Abstract
Triboelectric nanogenerators are aspiring energy harvesting methods that generate electricity from the triboelectric effect and electrostatic induction. This study demonstrates the harvesting of wind energy by a windrolling triboelectric nanogenerator (WRTENG). The WRTENG generates electricity from wind as a lightweight dielectric sphere rotates along the vortex whistle substrate. Increasing the kinetic energy of a dielectric converted from the wind energy is a key factor in fabricating an efficient WRTENG. Computation fluid dynamics (CFD) analysis is introduced to estimate the precise movements of wind flow and to create a vortex flow by adjusting the parameters of the vortex whistle shape to optimize the design parameters to increase the kinetic energy conversion rate. WRTENG can be utilized as both a selfpowered wind velocity sensor and a wind energy harvester. A single unit of WRTENG produces opencircuit voltage of 11.2 V and closedcircuit current of 1.86 μA. Additionally, findings reveal that the electrical power is enhanced through multiple electrode patterns in a single device and by increasing the number of dielectric spheres inside WRTENG. The windrolling TENG is a novel approach for a sustainable winddriven TENG that is sensitive and reliable to wind flows to harvest wasted wind energy in the near future.
Introduction
The development of renewable energy sources is one of the most important challenges in today’s world due to the rapid increase in industries and fossil fuel consumption. As wind energy harvesting is a sustainable and clean power source, kinetic energy from windmills was used to generate electricity through wind turbines and to thereby harvest wind energy. Current methods to generate electricity include conventional wind turbines using electromagnetic generators. However, they have numerous drawbacks such as complicated fabrication, inconstant electricity production, and limited construction location^{1,2,3}. Recently, several methods such as utilizing piezoelectric and triboelectric effects were proposed to overcome the difficulties of the wind turbines^{4,5,6,7,8,9,10,11,12,13,14,15,16}. Among the wind energy harvesting methods, a triboelectric nanogenerator (TENG) is a very important generating mechanism due to its simple design, input sensitive output, and high power density. TENG converts mechanical movements such as rotation and reciprocation to electrical energy by the contact electrification between two triboelectric materials and the electrostatic induction effect. Various studies indicated that TENGs could be used as wind power generators and selfpowered sensors by inducing rotation or vertical contactseparation motions using windmills^{13,14,15} and the fluttering behavior of flexible substrates^{4,6,7,8,9,16}. However, the robustness and durability of the winddriven TENGs continue to pose challenges due to the wear and fatigue failure caused by the friction between two dielectric materials and a repeatedly applied load during flutter motion. Furthermore, predicting the precise movement of wind flow is essential to achieve a wind flow sensitive TENG that can harvest even the slightest wind. Nevertheless, there were only a few attempts to force the movement of devices by an enclosed chamber or blades^{17} and to observe the movement of the fluttering material through a highspeed camera^{6}. There are no studies yet demonstrating a dynamic analysis of the wind flow in conjunction with an optimized design of winddriven TENGs. A new design and analysis for wind flow in the devices is necessary for sustainable and robust winddriven TENGs with sensitive wind flow outputs.
This study demonstrated and optimized parameters for a windrolling triboelectric nanogenerator (WRTENG) by using dynamic simulation to harvest a wide range of inputs. The vortex whistle design has an entrance and an exit that created a large vortex flow as air passed through the device^{17}. In addition to the wind flow, a lightweight sphereshaped dielectric moved along with the vortex flow and contacted the electrode on the inner surface of the whistle in the cases with strong winds and weak winds. In the study, computation fluid dynamics (CFD) simulations are used for the first time to optimize the flow inside the TENG and the movement to efficiently convert wind energy to kinetic energy. CFD can visualize the complex vortex flow and provide design parameters for improving the stability of the orbit of the lightweight sphereshape dielectric. Moreover, electrodes inside the device are patterned to have multiple outputs in a single rotation to increase the spaceefficiency of the WRTENG. The selfpowered anemometer based on the WRTENG with multiple electrodes generated linear signals of electrical output ranging from a weak wind (2 m/s) to a strong wind (exceeding 30 m/s). Furthermore, the whistleshaped wind energy harvester with freestanding mode TENGs and multiple sphere dielectrics could generate a maximum rectified opencircuit voltage (V_{OC}) of 11.2 V and a shortcircuit current (I_{SC}) of 1.86 μA. This new approach is a potential solution for winddriven TENG design, and for practical applications of a compact and lightweight wind power generator.
