href="#fb3_img_img_2942d3fc-6d47-571a-a85e-c3bf1c8cc24a.png" alt="equation"/>
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 environment to transform wind energy to electricity generation because their sources are close to the electrical load. Recently, there are some systems that have been integrated into the ambient environment. For example, micro‐wind turbines, which can be mounted on existing homes, form one of the techniques for application in urban environment (Figure 2.1c) [30]. On urban expressways, novel turbines are used to capture the energy generated by driving cars at high speeds (Figure 2.1d) [30,31].
2.3 Triboelectric Nanogenerators for Scavenging Wind Energy
In January 2012, the TENG was first introduced to harvest mechanical energy. Owing to its low cost and high efficiency, TENG provides a novel way for scavenging wind energy in our daily living environment. There are many types of TENGs to harvest wind energy. Among them, vibrating plate‐based TENGs have been widely used because of the high output performance.
2.3.1 Fundamental Modes and Structure
Currently, TENGs mainly depend on four modes, including vertical contact‐separation mode, in‐plane sliding mode, single‐electrode mode, and freestanding triboelectric‐layer mode. In consideration of the application, vertical contact‐separation mode was more suitable for harvesting wind energy.
2.3.1.1 Vibrating Plate‐Based TENGs
Yang et al. proposed vibrating plate‐based TENGs to harvest wind energy in our daily living environment, which consists of two aluminum (Al) foils and a fluorinated ethylene‐propylene (FEP) film in a rectangular acrylic tube, as shown in Figure 2.2a [32]. The Al foils were stuck on the upper and lower ends, which can be used as both electrodes and triboelectric surfaces. To vibrate along with the wind, one side of the FEP film was clamped to the rectangular acrylic tube. The periodic contact between the FEP film and the Al foils can generate an output current across an external circuit. The side view of the fabricated plate‐based TENG shows that the device is light and handy (Figure 2.2b). Figure 2.2c shows the front view of the device.
Figure 2.2 Diagram of the plate‐based TENG. (a) Schematic diagram of the TENG. (b,c) The side and front views of the TENG.
Source: Reproduced with permission from Yang et al. [32]. Copyright 2013, American Chemical Society.
The working mechanism of the plate‐based TENG is schematically represented in Figure 2.3. At the initial state, contact between the surface of the Al foil and the FEP film led to electrons transfer from Al to FEP, resulting in positive charges on the surface of the Al foil and negative charges on the surface of the FEP. There is no current in the external circuit because the positive and negative triboelectric charges balance each other (Figure 2.3a). When the FEP film segregates the bottom Al foil due to wind‐induced vibration the balance is destroyed, leading to the opposite current at the two Al foils (Figure 2.3b). The electrostatic‐induced current will be maintained consistently until the negative triboelectric charges on the FEP film are completely screened from the positive charges, where the current signals disappear at the foils, as shown in Figure 2.3c. Subsequently, the FEP film was close to the bottom Al foil, leading the influent and the effluent current signals to the top and the bottom Al foils, respectively, as depicted in Figure 2.3d. This process finishes once the FEP film makes contact with the bottom Al foil due to the balance formed between them, as shown in Figure 2.3e. Figure 2.3f illustrates that the FEP film is close to the top Al foil, leading to the opposite current signals compared with those in Figure 2.3d. Consequently, plate‐based TENG, which can be driven between the two Al electrodes under vibration situation, can be used to harvest wind energy.
2.3.1.2 Enhanced Plate‐Based TEGs
The device structures and materials can affect the output performance of triboelectric generators (TEGs). To increase the actual efficiency for harvesting wind energy, some improved structures have been invented by optimizing the structures of the devices. For example, Yang's group proposed enhanced plate‐based TEG, which consists of two polytetrafluoroethylene (PTFE) films, two Al foils, and a Kapton film coated with Al electrodes on its surfaces (Figure
