# Energy Harvesting Methods for Transmission Lines: A Comprehensive Review
## 4 State-of-the-Art Energy Harvesting Methods for Transmission Lines
### Vibration Energy Harvesting  
Vibration energy harvesting transforms mechanical vibrational energy into useful electric energy [21]. Therefore, mechanical vibrations in power lines or induced by them can be converted into electrical energy. Different energy extraction methods from vibrations are possible, such as piezoelectric [1; 54; 48; 49], inductive [26], or electrostatic [25; 50; 51] approaches.  
Piezoelectric EH is based on the use of piezoelectric materials, which generate electric charges, and thus voltage, when the material is subjected to mechanical stress [25]. Piezoelectric harvesters can be designed and manufactured at the scale of microelectromechanical systems (MEMS) to take advantage of ambient vibrations with a frequency usually less than 200 Hz [52]. In [53], a piezoelectric energy harvester was presented using a magnetic metal as the tip mass that vibrates due to the action of the AC magnetic field generated by a power conductor. The piezoelectric ceramic element is composed of Ba\({}_{0.85}\)Ca\({}_{0.15}\)Ti\({}_{0.90}\)Zr\({}_{0.10}\)O\({}_{3}\) + CuO 0.3 wt\(\%\), which can produce an instantaneous maximum power of 8.2 mW in a weak magnetic field of 250 \(\upmu\)T, and exhibits an energy density of 107.9 mW/cm\({}^{3}\). In [54], a device for harvesting the energy of a galloping conductor was proposed, using a swing piezoelectric cantilever beam collision structure and a pendulum with a swinging ball, whose motion was caused by the low-frequency vibration of the galloping conductor. Similarly, in [55], a linear Halbach array mounted on the free end of a piezoelectric cantilever beam is used to harvest the energy from the AC magnetic field generated by a power line. The presented results show that this harvester can generate 897 \(\upmu\)W across a 212 kOhm resistor when mounted on a two-wire cable carrying opposite currents of 10 A. In [56], a cantilever (piezoelectric) beam was used together with a miniaturized permanent magnet attached to the tip, which vibrates due to the interaction with the magnetic field of the AC line, as shown in Figure 3a. In [57], a nickel-zinc ferrite ring-type core, with a piezoelectric ceramic toroidal solenoid wound around it, was presented. The nickel-zinc ferrite ring was required to convert the 50 Hz circumferential magnetic field into a 50 Hz circumferential strain, transmitted to the piezoelectric layer, generating a voltage.  
Inductive EH from vibrations is based on the voltage generated due to the relative motion of a permanent magnet and a harvesting coil [26; 58], as shown in Figure 3b. The amplitude of the induced voltage in the coil depends on the amplitude of the beam vibration, the number of turns, the line frequency, and the flux gradient. To increase the magnitude of the flux gradient and the flux density, [59] proposes the use of an array of tiny and powerful permanent magnets with opposite polarities attached to the vibrating beam.  
The electrostatic method (see Figure 3c) is based on the capacitance change of a variable capacitor or electret due to relative motion between the two plates [11; 25; 50; 60], although different methods can be combined to improve performance [61]. The main drawback of electrostatic EH is related to the need for an external voltage source during operation [25].  
The energy produced by these harvesters increases with their volume [50]. In [51], an electrostatic energy harvester that produces 45 \(\upmu\)W for an acceleration of 0.6 m/s\({}^{2}\) at 50 Hz with a volume of 150 mm\({}^{3}\) is presented.  
Other methods, such as airflow energy harvesting, are not considered in this section due to the incompatibility of using rotating elements in power lines.