Result and Discussion
As shown as Fig. 1a, a windrolling triboelectric nanogenerator (WRTENG) was based on a 100 mm diameter vortex whistle. An EPS sphere was used as a dielectric to easily obtain the kinetic energy from the wind energy to induce a rolling motion along the inner surface of the vortex whistle. The density of an EPS is approximately 115 times lower than that of fluoropolymer materials (EPS = 19.1 kg/m^{3}, PTFE = 2200 kg/m^{3}), which are among the common dielectrics used in TENGs, and this enables WRTENGs to generate electricity even in the presence of a slight wind. The lightweight expanded polystyrene (EPS) has a low centripetal force and low friction for the WRTENG to work in high wind velocity without sustaining critical damage that could lead to the fracture of the device^{18,19,20,21,22,23}. The creation of the wind flow in the device needs both an inlet and outlet to make a path for the wind flow. A throat with a constricted area less than the inlet functions as the nozzle inside the WRTENG to increase the wind velocity. In the event when wind is blown to an inlet, the Venturi effect causes the wind velocity to increase along the path leading from the inlet to the throat. Although real fluids including air are compressible flows with significant changes in fluid density, the compressible flows are considered as incompressible flows when the Mach number is smaller than 0.3 (air flow velocity < 102 m/s) by empirical practices. Therefore, the incompressible fluid characteristics follow Equations (1) and (2) as given below:
where Q is volume flow rate, A is crosssectional area, v is wind velocity, g is the gravity acceleration, z is the elevation of the point, p is the pressure, ρ is the density of the fluid, and c is a constant value. In order to consider the fluid velocity at the inlet and after the throat, the fluid characteristic at the nozzle satisfies the following Equation (3):
Hence, the flow velocity increases due to the principle of mass continuity when wind passes through the throat. This decreases its static pressure in accord with the conservation of mechanical energy principle. Figure 1b shows photographs of the EPS sphere movement when the air flow velocity is 2.2 m/s. As shown in the photographs, the EPS sphere rotates inside the vortex whistle cylinder. The wind flow velocity increases as the wind passes through the throat, and the direction continuously changes due to the cylinder packaging. Thus, the ambient wind turns into a rapid vortex flow in the cylindrical vortex whistle creating the rotating orbit for the EPS sphere.
In the WRTENG, a negatively charged EPS sphere rotates over positively charged Ag electrodes printed on the inner surface of the WRTENG. As shown as Fig. 1c, two motions of the EPS sphere, namely rolling motion, and bouncing motion explain the working mechanism of the WRTENG. The drag force created as the wind rotates through the vortex whistle causes the EPS sphere to move along the wind flow. This drag force induces the rolling and rotating motions of the EPS sphere inside the cylinder. In the rolling mode, the negatively charged EPS sphere contacts with the silver electrode as it rolls through the inner surface of the cylinder, and reaches an electrical equilibrium by gaining electrons from the ground in a singleelectrode mode^{21} or from the connected electrode in a freestanding mode^{22}. After the EPS sphere passes through the silver electrodes, an electrical potential difference is generated on the silver electrode and electrons flow back to the ground or to other connected electrodes. Thus, alternative current is generated due to a continuous contactseparation of the EPS sphere during the rolling motion. Conversely, the EPS sphere separates from the inner surface of the cylinder when it reaches the throat due to shape of the throat and the highspeed wind flow passing through the nozzle. Following the separation of the EPS sphere, it bounces on the inner wall of the cylinder until the sphere is stabilized on the rotating orbit. An alternative current is also generated by the contactseparation between the EPS sphere and Ag electrodes due to the bounce motion.
