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.
Source: Reproduced with permission from Zhao et al. 2016 [61]. Copyright 2016, American Chemical Society.
Figure 2.12 Rectified output performance of the TENG. (a) Short‐circuit current of TEG 1. (b) Short‐circuit current of TEG 2. (c) The current of TEG 1 and TEG 2 connected in parallel. (d) The measured voltage of a 10 μF capacitor charged by TEG 1, TEG 2, and TEG 1and TEG 2 connected in parallel. (e) Rectified open‐circuit voltage of TEG 1 and TEG 2. (f) Dependence of the output current and the corresponding power.
Source: Reproduced with permission from Wang et al. [33]. Copyright 2015, John Wiley and Sons.
Figure 2.13 Output performance of the TENG. (a) The output voltage and the current of the device with different heights. (b) The output current and the corresponding power of the device with different heights. (c) The output voltage and the current of the device with different lengths of the Kapton film. (d) Dependence of the measured output current and the corresponding power on the length of the Kapton film. (e) Output performance of the device with different device lengths. (f) Dependence of the output current under the corresponding power on the device length. (g) The working frequency of TEG 1 under different flow rates. (h) The working frequency of TEG 2 under different flow rates.
Source: Reproduced with permission from Wang et al. [33]. Copyright 2015, John Wiley and Sons.
Nanostructure‐like morphology on the friction layers in the TENGs could effectively improve the output performances of the devices. Dudem et al. found that nanopillar architectures distributed on the surface of PDMS could enhance the output performance of the TENG [62]. The TENG based on a flat PDMS layer generated an output voltage of about 295 V. By introducing the nanopillar architectures on the surface of PDMS layer, the output voltage can be increased to about 443 V. On the other hand, ambient humidity is a critical factor that could affect the output performances of the TENG. According to Guo et al. output current signals of the airflow‐induced TENG could be decreased by increasing the ambient humidity [63]. The output current after rectification was about 3.8 μA at a relative humidity of about 20%, but it was only 0.5 μA at a relative humidity of about 100%. This could be due to the impairment of triboelectrification and electrostatic induction.
The relationship between electric output and the fluttering behavior was investigated by Bae et al. [37]. Comparing the profiles of the electrical signal and the fluttering images, they found that output current signal gradually increased when the condition of the vibrating film started from primary contact of the curved area to complete contact. A decrease in contact area then would appear at separation, leading to reduced output current signals of the TENG. In addition, the output current could be enhanced by increasing flow velocity, reaching a maximum value of about 240 V at 22 m/s. The output performance of the TENG based on the soft friction mode was explored by Wang et al. [40]. The wind flow had a light effect with the open‐circuit voltage and the transferred charge. The reason could be the deformation degree of the FEP film. The electrodes were radially arrayed on the stator, where the parts of the center are small, resulting in the possibility of mistaken connection for the friction film under lower wind speed. This problem could be partly dispelled under high wind speed condition, where the buoyancy generated by the air flow will partly balance the gravity, leading to full contact between the electrode and the film. Zhang et al. developed a lawn‐structured TENG and investigated output performance of the device under different gap distances between two strips [41]. The output current signals were enhanced by increasing the gap distance from 0 to 7 m. Subsequently, the electric output decreased as the gap distance increased. The maximum value of the output current of one cell comprised of two strips was about 10 μA. By connecting four cells in parallel, the value could be increased to about 40 μA.
2.3.4 Applications
Wind is one of the familiar energy sources in the surrounding environment. WD‐TENGs can effectively harvest wind energy, converting mechanical energy to electrical energy. Owing to their advantages such as small scale, low cost, and easy fabrication, WD‐TENGs can be used in many fields, especially for portable electronics. With the tremendous increase in low power devices, combining TENGs with other devices is vitally important because the combined systems can eliminate dependence for
