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 external energy storage by harvesting mechanical energy in the environment.
2.3.4.1 Self‐Powered Printer
The printer is a common office equipment, and has been used widely in offices, schools, and supermarkets. However, immobility and high energy consumption are two main drawbacks of current printers. Therefore, Chen et al. developed a self‐powered printer system, which consists of a simple handheld printer and a transparent TENG. The handheld system was assembled with a syringe and an acrylic holder [51]. The TENG could harvest wind energy, and then generate electrical energy to power the printer.
To provide unidirectional voltages to power the printer, a rectifier was used to adjust the primary output voltages generated by the TENG. Figure 2.14a shows the working mechanism of the self‐powered printer system. The wind‐driven TENG system can generate a DC voltage of about 130 V under a wind speed of 20 m/s, which could form an electric field between the nozzle and the copper sheet. The droplet surface could accumulate positive charges under this condition. The droplet was exposed to gravity and the downward electric field force, and could fall toward the copper sheet when the downward collective force was larger than the van der Waals force between the nozzle and the color ink droplet. The separating process of the droplet was illustrated in Figure 2.14b–d. Figure 2.14e shows that the letters of "TENG" were successfully color printed by the self‐powered printer system. In addition, this system can be used to fabricate a flexible conductive path comprised by printing Ag nanowire suspension, as shown in Figure 2.14f. A light‐emitting diode fixed on the path can be lightened by applying a 2.5 V DC voltage. Figure 2.14g shows a handheld printing system, which can be powered by human mouth‐blown air. In this condition, the value of output voltage was about 55 V. The handheld printing system can be used to print discrete points by harvesting mouth‐blown wind energy, as shown in Figure 2.14h.
Figure 2.14 The working principle and a photograph of the fabricated self‐powered printer. (a) The schematic diagram of the printer working principle. (b–d) Images of the separating process of the ink droplet. (e) The conductive circuit was printed by the printer and Ag nanowire suspension. (f) The letters were printed by the printer. (g) A photograph of the handheld printing system. (h) The curve printed by harvesting mouth‐blown wind energy.
Source: Reproduced with permission from Chen et al. [51]. Copyright 2018, Elsevier.
2.3.4.2 Wind Gauging System
A wind vector sensor system, which can detect wind speed and direction, has been widely used in weather forecasting. Traditionally, wind sensors need external power sources or conventional batteries, which limit their long‐term work. Wang et al. developed a self‐powered wind sensor system based on an anemometer TENG and a wind vane TENG. This system can simultaneously detect wind speed and direction