Although the wind flow is an essential parameter for the EPS sphere to rotate along the vortex whistle, the unconstrained nature of the wind flow makes it hard to analyze in complex designs. Therefore, the wind vortex flow is visualized by computational fluid dynamics (CFD) software and the substrate of the vortex whistle is optimized by the simulation results. The 3D model used in the CFD is illustrated in Fig. 2a,b and its parameters are listed in Table 1. In the simulation, the wind velocity is measured at the points v_{1,} v_{2,} v_{3} and v_{4} as shown in Fig. 2c. The direction of tangential and orthogonal velocities at each point is illustrated in Figure S1 (Supplementary information). Given that the wind goes through the vortex whistle and that the EPS sphere is pushed by the rotating wind, the drag force acting upon the EPS sphere is the main cause of the EPS sphere rotation along the inner cylinder. The drag force could be explained by Equation (4) given below:
where F_{d} is the drag force, ρ is the density of air, v is relative velocity between the object and the air, C_{d} is the drag coefficient, and A is the crosssectional area. As shown in Equation (4), the wind velocity is a crucial factor in understanding the motion of the EPS sphere inside the WRTENG. As the performance of the general TENG is known to improve in high frequency movements in both vertical contactseparation and sliding TENGs^{24,25}, the velocity of the EPS and the stability of the EPS movement orbit is examined through the simulation. Figure S2a (h = 8 mm, d = 20 mm) visualizes the flow in the WRTENG, and it indicates that the air is blown to the inlet, rotates along the wall forming vortex flow, and then exits through the outlet. Specifically, the tangential wind velocity is the main parameter for the accelerating EPS sphere, and the orthogonal wind velocity is a factor that disturbs the orbital motion of the EPS sphere. In order to determine the effect of the two factors, the tangential and orthogonal wind velocities are calculated depending on various heights of the throat h and the diameters of outlet d though the CFD analysis. As shown in Fig. 2d,a reduction in h increases the tangential velocity due to the Venturi effect while air flows through the nozzle. With respect to the orthogonal velocity, the points v_{2},v_{3},v_{4} indicate almost no change in orthogonal velocity, but the orthogonal velocity of point v_{1} tends to be higher than other points, particularly until h = 8 mm. This is because the point v_{1} is located near the entrance where the slope of the bottom surface of the inlet forms a nozzle shape increasing the wind flow velocity.
The wind flow converges to the top of the whistle surface due to the nozzle shape, and spreads out to a large cylindrical cavity disturbing the stability of the EPS sphere rotation orbit (Figure S2b). As shown as Figure S2b–ii, iii, increasing the orthogonal wind velocity increases the disturbance of the EPS sphere rotation orbit when the height of the throat h is smaller. At points v_{2},v_{3},v_{4}, the wind flow stably forms a vortex orbit while moving along the inner surface of the vortex whistle. The location of the outlet is also considered to form a proper vortex flow. There is interference in the rotation orbit of the EPS sphere if the outlet is located on the side of the cylinder substrate due to the air exiting from the cylinder and the disturbance of the rotation orbit caused by the outlet itself (Figure S3a). Therefore, in order to form a fully developed vortex flow, the outlet of the WRTENG is located on the central axis of the cylindrical substrate. In Fig. 2e, the inner diameter of the outlet d is analyzed from 20 mm to 100 mm depending on the CFD for optimization. In this simulation, h is fixed at 16 mm that equals the height of the inlet H. As the inner diameter of the outlet d increases, the tangential velocity at the points v_{1},v_{2},v_{3},v_{4} decreases because the flow exits through the outlet before it forms a fully developed vortex (Figure S3b). The size ratio between the electrode and dielectric material is an important factor for the spaceefficiency design. The dielectric material used in the WRTENG has a spherical shape where the electrical performance efficiency depends on the ratio of the Ag electrode width t (1~35 mm) and the diameter D of the EPS (20 mm) (Fig. 2f). The electrical performance of the WRTENG depends on the sphereelectrode size ratio as illustrated in Fig. 2g,h. Both the outputs increase continuously until the size ratio reaches 0.5, and then they gradually saturate owing to the spherical shape of the dielectric material. Furthermore, the size of the EPS sphere is the one of the most important factor for the performance of device. In general case, the maximum diameter ratio is 0.2 and its detailed in Supplementary Information, Section 21.
