the side acrylic substrates to modulate the rotator weight. As shown in Fig. S3, the acrylic boards were fixed at three different positions to adjust the mass center of the rotator, where the distances to the pivot point were r1 = 0 mm, r2 = 15 mm and r3 = 30 mm, respectively. As illustrated in Fig. 3d, the QSC increases with the growing of additional weight and the rotation radius, which matches well with the theoretical prediction. The maximum QSC appears at r3 = 30 mm. Compared with the original value (90 nC) without additional objects, the change of QSC is defined as ∆Q, and for the three different loading weights, maximum ∆Q can achieve 7 nC, 12 nC and 17 nC, respectively. Although the maximum QSC is realized at the heaviest additional objects, the increase of electrode weight alone is not a beneficial and harmless strategy, because it will directly influence the other important parameter
, which will further affect the
oscillation performance of FIV-TENG system in the open space. In this work, the as-fabricated device can be simplified to be a spring-damper-mass system [20], as depicted in Fig. 3e. The vibrating direction of the cylinder (parallels to the -direction) is perpendicular to the flow direction (along with
-direction), and the
system oscillator motion can be presented as [40]: +
+
=
(3)
where mt is the total mass of the oscillator, c denotes the viscous damping, k represents the cantilever stiffness, Y stands for the displacement of the oscillatory system in the -direction,
and
are velocity and acceleration, respectively; F is
the fluid force imposed on the structure. According to the equation, at a certain F, the changes of
and
have significant impacts on the oscillation characters of the
system, including vibration frequency and amplitude, which will finally influence the FIV-TENG output performance. To further explore the relationship between mt, k and the oscillation activity of the as-fabricated device, a unique measurement system was designed as exhibited in Fig. S4a. Briefly, the test system consists of a magnet mounted on the bottom of a sealed trapezoid cylinder, and a copper coil (turns = 126) placed right below the magnet. Once the FIV-TENG begins to oscillate, there will generate an electrical output based on the electromagnetism induction principle, and the current waveforms in time domain can directly reflect the oscillation frequency of the tested device. Herein, different weight objects ranging from 5 g to 30 g were used to alter cantilever stiffness
. The
was adjusted by changing the cantilever length (L) as shown in
Fig. S2c, and three different lengths (L1 = 30 mm, L2 = 45 mm and L3 = 60 mm, respectively) were utilized for measurements. The current signals of copper coil were monitored at a wind speed of 2.9 m/s, and corresponding results are depicted in Fig. S5b-d. With the utilization of Fast Fourier Transform (FFT), signals in time spectrum can be converted to a frequency spectrum, so that the results of oscillation frequency versus
and
can be obtained. As illustrated in Fig. 3f, a higher vibration
frequency, which is beneficial to realize a higher current output of the FIV-TENG device, can be obtained at a smaller
and shorter cantilever length. According to
the above results, we can draw a conclusion that the increase of rotator weight can improve the QSC, but paradoxically, the higher weight will reduce the system oscillation frequency. Therefore, the ratio of ∆Q to ∆m (additional weight) is proposed to quantify the output improvement efficiency, and the maximum value is 7 when ∆Q = 7 nC and ∆m = 1 g. Accordingly, the follow-up tests and applications of FIV-TENG are carried out under the ∆m = 1 g and r3 = 30 mm. As to the impact of cantilever length, the output performance of FIV-TENG has been systematically measured at wind speeds from 2.9 to 7.8 m s−1, which are between light breeze and moderate gale in the open space [2]. From Fig. 4a-c, it can be seen that electric outputs of FIV-TENG gradually increase with wind speed, resulting from the boosting of system vibration frequency as well as the angular velocity of the electrode. Notably, the difference in cantilever length has a significant impact on the device performance. Although the FIV-TENG shows the optimal current output at L1 = 30 mm, as shown in Fig. 4c, which matches well with the above discussion, the VOC and QSC signals suffer from nonlinearity. By comparison, L = 45 mm is defined to be the optimal cantilever length for the follow-up measurements due to its excellent performance as well as linear electrical output. Based on the excellent linear correlation between wind speed and VOC as depicted in Fig. S5a, a self-powered wind speed sensor was implemented, which can make an accurate measurement of wind speed in real time as shown in Video S5. Interestingly, the ISC
value can reach a saturation point at the wind speed above 6.8 m/s (red curve in Fig. 4c), because the speed of charge transferring no longer increases as shown in Fig. S5b. The detailed output performances of L = 45 mm are depicted in Fig. S5c-e. As can be seen, at a low wind speed of 2.9 m/s, VOC, ISC and QSC of one TENG unit can reach near 270 V, 7.6 µA and 98 nC, respectively. Moreover, to verify the stability of FIV-TENG in wet atmosphere, the device was tested under both normal and rainy (with a rainfall intensity of 33.9 mm per hour, which could be defined as heavy rain) conditions. As shown in Fig. S4d and Video S1, no distinct dissimilarities of the VOC values and the capability of lighting LEDs can be observed under two different conditions, implying the as-fabricated FIV-TENG has a desirable stability even in a harsh environment. To comprehensively characterize the FIV-TENG as a power source, the output voltages and currents on varied load resistances have been measured. As illustrated in Fig. 4e and 4f, the voltage increases with the increasing load resistance while the current presents an opposite trend. When the wind speeds are respectively 2.9 and 7.8 m/s, the maximum power can separately reach up to 0.9 and 1.3 mW with the load resistance of 44 MΩ. The above results prove that the FIV-TENG manifests excellent performance to harvest low-speed airflow energy.
2.3 Practical application of the FIV-TENG To demonstrate the capability of the FIV-TENG as a power supply, a commercial electric fan was applied to serve as the wind source, and as-fabricated FIV-TENG was utilized for powering various electronics under a simulated air flow. As shown in Fig.
