Hybridized and Coupled Nanogenerators. Ya Yang

TENG, and studied different modes via the finite element method [41]. The strips could be bent to contact together at the origin state under wind blow. The contact process could occur again when the atmospheric pressure exceeded the elastic restoration.

The wind flutter effect could affect the performance of the TENG. Wang et al. explored the interaction between the flow and the film by simulating the structure–fluid model [34]. This model consists of a fluid part and a solid mechanic part (solved with the Navier–Stokes equations). The results show that the bending mode was the main working condition for the film at different inlet wind speeds, as illustrated in Figure 2.10a–c. Figure 2.10d,e shows pressure and wind flow velocity distributions at a wind speed of 15 m/s. It was found that the vibrating film could work stably when the wind speed exceeded 10 m/s. On the other hand, the vibrating film worked on twist motion at low inlet wind speed (5 m/s), where the edge area of the thin film deformed more than the trailing edge. The twist motion would fade and fluttering mode form when wind speed approaches the critical wind velocity, which could be estimated by the frequency ratio of first order bending and twist mode and the respective damping ratio. Figure 2.10f shows the natural frequencies and the mode shapes of the mode shapes of first six order vibration mode, which coincides with the actual vibrations of the film observed via a high‐speed camera.

Figure 2.10 Simulation of the vibration film. (a–c) The displacement distributions of the Kapton film at different air‐flow rates. (d) Pressure distribution of the film. (e) Velocity magnitude distribution of the film. (f) Six vibration modes of the film.

      Source: Reproduced with permission from Wang et al. [34]. Copyright 2015, American Chemical Society.

Surface morphology could affect the mechanical performance of the TENG. Dudem et al. explored the performance of PDMS‐based TENG with different nanopillar‐array structures [62]. It was found that increasing the period of nanopillar architectures on the surface of PDMS could enhance the uniaxial normal stresses, but the contact area was decreased. The contact force across the PDMS and electrode was enhanced in the period of nanopillars from 100 to 200 nm, and subsequently decreased. On the other hand, the contact area also could be enhanced by increasing the diameter of nanopillar architectures.

      2.3.3.2 Electrical Output

The electrical output of TENGs usually includes alternative signals due to the coupling of triboelectrification and electrostatic induction. The elasto‐aerodynamics‐driven TENGs could deliver two part signals, where a short‐circuit current of about 80 μA and an output voltage of about 240 V were generated by the top TENG, and a short‐circuit current of about 60 μA and an output voltage of 220 V were generated by the bottom TENG [34]. It was found that the measured output voltage under the different loading resistances exhibited the largest power density of about 3 kW/m³ for the top TENG and 2 kW/m³ for the bottom TENG. It was found that the air gap could affect the output voltage, which could attain the maximum value when the distance of the gap was about 1.6 mm. On the other hand, increasing the length of the device could reduce the working frequency while increasing the power density of the TENG. The output current of the elasto‐aerodynamics‐driven TENG did not show any obvious change after the Kapton film vibrated to touch the PTFE film for 4.1 ×10⁶ times, illustrating good stability.

Wind speed is an important factor to affect output performance of the wind‐driven TENG. Jiang et al. found that output voltage could be enhanced with increasing the wind speed [67]. High wind speed could form more mechanical energy to generate friction between two triboelectric layers, leading to the generation of more electric energy. Xie et al. explored the output performance of the rotary TENG. The output voltage signals jumped from 0 to a maximum value of about 250 V when the TENG was blown at a wind speed of about 15 m/s [35]. The output current was small under low wind speed condition, and it could attain a saturation value of 200 μA under a wind speed of about 20 m/s. Zhao et al. explored the output performance of superhydrophobic‐surface‐based TENGs [61]. Figure 2.11a,b shows that the TENG can deliver an output voltage of about 218 V and an output current of about 30 μA at a wind speed of 12 m/s. The corresponding largest output power of this TENG is about 2.2 mW, as shown in Figure 2.11c. By the triboelectric charging process, the time to attain the saturated output current is about 100 seconds, as illustrated in Figure 2.11d. Increasing the wind speeds could enhance both the output performance and the corresponding working frequency, as shown in Figure 2.11e,f. On the other hand, wind direction also affects the output performance. It was found that no output signals can be measured when the angles between the wind direction and the horizontal direction of the device are under 90°. The TENG can generate a high output current when the angles are at 0° or 180°.

To realize entirely positive output signals, Wang et al. used a bridge rectification circuit to adjust the primary output signals generated by the flow‐driven TEG, which contain two single TEGs [33]. The rectified output current of TEG 1 is about 50 μA, and the current of TEG 2 is about 30 μA, as shown in Figure 2.12a,b. The output current could be enhanced by connecting the two TEGs in parallel, as shown in Figure 2.12c. Figure 2.12d shows that the charging performance of a 10 μF capacitor for TEG 1//TEG 2 is better than that of TEG 1 or TEG 2. The rectified open‐circuit voltage of the device is about 300 V, as shown in Figure 2.12e. The maximum output power of this device is about 2.35 mW at a loading resistance of 3 MΩ, as illustrated in Figure 2.12f. The height of the flow‐driven TEG affects the output performance, which can attain the maximum value when the height is adjusted to 10 mm, as shown in Figure 2.13a,b. Increasing the length of the Kapton film could also enhance the output performance, as shown in Figure 2.13c,d. Then, the length of the device also can affect the output performance, as illustrated in Figure 2.13f. On the other hand, wind speeds can enhance the frequency of both TEG 1 and TEG 2, as shown in Figure 2.13g,h. Ahmed et al. explored the output performance of TENG based on the sliding mode [74]. The maximum output voltage signals and transferred charge were about 600 V and 0.15 μC when the wind flow was regulated at a rotational speed of 50 rad/s. It was found that increasing the wind speed could enhance the output current signals of the TENG. It was found that the diameter of the rotator and grating number also could increase the performance of the TENG.

Figure 2.11 Output performance of the TENG. (a) Output voltage signals. (b) Output current signals. (c) Output current and the corresponding power of the TENG under the different loading resistances. (d) Measured original output current signals of a new TENG. (e) Output voltage/current under different wind speeds. (f) Working frequency of the TENG under different wind speeds.

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