The CFD results showed the wind sensitive characteristics of WRTENG indicating that it could be designed as a selfpowered anemometer. The relationship between the wind flow velocity and angular velocity of EPS sphere rotation was used as a wind velocity sensing parameter (Fig. 3a) as the rotation of the EPS sphere was driven by the wind. The center of the Ag electrodes was located at the point 90° (v_{2}) and 180° (v_{3}) from the top of the cylinder with low orthogonal wind velocities (Fig. 2d,e). The ratio of the EPS sphere and the Ag electrode used to fabricate the optimized WRTENG was 0.75 (diameter D = 20 mm, width t = 15 mm) where the I_{CC} output was saturated (Fig. 2g,h). Signal 1 was generated at t_{1} when the negatively charged EPS sphere moved on the first electrode (Fig. 3a–i). Then, the EPS sphere rolled along the rotating orbit until distance l, and the center of the sphere moved from electrode 1 to 2 (Fig. 3a–ii). Signal 2 was generated at t_{2} when the sphere reached electrode 2 (Fig. 3a–iii). Both electrode 1 and electrode 2 are singleelectrode mode TENGs that are connected to a ground. As shown in Fig. 3b, the output generated in each electrode has points t_{1} and t_{2} where the wave form reaches 0 when the electric charge reverses direction. Therefore, using the distance between the electrode l and time t_{1} and t_{2} in the rotating motion, the sphere velocity v_{s} could be determined by the following Equation (5):
The plot between the measured wind velocity and the calculated sphere velocity according to this equation is illustrated in Fig. 3c, and the measured output based on wind velocity is depicted in Figure S4. As shown in the plot, the wind velocity and the sphere velocity have a linear relation. The experimental converting constant C indicates the relationship of the sphere velocity v_{s} and the wind velocity v_{w} as expressed in Equation (6) given below:
The linearity is a suitable characteristic for the wind velocity sensor where the signals of sphere velocity could be directly interpreted as wind velocities. Additionally, the WRTENG could be used for a wide range of wind velocities because of its high sensitivity (C = 1.4) due to the lightweight dielectric material.
The most influential factor for wind energy harvesting is the kinetic energy of the EPS sphere converted from wind energy. Therefore, the efficiency of converting wind energy to kinetic energy in the WRTENG is derived by the relation between the sphere and wind velocity. The kinetic energy of the EPS sphere is an essential parameter for the high efficiency of the WRTENG. The parameters utilized to calculate the efficiency are illustrated in Fig. 3d and Table 2. As discussed previously, the sphere velocity and wind velocity share a linear relationship between sensing points v_{2} and v_{3} when the wind continuously blows. Figure S5 shows that the average of the tangential wind velocity v_{s} at each point is between points v_{2} and v_{3}. Therefore, a converting constant C could be applied while the EPS sphere is rotated through the inner cylinder. The movement of the EPS sphere when the wind blows at a velocity of v_{w}, could be expressed as moving at a velocity of v_{s} and rotating at an angular velocity of ω (Fig. 3d–i). The kinetic energy of the EPS sphere could be expressed as the sum of the translational and rotational kinetic energies about the center of the mass Equation (7) given below:
The EPS sphere moves along the orbit of radius R′ (Fig. 3d–ii). Given that the EPS sphere gains kinetic energy through wind at v_{w} for the time t required for a rotation cycle, the work performed by the wind could be expressed according to Equation (8) as follows:
Based on Equations (7) and (8), the efficiency of converting the wind energy to kinetic energy could be expressed as Equation (9) (a detailed derivation is shown in Supplementary Information, Sections 22):
According to the results, the WRTENG indicates a high efficiency (38.28%) of converting wind energy to kinetic energy. Furthermore, the magnitude of the wind velocity is an independent parameter from the efficiency of converting wind energy to kinetic energy, and this suggests the availability of the wind sensor and generator for a wide range of wind velocities with a high efficiency.