5a, the particulars of charging commercial capacitors with different capacities by one TENG component have been recorded at the wind speed of 7.8 m/s, and the inset is the corresponding equivalent circuit. It takes 2 s to charge a 2.2 µF capacitor to 2 V, while charging to the same voltage in a 22 µF capacitor takes 43 s. Notably, in this FIV-TENG system, contact-separation motions of two TENG components are asynchronous, where one contacts while the other separates, so the QSC can superimpose. To prove that, a 47 µF capacitor was charged from 0 to 0.4 V by one TENG unit and two TENG components connected in series, respectively. From Fig. 5b, it can be observed that the charging times are 17 s and 9.4 s, respectively, which indicates that the charging speed is nearly doubled by the integration two TENG units. The QSC waveforms of every single TENG unit and two TENGs connected in series further confirm the output superimposition, as depicted in the inset of Fig. 5b. Moreover, the electric output of TENGs can be utilized for powering sensors, especially the nodes of Internet of Things (IoT) [27]. It is known that using temporary batteries or connecting power cables to drive wide-range installed sensors will require an astronomical budget. Thus, the FIV-TENG which can efficiently scavenge wind energy shows an enormous application potential in this field [41]. Here, a commercial capacitor (33µF) serving as an energy storage and buffer unit was employed to connect with the FIV-TENG for powering sensors at a wind speed of 4.8 m/s. A flam sensor and a rain-drop detector were separately driven by sole TENG and the whole FIV-TENG system because of their different power consumptions. The charging and discharging processes of the capacitor to power these two sensors are presented in Fig.
5c and d, respectively. As can been seen, a voltage drop appears when the measured sensor starts operating, and the sensors can be continuously powered as long as the FIV-TENG works. Once there is on fire or rain, the light alarms will be transmitted immediately as shown in the insets of Fig. 5c, d and Supporting Videos (Video S2 and S3). Besides, the FIV-TENG is capable of acting as a direct power source to drive LEDs. As demonstrated in Fig. 5e and Video S4, more than 200 LEDs can be driven by FIV-TENG at a wind speed of 2.9 m/s. These results imply that the FIV-TENG can be used not only to harvest wind energy, but also to realize a self-powered system.
3. Conclusion In summary, a novel-designed rotational contact-separation TENG driven by FIV effect is developed for the first time to solve the existing problems of wind energy harvester based on TENGs including high wind speed requirement, low conversion efficiency, weak stability and poor durability. The device structure is quite simple, and its working states highly depend on the designed parameters including electrode configuration, electrode weight, rotational radius and cantilever length. Benefiting from the unique design, this FIV-TENG is able to deliver an excellent output at wind speeds ranging from light breeze to moderate gale, and shows a desirable stability even in rainy conditions. The FIV-TENG is capable of directly driving more than 200 LEDs, charging commercial capacitors and powering some electronic sensors. This work not only puts forward a highly efficient approach to capturing wind energy, but also promotes the development of a self-powered system based on TENGs.
4. Experimental section Fabrication of the FIV-TENG: In this FIV-TENG device, two identically designed TENG units were mounted face-to-face at the end of a steel cantilever beam (12.7 mm* 100 mm* 0.15 mm) and sealed in a unique trapezoid cylinder shell (100 mm in height with a cross-section bases of 40 mm* 80 mm* 50 mm) folded by PET film (0.125 mm in thickness). The detailed fabrication process of the TENG units is as follows: First, two Al foils (40 mm* 90 mm* 0.03 mm) were attached to both sides of an acrylic sheet (40 mm* 90 mm* 1 mm) acting as the contact electrodes. Secondly, PTFE film (0.03 mm), Al foil and PET film were sequentially attached to each other layer-by-layer, and fixed on the inner surfaces of the two side acrylic substrates (40 mm* 90 mm* 1 mm). Six rectangular holes (0.5 mm* 10 mm) were symmetrically arranged along the long edges of the acrylic sheets for installation of PTFE/ Al/ PET film, and distance between the two groups of the holes was 38mm. The electrode curvature could be easily adjusted by changing the width of PTFE/ Al/ PET film. Then, four pieces of rubber films (8 mm* 15 mm* 0.2 mm) were utilized to connect the middle acrylic sheet (contact electrodes) and the two side acrylic substrates (composite film) together to form a symmetrical structure. Due to the flexibility of rubber films, the two side acrylic substrates can rotate along the fixed border of the middle acrylic sheet. Lastly, the symmetrical structure was put into the trapezoid cylinder shell and fixed at one end of the spring steel sheet by a mounting hole made
by acrylic board. In order to achieve its waterproof and dustproof performance, the edges of trapezoid cylinder shell were sealed by waterproof adhesive plaster. Characterization of the FIV-TENG: An electrical fan was employed to simulate air flow, and a commercial anemometer (UNI-T UT363BT) was applied to measure the wind speeds. The output signals of the FIV-TENG were measured by a voltage preamplifier (Keithley 6514 System Electrometer), and the electric potential distribution of FIV-TENG was simulated by using Comsol Multiphysics software.
Acknowledgment The authors gratefully acknowledge the financial support from the Fundamental Research
Funds
for the Central
Universities (Grant Nos.
CYFH201821,
2018CDQYWL0046, 2019CDXZWL001), the Chongqing University Postgraduates' Innovation Project (Grant No. CYB18061), the Natural Science Foundation of Chongqing (Grant No. cstc2017jcyjAX0307), the National Natural Science Foundation of China (Grant No. 51402112), and the Large-Scale Equipment Sharing Fund of Chongqing University.
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Figures
Fig. 1. The design inspiration and structure of the FIV-TENG. a) The schematic illustration of wind blowing against a tree. b) Detailed structure of the FIV-TENG
with its photographs shown in the insets. c) The array assembling of FIV-TENGs with different cantilever lengths. d) The schematic diagram of FIV-TENGs network
Fig. 2. The operation mechanism of FIV-TENG. a) Simulations of velocity field distributions for the trapezoid column at the wind speed 2.9 m/s. b) The diagrams of positions of two points on the both sides of the trapezoid column, pressure waveform of point A and point B at different instants of time, Three typical states of FIV-TENG activity. c) Schematic diagram for the working principle of FIV-TENG. d) Finite element simulations of the electric potential distribution.
