# Energy Harvesting Methods for Transmission Lines: A Comprehensive Review
## 4 State-of-the-Art Energy Harvesting Methods for Transmission Lines
### Electric-Field Energy Harvesting  
The electric field generated by a transmission line is independent of the current level since it only depends on the applied voltage. Therefore, electric field energy harvesting is the only method that can allow effective EH at any time the line is energized, even when not carrying any current. It makes electric-field EH the most feasible option to energize sensors from the point of view of predictability, availability, and controllability [14]. The idea of harvesting energy from the electric field is not new [11]. Electric-field energy harvesting does not depend on environmental variables, unlike many conventional harvesting methods [82], so it provides a more durable and reliable operation since it allows operation with any conductive material to which voltage is applied, making it ideal for applications requiring a certain quality of service. Since the frequency and voltage of transmission lines are tightly regulated, the electric field they generate is stable, allowing predictable amounts of energy to be harvested due to the constant rate of power harvested [14]. Electric-field EH is well suited for high-voltage transmission lines due to the strong associated electric fields, although different works have shown that it is also feasible in low-voltage applications [83; 84] using low-power electronics and switches.  
Figure 4: Magnetic field energy harvesting concept in power conductors, adapted from [75]. (**a**) Clamped topology. (**b**) Clampeds topology.  
Any energized conductor generates a radial electric field. In the case of alternating current (AC) lines, the time-varying electric field produces a displacement current that can charge a nearby capacitor so that the energy stored in this capacitor \(E_{C}\) can be expressed as,  
\[E_{C}=\frac{1}{2}CV^{2} \tag{2}\]  
\(C\) being the capacitance, and \(V\) the voltage accumulated between the armatures of the capacitor terminals.  
As shown in Figure 5, the EH unit includes diodes to rectify the generated voltage and prevent the scavenged energy from back feeding, capacitors or supercapacitors, and a controlled switch to regulate energy usage. The switch allows automatic charging of the capacitor (position 1) when the voltage is below a specific value and connects the capacitor to the load (position 2) when the stored energy is high enough for transmission. Therefore, it Is essential to use highly efficient rectifiers, microcontrollers, and regulators to optimize the overall efficiency of EH [14].  
In [85], a wire wound around an insulated single-phase 3-wire 220 V cable was used to harvest energy from the stray electric field, generating 680 nW on average. A sketch of this EH system is shown in Figure 5.  
In [86; 14], an EH system is proposed using a dielectric layer and a conducting sheath wrapped around the conductor that generates a stray capacitance, which is used to harvest energy from the electric field generated by the conductor. A multi-layer structure is also possible, as shown in Figure 6. A similar approach, shown in Figure 6a, was applied in [84] using a 220 V power line as a reference, showing an average power extracted of about 47 uW. In [87], using a copper sheet wrapped around a 230 V power line, the authors harvested 367.5 uW. In [88], a similar EH method was applied using a power line insulator, the authors stating that a continuous power of up to 17 mW can be extracted from a 12.7 kV medium voltage power line. Similar circuits can be applied using metallic plates instead of wrapping a conductive sheath around the conductor [85; 89]. In [90], it has been shown that 2.5 uW can be extracted from a 120 V power line using a metallic plate. A similar approach is proposed in [91], showing that a displacement current of fractio  
Figure 5: Electric-field energy harvesting using a wire wound around a conductor adapted [85].