Figure 4a shows the patterned electrode of the WRTENG with 14 electrodes aligned along the inner cylinder used in the experiment. Every two electrodes along the cylinder are connected as a unit of the freestanding mode TENG. The freestanding pattern electrodes were used to increase the output of the WRTENG when compared to a singleelectrode mode as shown in Figure S6. Each freestanding generating unit was connected to a rectifier to show a positive output and to avoid any cancellation of electrical outputs caused by a phase difference between each unit. The integrated WRTENG could light up 30 LEDs instantaneously when wind was blow at 22.5 m/s (Fig. 4b, Video S1 in Supplementary information). Figure 4c,d indicates opencircuit voltage (V_{OC}) and closedcircuit current (I_{CC}) of the multiple patterned WRTENG at a wind velocity of 22.5 m/s. As shown in the plot, the WRTENG generates electrical power output when the wind was blown. The high peak point when the wind was blown initially was due to a sudden pressure change in the wind flow. Following this, the output of the WRTENG was stabilized. The average V_{OC} and I_{CC} of the generated output peaks were 8.05 V and 1.59 μA, respectively. Despite the ceasing of the wind, the EPS sphere continued to rotate inside the vortex whistle and generate additional output because of its inertia. Figure 4e,f illustrates a magnified plot of V_{OC} and I_{CC} generated by the WRTENG. As the EPS sphere rotated along the inner surface of the vortex whistle, the sphere contacted and separated with every electrode in the whistle. As shown in both the figures, there are 14 peaks generated by each electrode fabricated inside the cylinder. As discussed previously, the mechanism of the WRTENG could be divided into the rolling motion caused by the wind flow and the bouncing motion near the whistle inlet. The first two output peaks shown in Fig. 4e,f were generated through the small bouncing motion of the EPS sphere. The electrical output for the bouncing motion was mainly dependent on the amount of the kinetic energy of the dielectric materials. In the bouncing region, the EPS sphere bounced off from the inner cylinder due to the inlet and the incoming wind flow. With respect to the kinetic energy perspective, the EPS sphere instantaneously loses kinetic energy causing a decrease in the velocity when it bounced on the inner surface (electrodes 1–2). In the rolling region, the sphere gained higher kinetic energy as the wind pushed the EPS sphere to roll around the inner cylinder and generated a higher output (electrodes 3–14). Hence, the electrical output of the vertical contactseparation motion was smaller when compared to the average output of the EPS sphere rotation cycle. As the wind velocity was the most important factor, Fig. 4g,h shows the V_{OC} and I_{CC}outputs of the WRTENG according to the wind velocity. The velocity of the rotating ball increased the centrifugal force that the EPS sphere received in the cycle causing an increase in the effective contact area^{26,27}. Therefore, both V_{OC} and I_{CC} increased as the wind velocity increased. Figure S7 is a plot of the voltage and current output based on the resistance, and the power output according to the voltage and current output. The electrical power output measured in the plot was based on a single unit inside the WRTENG. The multiple patterns inside the WRTENG generated multiple outputs in a single rotation of the EPS sphere that could multiply the total power generated in the WRTENG by the number of generating units inside.
The number of EPS spheres could also be a significant factor for the WRTENG in case of generating multiple outputs. In Fig. 5a,b, the V_{OC} and I_{CC} outputs of multiple EPS spheres in the WRTENG are shown. Every two electrodes were connected as a freestanding mode similar to that detailed in the previous paragraph. In the case when there was only one EPS sphere rolling inside the WRTENG, the peaks formed a uniform wave pattern due to the output reduction on the wind flow entrance. As the number of spheres increased, the output of the WRTENG increased accordingly because the electrical power peak generated from each EPS sphere converged and formed higher peaks. As shown in the plot, both the V_{OC} and I_{CC} outputs tended to have more electrical power peaks until there were 3 EPS spheres inside the cylinder. After 4 EPS spheres were inside the WRTENG, the EPS spheres collided with each other and disturbed the rotating orbit themselves. The kinetic energy of each EPS sphere decreased when the spheres collided and the output of the WRTENG decreased. Figure 5c shows the WRTENG charging a 100 μF capacitor based on number of balls inside. As shown in the plot, 3 spheres inside the WRTENG indicated the highest charging rate. Although a single sphere output of the WRTENG had a higher output than the 4 spheres inside, the number of peaks generated in a single cycle was lower than 4 spheres causing a single sphere to have the lowest rate in charging a capacitor.