Fig. 3. FIV-TENG configuration optimization. a) The change of transfer charge between two electrodes with a flat structure. b) Transfer charge and c) open-circuit voltage with different height of arched structure. d) Comparison of transfer charge under diverse additional weights and positions. e) The diagram of the physical model of oscillatory system. f) Vibration frequency histogram under different load weight and the cantilever length.
Fig. 4. The output performance of FIV-TENG. a-c) Open-circuit voltage, short-circuit current and transfer charge of FIV-TENG of different cantilever lengths at the wind speed range from 2.9 to 7.8 m/s. d) Open-circuit voltage waveforms under normal and rainy conditions. e) At wind speed of 2.9 m/s，f)at wind speed of 7.8 m/s, voltage, current and corresponding output power of the FIV-TENG under different external load resistance.
Fig. 5. Electron devices driven by as-fabricated FIV-TENG. a) The measured voltage curves of capacitors (2.2; 3.3; 4.7; 10; 22#F;) charged by one TENG component, and the inset shows the equivalent circuit. b) Voltage profile of 47 #F capacitor charged by one TENG unit and two serial TENG components, the inset reveals the transfer charge curves. c) A flame sensor is driven by a 33 #F capacitor charged by a TENG unit, the inset shows the photo of system operation. d) A 33 #F capacitor stores the electricity generated by serial TENG components to drive a raindrop sensor, and the inset displays the LED alarm. g) Picture of rows of LEDs lighted up by a unit of FIV-TENG under wind speed of 2.9 m/s.
Qixuan Zeng received his B.S. degree in Marine Technology from Ocean University of China in 2016. Now, he is a postgraduate student in Chongqing University. His current research interests are mechanism of triboelectric nanogenerator and self-powered sensor based on triboelectric nanogenerator.
Yan Wu received her B.S. in Physics (2017) from China West Normal University. Now, she is a postgraduate student in Chongqing University. Currently, her research interests are mainly focused on triboelectric nanogenerator.
Qian Tang received his B.S. in Physics (2016) from China West Normal University. Now, he is a postgraduate student in Chongqing University. Currently, his research interests are mainly focused on energy harvesting for self-powered system.
Wenlin Liu received his B.S. degree in Applied Physics (2017) from Chongqing University. Now, he is a PhD student in Chongqing University. His research interests are mainly focused on nanogenerator.
Jun Wu received his B.S. in Physics (2018) from Ludong University. Now, he is a postgraduate student in Chongqing University. Currently, his research interests are mainly focused on triboelectric nanogenerator
Ying Zhang received her B.S. in Physics (2018) from China West Normal University. Now, she is a postgraduate student in Chongqing University. Currently, her research interests are mainly focused on triboelectric nanogenerator
Guoying Yin received his B.S. degree in Marine Technology from Ocean University of China in 2016. His current research interests are mainly focused on signal processing.
Huake Yang received his bachelor degree from Chong Qing University, with awards of Excellent bachelor degree dissertation.
Songlei Yuan received his B.S. in Physics (2019) from Hunan University of Science and Technology. Now, he is a postgraduate student in Chongqing University. Currently, his research interests are mainly focused on nanogenerator.
DuJuan Tan received her B.S. in physics (2019) from China West Normal University. Now, she is a postgraduate student in ChongQing University. Currently, her research interests are mainly focused on triboelectric nanogenerater.
Chenguo Hu Chenguo Hu is a professor of physics in Chongqing University and the director of Key Lab of Materials Physics of Chongqing Municipality. She received her Ph.D. from Chongqing University in 2003. Her research interests include the investigation of morphology and size dependent physical and chemical properties of nanomaterials, and design and fabrication of electronic devices, such as nanogenerators and self-powered sensors.
Xue Wang received her Ph. D degree in Condensed Matter Physics in 2012 from
Chongqing University, China. Now, she is an associate professor of Chongqing University. Her research interests are related to the designs and synthesis of functional nanomaterials for energy conversion and storage, including nanogenerators and supercapacitors.
Research Highlights
1. Triboelectric nanogenertor driven by flow-induced vibration effect is fabricated for high-efficient low-speed wind energy harvesting. 2. This design dramatically reduces the material friction wear and ensures a prolonged device service life. 3. Arched structural design of the electrodes is beneficial to overcome the electrostatic adsorption and facilitates the contact-separation activity. 4. The full-packaged device has a high practicability and steady electric output without additional process step, even in a rainy condition.
S2211-2855(20)30081-1
DOI:
https://doi.org/10.1016/j.nanoen.2020.104524
Reference:
NANOEN 104524
To appear in:
Nano Energy
Received Date: 11 December 2019 Revised Date:
7 January 2020
Accepted Date: 20 January 2020
Please cite this article as: Q. Zeng, Y. Wu, Q. Tang, W. Liu, J. Wu, Y. Zhang, G. Yin, H. Yang, S. Yuan, D. Tan, C. Hu, X. Wang, A high-efficient breeze energy harvester utilizing a full-packaged triboelectric nanogenerator based on flow-induced vibration, Nano Energy, https://doi.org/10.1016/ j.nanoen.2020.104524. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.
BRIEFS A full-packaged triboelectric nanogenerator based on flow-induce vibration effect can convert low-speed air flow energy into stable electrical output without additional process, even in rainy days. TOC
A high-efficient breeze energy harvester utilizing a full-packaged triboelectric nanogenerator based on flow-induced vibration Qixuan Zeng †, Yan Wu †, Qian Tang, Wenlin Liu, Jun Wu, Ying Zhang, Guoying Yin, Huake Yang, Songlei Yuan, Dujuan Tan, Chenguo Hu, Xue Wang*
Department of Applied Physics, State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing, 400044, P. R. China. †
These authors contributed equally to this work.
