32 32 Wang, S., Mu, X., Yang, Y. et al. (2015). Flow‐driven triboelectric generator for directly powering a wireless sensor node. Adv. Mater. 27: 240.
33 33 Chen, B., Yang, Y., and Wang, Z.L. (2017). Scavenging wind energy by triboelectric nanogenerators. Adv. Energy Mater. 8: 1702649.
34 34 Glass, A.M., Von der Linde, D., and Negran, T.J. (1974). Multiphoton photorefractive processes for optical storage in LiNbO3. Appl. Phys. Lett. 25: 233.
35 35 Carnicero, J., Caballero, O., Carrascosa, M., and Cabrera, J.M. (2004). Superlinear photovoltaic currents in LiNbO3: analyses under the two‐center model. Appl. Phys. B 79: 351.
36 36 Arizmendi, L. (2004). Photonic applications of lithium niobate crystals. Phys. Status Solidi A 201: 253.
37 37 Zhang, J., Su, X., Shen, M. et al. (2013). Enlarging photovoltaic effect: combination of classic photoelectric and ferroelectric photovoltaic effects. Sci. Rep. 3: 2109.
38 38 Yang, X., Su, X., Shen, M. et al. (2012). Enhancement of photocurrent in ferroelectric films via the incorporation of narrow bandgap nanoparticles. Adv. Mater. 24: 1202.
39 39 Ichiki, M., Maeda, R., Morikawa, Y. et al. (2004). Photovoltaic effect of lead lanthanum zirconate titanate in a layered film structure design. Appl. Phys. Lett. 84: 395.
40 40 Koch, W.T.H., Munser, R., Ruppel, W., and Würfel, P. (1975). Bulk photovoltaic effect in BaTiO3. Solid State Commun. 17: 847.
41 41 Xing, J., Jin, K.J., Lu, H. et al. (2008). Photovoltaic effects and its oxygen content dependence in BaTiO3−δ/Si heterojunctions. Appl. Phys. Lett. 92: 71113.
42 42 Liu, F., Fina, I., Gutiérrez, D. et al. (2015). Selecting steady and transient photocurrent response in BaTiO3 films. Adv. Electron. Mater. 1: 1500171.
43 43 Yi, H.T., Choi, T., Choi, S.G. et al. (2011). Mechanism of the switchable photovoltaic effect in ferroelectric BiFeO3. Adv. Mater. 23: 3403.
44 44 Bhatnagar, A., Chaudhuri, A.R., Kim, Y.H. et al. (2013). Role of domain walls in the abnormal photovoltaic effect in BiFeO3. Nat. Commun. 4: 2835.
45 45 Yang, S.Y., Seidel, J., Byrnes, S.J. et al. (2010). Above‐bandgap voltages from ferroelectric photovoltaic devices. Nat. Nanotechnol. 5: 143.
46 46 Ma, N., Zhang, K., and Yang, Y. (2017). Photovoltaic–pyroelectric coupled effect induced electricity for self‐powered photodetector system. Adv. Mater. 29: 1703694.
47 47 Wang, S., Wang, Z.L., and Yang, Y. (2016). A one‐structure‐based hybridized nanogenerator for scavenging mechanical and thermal energies by triboelectric–piezoelectric–pyroelectric effects. Adv. Mater. 28: 2881.
48 48 Ji, Y., Zhang, K., and Yang, Y. (2017). A one‐structure‐based multi‐effects coupled nanogenerator for simultaneously scavenging thermal, solar, and mechanical energies. Adv. Sci. 5: 1700622.
Note
1 *Corresponding author: [email protected]
2 Wind‐Driven Triboelectric Nanogenerators
2.1 Introduction
Wind, a sustainable source of clean energy, has been widely used to conquer global warming and energy crisis, and about 4.3% of global electricity came from wind power by 2015 [1]. The electromagnetic wind turbine based on the electromagnetic effect is the main type of wind energy harvester [2–10]. However, this wind harvester can hardly be applied in our daily living environment owing to its inherent drawbacks, such as the complex structure, the large volume, and the requirement for high wind speed [11–17].
In order to adequately exploit wind energy and enforce the wind harvester on small electronic devices, it is necessary to develop new techniques to effectively utilize weak wind on miniaturized devices. Triboelectric nanogenerators (TENGs) based on a combined mechanism of contact electrification and electrostatic induction have been widely used to convert multiform mechanical energy to electric energy and showed outstanding advantages including simple structure, low cost, and high power density [18–27]. The wind‐driven triboelectric nanogenerator (WD‐TENG), an important portion of TENG family, is an ideal harvester for utilizing wind energy in the living environment. In this chapter, we will first review conventional wind energy harvesters and their applications. Special emphasis is given to the WD‐TENG including fundamental structures, materials, performance, and applications. We will conclude this chapter with a comparison between conventional wind energy harvesters and WD‐TENGs.
2.2 Conventional Wind Harvester
2.2.1 Working Mechanisms and Devices Structure
Horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs) are two main types of conventional wind turbines (Figure 2.1a,b) [28,29]. They usually consist of three main parts, including a wind turbine, a cabin, and a tower. In the wind turbine, blades are very important, which can transform wind energy into the driving force of the turbine rotation. The mechanical energy from the driving force can be converted to electric energy by the generator. The tower not only bolsters the cabin but also limits the vibration produced by the change in the wind speed. The wind energy is defined as
(2.1)
where Ek is the wind energy, m is the wind mass, ν is the wind velocity, ρ is the air density, A is the area of the wind wheel, R is the length of the blade, and d is the thickness of the wind disk. According to the theoretical derivation, wind velocity is one of the key factors that affect wind power, and the power is given by
(2.2)
Figure 2.1 Main types of conventional wind turbines. (a) Horizontal axis wind turbines. (b) Vertical axis wind turbines. (c) Applications of micro‐wind turbines on homes.
Source: Reproduce with permission Ayhan and Sağlam [30]. Copyright 2012, Elsevier.
(d) Applications of turbines installation on urban expressways.
Source: Reproduced with permission from Ishugah et al. [28]. Copyright 2014, Elsevier.
HAWTs, which have the axis of rotation of the blades in a horizontal position, are widely used in urban environment because of their high efficiency. To effectively harvest wind energy, the propeller‐type rotor, which is mounted on a horizontal axis, uses a yaw motor to face the wind direction. The primal problems for these types of wind harvesters are the dangers to birds and aircraft due to their big blade sizes. The VAWTs adopt a vertical axis of rotation of blades, which is perfectly suitable for harvesting wind energy with different directions. In addition, the generator and gearbox of VAWTs can be mounted at ground level, making them easy to modulate and repair. Smaller VAWTs have been widely used in the urban environment because of their advantages of simple structures and low manufacturing cost.
2.2.2 Applications
In some ground locations or elevated locations, standalone wind turbines have been used to harvest wind energy. To realize broader applications, some researchers expect to design small‐scale standalone wind power sources, and they must conquer some key questions, such as challenges of low capacity factor, high costs, and finite capacity to store electricity. HAWTs and VAWTs have been widely used in the urban