In order to measure the durability of the WRTENG, the amount of the centrifugal force applied by the EPS sphere for a wind velocity of 22.5 m/s is expressed as Equation (10) given below:
According to the parameters listed in Table 3, the calculated F was 1.01 N. Although the initial contact of EPS sphere and electrode was a dot contact, it contacted the Ag electrode as a circular shape due to the centrifugal force applied on the EPS sphere. The radius of the circle a contacted by the EPS sphere with the Ag electrode can be expressed as Equation (11) as follows:
where, E^{*} follows Equation (12) given below:
The calculated a with the parameters of the WRTENG was 0.09357 mm. The maximum contact pressure occurred in the center of circle, and it could be expressed as Equation (13):
Given the calculated F and a, the maximum contact pressure caused by the EPS sphere was 552.16 kPa. The practical use of the WRTENG in order to avoid the loss of function necessitates considering the factor of safety, which is a term for the capacity to allow for uncertainties. The factor of safety can be described as Equation (14) given below:
The safety factor for the WRTENG is 99.61 considering that yield strength of silver is 55 MPa. With respect to the EPS perspective, deformation occurred at the outer surface of the sphere that was in contact with the inner surface of the vortex whistle since the yield strength of the EPS was 208.23 kPa. The contact surface area increased as the deformation occurred, and the maximum contact pressure caused by the centrifugal force decreased. Additionally, there were elastic, plateau, and densification regions without fracture over the yield strength when the EPS was compressed^{28,29,30}. Therefore, there was no fracture or critical damage even if the EPS spheres deformed due to the contact pressure. Furthermore, even after 10^{6} cycles of rotation, the WRTENG showed a steady output as illustrated in Fig. 5d. This result indicated that the design of the WRTENG was highly reliable for practical applications.
Conclusion
In summary, a novel winddriven TENGbased was developed on a vortex whistle. The WRTENG indicated wind flow sensitive characteristics due to the nozzle design and the lightweight dielectric material that could be utilized as both a selfpowered wind velocity sensor and a wind powered generator. The wind velocity could be calculated by using a converting constant C that uses the linear relationship of the sphere and wind velocity in the cylinder. The efficiency of converting the wind energy to the kinetic energy of the EPS sphere was shown as approximately 40%. A multiple patterned electrode was fabricated to increase the output efficiency in a single cycle. Moreover, optimized results according to the number of spheres were examined. Theoretical analysis and experimental results indicate a generating output without degradation even after 1 million cycles at wind velocity of 22.5 m/s and a safety factor of 99.61, thereby indicating that the design of the WRTENG is reliable for practical applications. This study demonstrates a novel approach for developing the performance of sensitive and robust winddriven TENGs, and a practical method for utilizing wasted wind energy.
Methods
Fabrication of the Windrolling TENGs
The polyvinyl chloride (PVC) substrate of the WRTENG with a thickness of 300 μm was used to control the wind flow to convert wind energy to kinetic energy with its enclosed design. The PVC sheet was attached without a gap to form an inlet and outlet of the vortex whistle. A 20 mm diameter EPS sphere was placed inside the whistle, and it rotates around the inner surface of the vortex whistle when wind is blown through the whistle. The silver paste (AgIC conductive ink) electrodes that is printed on PET sheet by circuit printer were patterned on the inner surface of whistle cylinder. For enhanced reliability of WRTENG, the electrodes were located the specific point like Figs 3a and 4a.
Analysis of the vortex flow by Computation Fluid Dynamics (CFD)
In this study, for optimization, the flow in vortex whistle was analyzed by Computation Fluid Dynamics (Autodesk CFD). Utilized materials PVC is adapted on vortex whistle and inside is full of air. For the setting boundary condition, two section inlet A and outlet B were selected with the properties is illustrated in Fig. 2a and Table 1. All of the result of point v_{k}(k = 1 to 4) has the same constant that middle surface of cylinder.
Electrical Measurement of the TENGs
The output voltage and current signals were measured by a digital oscilloscope (Tektronix MDO3012) and a current preamplifier (Stanford Research Systems SR570).
Additional Information
How to cite this article: Yong, H. et al. Highly reliable windrolling triboelectric nanogenerator operating in a wide wind speed range. Sci. Rep. 6, 33977; doi: 10.1038/srep33977 (2016).
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Acknowledgements
This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (No. 2016R1A4A1012950) and Nano·Material Technology Development Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and Future Planning (NRF2016M3A7B4910532).
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H.Y., J.C., M.C. and S.L. conceived the idea and design the experiment. H.Y. and J.C. fabricated the experimental device. D.C. analysis the wind sensor and efficiency. H.Y. and D.J. analysis the vortex flow by Computational Fluid Dynamics (CFD). H.Y., J.C., M.C. and S.L. wrote the paper and discuss about the contents.
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Yong, H., Chung, J., Choi, D. et al. Highly reliable windrolling triboelectric nanogenerator operating in a wide wind speed range. Sci Rep 6, 33977 (2016). https://doi.org/10.1038/srep33977
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DOI: https://doi.org/10.1038/srep33977
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