*Corresponding author: [email protected]
Keywords: flow-induced vibration, triboelectric nanogenerator, breeze energy harvesting, full-packaged
Abstract Wind energy is an important and promising renewable energy for the sustainable development of society. However, considering the limitations of device structures, collection of low-speed wind energy by triboelectric nanogenerators (TENGs) still faces some challenges such as huge energy loss, destructive friction wear and restrictions of onset wind speed. To solve these problems, we have developed a novel-designed TENG based on flow-induced vibration (FIV) effect (FIV-TENG). Distinguishing from previous wind-driven TENGs, the TENG components of this
device are packaged in a bluff body and connected with a cantilever beam. This unique design not only separates the TENG units from the wind-driven part to free from environmental disruptions, but also avoids the great rotation resistance and friction wear in the ordinary designed TENG-based wind energy harvesters. Benefiting from the novel configuration, this FIV-TENG can be easily triggered by wind and delivers an excellent electrical output. The output performance of the as-fabricated device is systematically investigated under wind speeds ranging from light breeze to moderate gale, and a theoretical model is constructed to further understand the working mechanism and oscillating behaviors of the FIV-TENG. Additionally, structural parameters of the device have been optimized to achieve an optimal energy production, and the stability when working in harsh environment is also investigated. The creative device structure realizes the superior robustness and reliability, and also provides an efficient approach to realizing practical wind energy harvesting and applications.
1. Introduction Wind is deemed to be one of the most promising renewable energy resources with advantages of wide distribution, clean nature and environmental friendliness [1, 2]. Wind energy is converted to electrical energy by the wide use of conventional electromagnetic generators (EMG) on the basis of electromagnetic induction principle and turbine structure. Although it is an efficient and reliable power generation technology, EMG is often criticized by its disadvantages of bulky volume, massive
weight, high cost, and low efficiency at low wind speeds [2, 3]. Therefore, new technology for harvesting widely-distributed low-frequency breeze energy with reduced device volume and weight, lower cost and increased energy production is highly desired and still poses challenges. As an emerging energy-conversion technology, triboelectric nanogenerator (TENG) exhibits great potential as a wind energy harvester due to its easy fabrication, lightweight, good scalability and low cost [4-8]. With regard to these advantages, some TENGs designed to scavenge wind energy have been reported lately, which usually adopt rotational configurations based on freestanding mode or flutter-driven structures utilizing contact-separation mode [9-12]. These TENGs with ordinary designs demonstrate ideal performances to harvest miniature wind energy. However, until now, they are not widely used in application domains, due to some obstacles such as severe requirement on durability of friction materials, and restrictions of cut-in wind speed and working environment [5, 13-16]. Moreover, according to the literature, the global average wind speed is 3.28 m/s at the height near the ground [10, 17, 18]. Therefore, it is of great significance to develop feasible TENGs for collecting breeze energy in that wind-speed range with desirable output performance, long-term stability. Flow-induced vibration (FIV) effect is a kind of widespread physical phenomenon of fluid-structure interaction. Elastically mounted, rigid, bluff bodies that are long in one direction perpendicular to a free stream are susceptible to this phenomenon [19, 20]. FIV is always treated as a negative effect in the engineering field because large
transverse vibrations may lead to fatigue failure of related facilities [20-22]. On the contrary, the power of FIV contains vast amount of kinetic energy which can be converted into electricity. Although some related research of FIV for scavenging energy from air flows and water flows have been reported [23, 24], relevant studies on FIV driving TENGs for flow energy harvesting are still very rare. In this study, to realize a high-efficient and practical wind energy harvester, we have developed a novel TENG driven by FIV effect (named as FIV-TENG). In this structure, two identically designed TENG units were mounted face-to-face at the end of a steel cantilever beam and sealed in a unique trapezoid cylinder shell. This specific design can amplify the system vibration, enhances the TENG electrical production at a relatively low wind speed and protects the device against the disturbance of external environment. A comprehensive study of the impacts of structural parameters on the output performance of the FIV-TENG was performed experimentally, and the electrical output of the as-fabricated device was systematically studied under wind speeds ranging from light breeze to moderate gale range. At a low wind speed of 2.9 m/s, open-circuit voltage (VOC)，short-circuit current (ISC) and transfer charge (QSC) of one TENG unit could reach around 270 V, 7.6 µA and 98 nC, respectively. The maximum output power of FIV-TENG could reach 0.9 and 1.3 mW at a load resistance of 44 MΩ when wind speed was 2.9 and 7.8 m/s, respectively, implying the FIV-TENG could demonstrate a superior output performance than the previous wind energy harvesters based on TENGs [13, 25, 26]. Furthermore, the stability of FIV-TENG working in a harsh environment such as rainy condition was also
investigated. Electrical energy converted by the FIV-TENG could be stored or directly used for powering the distributed sensors or smart devices, and could also be utilized for self-powered wind speed monitoring [27]. This work demonstrates an efficient and reliable strategy for capturing breeze energy, and marks a step toward practical application of TENG for wind energy harvesting.
