sensor system based on four TENGs [32]. To detect wind flows from multiple directions, four independent wind‐driven TENGs were fixed along four directions, as shown in Figure 2.15a. Figure 2.15b shows the external circuit diagram. The output current signals of the wind‐driven TENGs could be enhanced by increasing the wind speeds. Thus, by measuring the current signals, the wind speeds could be in turn calculated, as shown in Figure 2.15c. The output voltage peaks have small fluctuations, which can be independently used to confirm the wind direction (Figure 2.15d). The output voltage signals were connected to 16 channels, forming a mapping figure. When no wind flowed along the four directions, the value of the output voltage was 0, and when the wind flowed along one direction, the wind direction can be confirmed by observing the mapping figures, as shown in Figure 2.15e–h.
2.3.4.3 Polarization of Ferroelectric Materials
Ferroelectric materials, such as BiFeO3, Bi2FeCrO6, and BaTiO3, have been widely used in photovoltaic devices due to their abnormal photovoltaic effect. Usually, these materials need a polarization process, which could induce most of the domains along a fixed direction by applying a high external electric field. Conventional polarization apparatus exhibits some drawbacks including large volume, high cost, and safety issues.
Liu et al. developed an original polarization method, which could harvest ambient wind energy through the wind‐driven TENG to generate high output voltage pulse (of about 1000 V) leading to the polarization effect [75]. For the BaTiO3, a large piezoelectric constant of about 150 pC/N and a pyroelectric constant of about 14 nC/(cm2 K) were achieved by this method. Figure 2.16a illustrates the working principle of the polarization system. When the FEP film vibrated between two Cu electrodes due to the wind‐induced vibration, alternate output signals were formed in the external circuit based on a combined mechanism of contact electrification and electrostatic induction. By a rectifier, the primary output violated signals can be transformed to DC voltage pulses, which were used to polarize ferroelectric material with Ag electrodes. Figure 2.16b shows a photograph of a handheld TENG‐based polarization device. The piezoelectric constant could be increased with increasing the polarization time, as shown in Figure 2.16c. The piezoelectric constant of the BTO film was about 82 pC/N after a polarization time of about 20 s, as shown in Figure 2.16d.
2.3.4.4 Self‐Powered Wearable Electronics
Together with the advancement of digital health, wearable electronics are attracting extensive attention. Currently, powering these electronic devices mainly depends on rechargeable batteries, limiting their further applications. To effectively extend the lifetime of the wearable electronics, a number of different research teams have begun to develop new technologies for harvesting energy from our living environment.
Figure 2.15 The self‐powered wind vector sensor system. (a) Photograph of the four TENGs orientation. (b) Schematic diagram of the circuit. (c) Measured output current. (d) Measured output voltage. (e–h) Measured output voltage mapping figures for different wind directions.
Source: Reproduced with permission from Yang et al. [32]. Copyright 2013, American Chemical Society.
Figure 2.16 The polarization system. (a) Schematic illustration of the working principle of the polarization system. (b) Photograph of the handheld TENG‐based polarization device. (c) Measured piezoelectric coefficient for different polarization times. (d) Photograph of the effective polarization result.
Source: Reproduced with permission from Liu et al. [75]. Copyright 2018, Elsevier.
Jiang et al. fabricated a smart WD‐TENG, which can be used to power some wearable electronics by harvesting various human motorial energies [67–69]. By dexterous designs, the TENGs can be used as self‐powered devices to monitor different human movements. Figure 2.17a shows that the TENG can be integrated into a shoe. The human motion state under the feet could be realized by measuring the output voltages under different leg movements, which respectively correspond to walking, jogging, and running. In addition, the electric energy generated by the device can power three light‐emitting diodes, as shown in Figure 2.17b. On the other hand, the wind‐driven TENG can also be processed into a self‐powered bracelet device. The mechanical energy of arm swing could be transformed to electric energy by the TENG. By measuring the output voltages, different arm movements can be monitored. Wang et al. fabricated a self‐powered healthcare monitoring device based on wind‐driven TENG [34]. This device can monitor human nose breath, as shown in Figure 2.18a. Figure 2.18b shows that the TENG can convert the flow from human breath to electrical energy, achieving an output voltage peak of about 30 V. By the up‐flow chip, the output current of the TENG can be increased to about 350 μA, as shown in Figure 2.18c. The electrical energy generated by the TENG can be used to charge a capacitor, as illustrated in Figure 2.18d. Figure 2.18e,f shows that the temperature sensor can be powered by harvesting human nose blown air flow via the TENG.
Figure 2.17 Wind‐driven wearable electronics. (a) TENG‐based shoe for monitoring leg movements.
Source: Reproduced with permission from Jiang et al. [67]. Copyright 2018, American Chemical Society.
(b) The electrical energy form the device can power LEDs. (c) The wind‐driven TENG‐based band for monitoring hand movements.
Source: Reproduced with permission from Jiang et al. [69]. Copyright 2017, American Chemical Society.
2.3.4.5 Others
WD‐TENGs, which possess the advantages of small scale, low cost, and easy fabrication, can be used as facile power sources to uninterruptedly power some small electronic devices under continuous wind flow. Yang et al. developed a small‐scale wind‐driven TENG, which can be used to power three light‐emitting diodes and an exit sign by harvesting human mouth blowing induced wind energy, as shown in Figure 2.19a,b [32].