2.Results and discussion
2.1. Structural design and working principle of the FIV-TENG
Fig. 1a illustrates the design inspiration of the FIV-TENG. It is a common phenomenon in nature that trees will sway when a breeze blows over, which implies treelike structure can capture the energy of breeze motion. In order to demonstrate the feasibility of this model, we have designed an FIV-TENG and its holistic structure is schematically showed in Fig. 1b. In general, the main frame of the device is made by a spring steel sheet with a sealed trapezoid polyethylene terephthalate (PET) cylinder mounted on the end, and digital pictures of the whole device with different view angles are displayed in the insets of Fig. 1b. In this structure, the spring steel sheet acts as a tree trunk to supply an elastic restoring force to maintain the system equilibrium. Meanwhile, the sealed trapezoid shell serves like the external contour of tree and plays a dual role: One is to effectively capture the wind energy by means of FIV effect, and the other is to isolate the interference of the external environment, such as humidity change, dust contamination and so on. Moreover, the energy
converting components of this system are two same-designed TENGs, which are symmetrically mounted at the end of the spring steel sheet, acting as the tree branches. The manufacturing process of the TENG components can be summarized as follows: Firstly, aluminum (Al) foils adhering to both sides of the middle acrylic sheet act as the contact electrodes of TENGs. Then, polytetrafluoroethylene (PTFE) film, Al foil and PET film were sequentially attached to each other layer-by-layer to form a sandwich-structured film, and fixed on the inner surfaces of the two side acrylic substrates. Since the width of composite film (PTFE/Al/PET) was slightly wider than the distance between mounting holes of the acrylic sheet (as shown in Fig. S1a), it could shape an arced structure here (see the real shot diagrams in Fig. S1b and S1c), which was beneficial to store potential energy and further facilitated the contact-separation activity of the electrodes. Additional details of the manufacturing processes are presented in the Experimental section. Furthermore, an idea of array assembling of FIV-TENGs is shown in Fig. 1c. As demonstrated in Fig. 1d, with the further scale-up of such array system, the network of FIV-TENGs can generate huge power output, so that it can serve as a robust power supply [28]. FIV effect is a double-edged sword, which can cause destructive vibrations as well as supplying valuable kinetic energy. Herein, we aim to study and develop a simple and practical device for converting wind energy into electricity based on the FIV effect. According to the fluid-structure interaction theory, when a blunt body placed in a fluid flow field, large-scale vortices can create an unsteady fluid loading on the body surface and further induce periodic lift forces to act on the cylinder, which can
eventually cause vibrations to the system [19, 29]. Here, we utilize COMSOL platform to simulate the vortex distribution surrounding the FIV-TENG system when airflow speed is 2.9 m/s, and results are depicted in Fig. 2a. The interval time between Fig. 2a-I and Fig. 2a-II is 0.5 s. It can be seen, as the flow field shifts, vortex velocity surrounding the trapezoid body will change and induce a changing fluid force loading on the body surface. Fig. 2b shows the detailed motion status and calculated pressure change process of the FIV-TENG system. At the very beginning, the device is at its equilibrium position (Fig. 2b-III, state 1). Then, when the wind blows across the trapezoid body, the pressures loading on both bevel sides (Fig. 2b-I, point A and B) will achieve amplitude value alternatively (as shown in Fig. 2b-II). Due to the flexibility of the spring steel sheet and the cantilevered structure of this system, the device will take a vibrational activity. A complete vibration cycle occurs when the body moves from one extreme position (Fig. 2b-III, state 2) to the other extreme (Fig. 2b-III, state 3), and moves back again. To further verify the motion mechanism, distributions of the pressure field are also calculated and results are illustrated in Fig. S2a, and the pressure field of FIV-TENG array is also simulated as shown in Fig. S2b, which is consistent with the trend of a single FIV-TENG. Apparently, pressure loading on both bevel sides of the cylinder alternatively reaches the maximum value with time change. As long as wind blows, the FIV-TENG will demonstrate a reciprocating oscillation, and the TENG components sealed in the cylinder will keep taking the contact-separation motion so as to generate a continuous electrical output.
The working principle of FIV-TENG is based on a rotational contact-separation mode, and its theoretical foundation is the coupling effect of electrification and electrostatic induction [30, 31]. The detailed charge generation and transfer process are schematically depicted in Fig. 2c. Since the two TENG units have the same structure and consequently demonstrate a same power generation process, we take one TENG to expound. When the device begins to capture wind energy and demonstrates vibration, a contact-separation activity will take place between the PTFE film and the top Al electrode. According to the triboelectric series [32, 33]. After the first contact (state I), the PTFE surface would be negatively charged, and the Al film would have an equal amount of positive charges [34, 35]. Subsequently, the two electrodes are separated at a certain angle by the external force (state II), and a potential difference will form between them. Therefore, the induced electrons will flow through the external circuit from the top Al electrode to the bottom Al electrode. When the angle between the two electrodes reaches the maximum (state III), the positive triboelectric charges on the bottom electrode will be completely neutralized by the induced electrons. Then, the external force drives two electrodes to gradually move back towards each other (state IV), and a current in the opposite direction would be produced. To demonstrate the working mechanism more clearly, the corresponding distribution of electric potential under the open-circuit condition is simulated via the finite element method [36], and results are illustrated in Fig. 2d.
2.2 Structural optimization of the FIV-TENG
It is interesting that the architecture of the electrode shows a significant impact on the output performance of TENGs in this FIV-TENG system. We firstly fabricated a simple device with a flat electrode configuration, where the PTFE/Al/PET composite film tightly adhered to the side acrylic substrate (as shown in the inset of Fig. 3a). To characterize the performance of the device, the electrical output was tested. As can be seen in Fig. 3a, the QSC gradually increased from 32 to 52 nC at the very beginning. However, after 180 seconds, the value reduced drastically to around 5.5 nC, which revealed that the contact-separation activity between two electrodes was no longer produced. The key reason for this phenomenon was that the moment of inertia could not overcome the strong electrostatic attraction between the Al electrode and PTFE film. To solve this problem, we further designed an arched structure, in which the PTFE/Al/PET composite film was mounted on the acrylic substrate with a certain radian (see the inset of Fig. 3b). Due to the flexibility of the composite film, the arched structure could present elastic deformation when it was squeezed, which could store elastic potential energy to promote the separation between Al electrode and PTFE film. Moreover, the arched structure, subjected to the deformation, could increase the effective contact area between two friction materials, so as to induce an improved output performance. It is interesting to know whether the variation of arch curvature could influence the system output performance. Therefore, we compared four arched structures with different curvatures to define an optimized design. As shown in the inset of Fig. 3c, for simplifying structure as well as facilitating analysis, the curvature was assumed to
be equal to the maximum distance from the acrylic sheet to the vertex of the laminated film (H), which varied from 0.5 mm, 1 mm, 1.5 mm to 2 mm, respectively. According to their electrical outputs displayed in Fig. 3b and c, the FIV-TENG with the H of 1 mm showed the best output performance compared with others. This result could be attributed to the change of the effective contact area between Al electrode and PTFE film. It has been recognized that the degree of materials’ mechanical deformation defines the device effective contact area and finally affect the output performance [37, 38]. When the curvature of the PTFE/Al/PET film was relatively small, system inertia could supply adequate external force to squeeze the composite film, and a larger contact area between two friction materials would be achieved at a larger curvature. However, when the film curvature was large enough, the force provided by the device oscillation could not overcome the huge elastic potential energy, resulting a less sufficient contact between the two electrodes. In order to further explore the intrinsic factors that influence the output performance of FIV-TENG, the detailed kinetic model of the system was investigated. In this work, the TENG components demonstrate a rotating contact-separation motion, and the larger rotational kinetic energy can help to induce a larger elastic deformation [39], which can finally increase the effective contact area between the two friction materials. Rotational kinetic energy Erotational and moment of inertia I can be separately expressed as:
=
=
(1) (2)
where
represents the angular velocity of the electrode. According to the above
equations, a larger effective contact area of FIV-TENG can be achieved by increasing and . Obviously, the value of radius
is proportional to rotator mass
and rotation
. Here, three acrylic boards with different weights (1 g, 2 g, and 3 g) served
as the additional objects adhering to the side acrylic substrates to modulate the rotator weight. As shown in Fig. S3, the acrylic boards were fixed at three different positions to adjust the mass center of the rotator, where the distances to the pivot point were r1 = 0 mm, r2 = 15 mm and r3 = 30 mm, respectively. As illustrated in Fig. 3d, the QSC increases with the growing of additional weight and the rotation radius, which matches well with the theoretical prediction. The maximum QSC appears at r3 = 30 mm. Compared with the original value (90 nC) without additional objects, the change of QSC is defined as ∆Q, and for the three different loading weights, maximum ∆Q can achieve 7 nC, 12 nC and 17 nC, respectively. Although the maximum QSC is realized at the heaviest additional objects, the increase of electrode weight alone is not a beneficial and harmless strategy, because it will directly influence the other important parameter
, which will further affect the
oscillation performance of FIV-TENG system in the open space. In this work, the as-fabricated device can be simplified to be a spring-damper-mass system [20], as depicted in Fig. 3e. The vibrating direction of the cylinder (parallels to the -direction) is perpendicular to the flow direction (along with
-direction), and the
system oscillator motion can be presented as [40]: +
+
=
(3)
where mt is the total mass of the oscillator, c denotes the viscous damping, k represents the cantilever stiffness, Y stands for the displacement of the oscillatory system in the -direction,
and
are velocity and acceleration, respectively; F is
the fluid force imposed on the structure. According to the equation, at a certain F, the changes of
and
have significant impacts on the oscillation characters of the
system, including vibration frequency and amplitude, which will finally influence the FIV-TENG output performance. To further explore the relationship between mt, k and the oscillation activity of the as-fabricated device, a unique measurement system was designed as exhibited in Fig. S4a. Briefly, the test system consists of a magnet mounted on the bottom of a sealed trapezoid cylinder, and a copper coil (turns = 126) placed right below the magnet. Once the FIV-TENG begins to oscillate, there will generate an electrical output based on the electromagnetism induction principle, and the current waveforms in time domain can directly reflect the oscillation frequency of the tested device. Herein, different weight objects ranging from 5 g to 30 g were used to alter cantilever stiffness
. The
was adjusted by changing the cantilever length (L) as shown in
Fig. S2c, and three different lengths (L1 = 30 mm, L2 = 45 mm and L3 = 60 mm, respectively) were utilized for measurements. The current signals of copper coil were monitored at a wind speed of 2.9 m/s, and corresponding results are depicted in Fig. S5b-d. With the utilization of Fast Fourier Transform (FFT), signals in time spectrum can be converted to a frequency spectrum, so that the results of oscillation frequency versus
and
can be obtained. As illustrated in Fig. 3f, a higher vibration
frequency, which is beneficial to realize a higher current output of the FIV-TENG device, can be obtained at a smaller
and shorter cantilever length. According to
the above results, we can draw a conclusion that the increase of rotator weight can improve the QSC, but paradoxically, the higher weight will reduce the system oscillation frequency. Therefore, the ratio of ∆Q to ∆m (additional weight) is proposed to quantify the output improvement efficiency, and the maximum value is 7 when ∆Q = 7 nC and ∆m = 1 g. Accordingly, the follow-up tests and applications of FIV-TENG are carried out under the ∆m = 1 g and r3 = 30 mm. As to the impact of cantilever length, the output performance of FIV-TENG has been systematically measured at wind speeds from 2.9 to 7.8 m s−1, which are between light breeze and moderate gale in the open space [2]. From Fig. 4a-c, it can be seen that electric outputs of FIV-TENG gradually increase with wind speed, resulting from the boosting of system vibration frequency as well as the angular velocity of the electrode. Notably, the difference in cantilever length has a significant impact on the device performance. Although the FIV-TENG shows the optimal current output at L1 = 30 mm, as shown in Fig. 4c, which matches well with the above discussion, the VOC and QSC signals suffer from nonlinearity. By comparison, L = 45 mm is defined to be the optimal cantilever length for the follow-up measurements due to its excellent performance as well as linear electrical output. Based on the excellent linear correlation between wind speed and VOC as depicted in Fig. S5a, a self-powered wind speed sensor was implemented, which can make an accurate measurement of wind speed in real time as shown in Video S5. Interestingly, the ISC
value can reach a saturation point at the wind speed above 6.8 m/s (red curve in Fig. 4c), because the speed of charge transferring no longer increases as shown in Fig. S5b. The detailed output performances of L = 45 mm are depicted in Fig. S5c-e. As can be seen, at a low wind speed of 2.9 m/s, VOC, ISC and QSC of one TENG unit can reach near 270 V, 7.6 µA and 98 nC, respectively. Moreover, to verify the stability of FIV-TENG in wet atmosphere, the device was tested under both normal and rainy (with a rainfall intensity of 33.9 mm per hour, which could be defined as heavy rain) conditions. As shown in Fig. S4d and Video S1, no distinct dissimilarities of the VOC values and the capability of lighting LEDs can be observed under two different conditions, implying the as-fabricated FIV-TENG has a desirable stability even in a harsh environment. To comprehensively characterize the FIV-TENG as a power source, the output voltages and currents on varied load resistances have been measured. As illustrated in Fig. 4e and 4f, the voltage increases with the increasing load resistance while the current presents an opposite trend. When the wind speeds are respectively 2.9 and 7.8 m/s, the maximum power can separately reach up to 0.9 and 1.3 mW with the load resistance of 44 MΩ. The above results prove that the FIV-TENG manifests excellent performance to harvest low-speed airflow energy.
2.3 Practical application of the FIV-TENG To demonstrate the capability of the FIV-TENG as a power supply, a commercial electric fan was applied to serve as the wind source, and as-fabricated FIV-TENG was utilized for powering various electronics under a simulated air flow. As shown in Fig.
5a, the particulars of charging commercial capacitors with different capacities by one TENG component have been recorded at the wind speed of 7.8 m/s, and the inset is the corresponding equivalent circuit. It takes 2 s to charge a 2.2 µF capacitor to 2 V, while charging to the same voltage in a 22 µF capacitor takes 43 s. Notably, in this FIV-TENG system, contact-separation motions of two TENG components are asynchronous, where one contacts while the other separates, so the QSC can superimpose. To prove that, a 47 µF capacitor was charged from 0 to 0.4 V by one TENG unit and two TENG components connected in series, respectively. From Fig. 5b, it can be observed that the charging times are 17 s and 9.4 s, respectively, which indicates that the charging speed is nearly doubled by the integration two TENG units. The QSC waveforms of every single TENG unit and two TENGs connected in series further confirm the output superimposition, as depicted in the inset of Fig. 5b. Moreover, the electric output of TENGs can be utilized for powering sensors, especially the nodes of Internet of Things (IoT) [27]. It is known that using temporary batteries or connecting power cables to drive wide-range installed sensors will require an astronomical budget. Thus, the FIV-TENG which can efficiently scavenge wind energy shows an enormous application potential in this field [41]. Here, a commercial capacitor (33µF) serving as an energy storage and buffer unit was employed to connect with the FIV-TENG for powering sensors at a wind speed of 4.8 m/s. A flam sensor and a rain-drop detector were separately driven by sole TENG and the whole FIV-TENG system because of their different power consumptions. The charging and discharging processes of the capacitor to power these two sensors are presented in Fig.
5c and d, respectively. As can been seen, a voltage drop appears when the measured sensor starts operating, and the sensors can be continuously powered as long as the FIV-TENG works. Once there is on fire or rain, the light alarms will be transmitted immediately as shown in the insets of Fig. 5c, d and Supporting Videos (Video S2 and S3). Besides, the FIV-TENG is capable of acting as a direct power source to drive LEDs. As demonstrated in Fig. 5e and Video S4, more than 200 LEDs can be driven by FIV-TENG at a wind speed of 2.9 m/s. These results imply that the FIV-TENG can be used not only to harvest wind energy, but also to realize a self-powered system.
3. Conclusion In summary, a novel-designed rotational contact-separation TENG driven by FIV effect is developed for the first time to solve the existing problems of wind energy harvester based on TENGs including high wind speed requirement, low conversion efficiency, weak stability and poor durability. The device structure is quite simple, and its working states highly depend on the designed parameters including electrode configuration, electrode weight, rotational radius and cantilever length. Benefiting from the unique design, this FIV-TENG is able to deliver an excellent output at wind speeds ranging from light breeze to moderate gale, and shows a desirable stability even in rainy conditions. The FIV-TENG is capable of directly driving more than 200 LEDs, charging commercial capacitors and powering some electronic sensors. This work not only puts forward a highly efficient approach to capturing wind energy, but also promotes the development of a self-powered system based on TENGs.
4. Experimental section Fabrication of the FIV-TENG: In this FIV-TENG device, two identically designed TENG units were mounted face-to-face at the end of a steel cantilever beam (12.7 mm* 100 mm* 0.15 mm) and sealed in a unique trapezoid cylinder shell (100 mm in height with a cross-section bases of 40 mm* 80 mm* 50 mm) folded by PET film (0.125 mm in thickness). The detailed fabrication process of the TENG units is as follows: First, two Al foils (40 mm* 90 mm* 0.03 mm) were attached to both sides of an acrylic sheet (40 mm* 90 mm* 1 mm) acting as the contact electrodes. Secondly, PTFE film (0.03 mm), Al foil and PET film were sequentially attached to each other layer-by-layer, and fixed on the inner surfaces of the two side acrylic substrates (40 mm* 90 mm* 1 mm). Six rectangular holes (0.5 mm* 10 mm) were symmetrically arranged along the long edges of the acrylic sheets for installation of PTFE/ Al/ PET film, and distance between the two groups of the holes was 38mm. The electrode curvature could be easily adjusted by changing the width of PTFE/ Al/ PET film. Then, four pieces of rubber films (8 mm* 15 mm* 0.2 mm) were utilized to connect the middle acrylic sheet (contact electrodes) and the two side acrylic substrates (composite film) together to form a symmetrical structure. Due to the flexibility of rubber films, the two side acrylic substrates can rotate along the fixed border of the middle acrylic sheet. Lastly, the symmetrical structure was put into the trapezoid cylinder shell and fixed at one end of the spring steel sheet by a mounting hole made
by acrylic board. In order to achieve its waterproof and dustproof performance, the edges of trapezoid cylinder shell were sealed by waterproof adhesive plaster. Characterization of the FIV-TENG: An electrical fan was employed to simulate air flow, and a commercial anemometer (UNI-T UT363BT) was applied to measure the wind speeds. The output signals of the FIV-TENG were measured by a voltage preamplifier (Keithley 6514 System Electrometer), and the electric potential distribution of FIV-TENG was simulated by using Comsol Multiphysics software.
Acknowledgment The authors gratefully acknowledge the financial support from the Fundamental Research
Funds
for the Central
Universities (Grant Nos.
CYFH201821,
2018CDQYWL0046, 2019CDXZWL001), the Chongqing University Postgraduates' Innovation Project (Grant No. CYB18061), the Natural Science Foundation of Chongqing (Grant No. cstc2017jcyjAX0307), the National Natural Science Foundation of China (Grant No. 51402112), and the Large-Scale Equipment Sharing Fund of Chongqing University.
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Figures
Fig. 1. The design inspiration and structure of the FIV-TENG. a) The schematic illustration of wind blowing against a tree. b) Detailed structure of the FIV-TENG
with its photographs shown in the insets. c) The array assembling of FIV-TENGs with different cantilever lengths. d) The schematic diagram of FIV-TENGs network
Fig. 2. The operation mechanism of FIV-TENG. a) Simulations of velocity field distributions for the trapezoid column at the wind speed 2.9 m/s. b) The diagrams of positions of two points on the both sides of the trapezoid column,
Fig. 3. FIV-TENG configuration optimization. a) The change of transfer charge between two electrodes with a flat structure. b) Transfer charge and c) open-circuit voltage with different height of arched structure. d) Comparison of transfer charge under diverse additional weights and positions. e) The diagram of the physical model of oscillatory system. f) Vibration frequency histogram under different load weight and the cantilever length.
Fig. 4. The output performance of FIV-TENG. a-c) Open-circuit voltage, short-circuit current and transfer charge of FIV-TENG of different cantilever lengths at the wind speed range from 2.9 to 7.8 m/s. d) Open-circuit voltage waveforms under normal and rainy conditions. e) At wind speed of 2.9 m/s，f)at wind speed of 7.8 m/s, voltage, current and corresponding output power of the FIV-TENG under different external load resistance.
Fig. 5. Electron devices driven by as-fabricated FIV-TENG. a) The measured voltage curves of capacitors (2.2; 3.3; 4.7; 10; 22#F;) charged by one TENG component, and the inset shows the equivalent circuit. b) Voltage profile of 47 #F capacitor charged by one TENG unit and two serial TENG components, the inset reveals the transfer charge curves. c) A flame sensor is driven by a 33 #F capacitor charged by a TENG unit, the inset shows the photo of system operation. d) A 33 #F capacitor stores the electricity generated by serial TENG components to drive a raindrop sensor, and the inset displays the LED alarm. g) Picture of rows of LEDs lighted up by a unit of FIV-TENG under wind speed of 2.9 m/s.
Qixuan Zeng received his B.S. degree in Marine Technology from Ocean University of China in 2016. Now, he is a postgraduate student in Chongqing University. His current research interests are mechanism of triboelectric nanogenerator and self-powered sensor based on triboelectric nanogenerator.
Yan Wu received her B.S. in Physics (2017) from China West Normal University. Now, she is a postgraduate student in Chongqing University. Currently, her research interests are mainly focused on triboelectric nanogenerator.
Qian Tang received his B.S. in Physics (2016) from China West Normal University. Now, he is a postgraduate student in Chongqing University. Currently, his research interests are mainly focused on energy harvesting for self-powered system.
Wenlin Liu received his B.S. degree in Applied Physics (2017) from Chongqing University. Now, he is a PhD student in Chongqing University. His research interests are mainly focused on nanogenerator.
Jun Wu received his B.S. in Physics (2018) from Ludong University. Now, he is a postgraduate student in Chongqing University. Currently, his research interests are mainly focused on triboelectric nanogenerator
Ying Zhang received her B.S. in Physics (2018) from China West Normal University. Now, she is a postgraduate student in Chongqing University. Currently, her research interests are mainly focused on triboelectric nanogenerator
Guoying Yin received his B.S. degree in Marine Technology from Ocean University of China in 2016. His current research interests are mainly focused on signal processing.
Huake Yang received his bachelor degree from Chong Qing University, with awards of Excellent bachelor degree dissertation.
Songlei Yuan received his B.S. in Physics (2019) from Hunan University of Science and Technology. Now, he is a postgraduate student in Chongqing University. Currently, his research interests are mainly focused on nanogenerator.
DuJuan Tan received her B.S. in physics (2019) from China West Normal University. Now, she is a postgraduate student in ChongQing University. Currently, her research interests are mainly focused on triboelectric nanogenerater.
Chenguo Hu Chenguo Hu is a professor of physics in Chongqing University and the director of Key Lab of Materials Physics of Chongqing Municipality. She received her Ph.D. from Chongqing University in 2003. Her research interests include the investigation of morphology and size dependent physical and chemical properties of nanomaterials, and design and fabrication of electronic devices, such as nanogenerators and self-powered sensors.
Xue Wang received her Ph. D degree in Condensed Matter Physics in 2012 from
Chongqing University, China. Now, she is an associate professor of Chongqing University. Her research interests are related to the designs and synthesis of functional nanomaterials for energy conversion and storage, including nanogenerators and supercapacitors.
Research Highlights
1. Triboelectric nanogenertor driven by flow-induced vibration effect is fabricated for high-efficient low-speed wind energy harvesting. 2. This design dramatically reduces the material friction wear and ensures a prolonged device service life. 3. Arched structural design of the electrodes is beneficial to overcome the electrostatic adsorption and facilitates the contact-separation activity. 4. The full-packaged device has a high practicability and steady electric output without additional process step, even in a rainy condition.