# Energy Harvesting Methods for Transmission Lines: A Comprehensive Review
## 4 State-of-the-Art Energy Harvesting Methods for Transmission Lines
### Magnetic Field Energy Harvesting  
Magnetic field EH harvesting has been extensively discussed in the literature. The magnetic field generated by a line conductor is directly proportional to the intensity of the current and decreases with distance from the conductor as,  
\[B=\frac{\mu_{0}I}{2\pi r} \tag{1}\]  
\(I\) being the intensity of the current, \(\mu_{0}\) the magnetic permittivity of free air, and \(r\) the distance from the measuring point and the center of the conductor.  
From (1), it is evident that the magnetic field strength of power lines is substantial only in their close proximity, which limits the possible locations of EHs close to AC conductors. Another limitation is that energy harvesters are based on transformer action, so usually, they have wound coils, with their internal resistance limiting the extraction of energy, so a highly efficient circuit is required [68]. For efficient use, they need to be clamped around the conductor, thus limiting their application because, in some cases, it is not practical [11].  
Current transformers are typically used for this purpose (see Figure 4), although power availability is highly dependent on the current level of the transmission line, so a minimum current flow is required [14].  
Figure 4 shows the basic principle for energy harvesting from the magnetic field generated by power lines. Similar designs are found in numerous scientific works [59; 69; 70; 71; 72; 73; 74].  
In [39], it was shown that using a multi-winding up-conversion current transformer configuration is able to scavenge energy with a current as low as 1 A, thus providing enough power to supply wireless sensors. In [68], two inductors were placed between two parallel wires in opposite directions, carrying 8.4 A each, showing that depending on the configuration, up to 850 \(\upmu\)W can be generated. In [76], it was shown that by using nano circular cut crystalline cores, high power densities could be achieved, around 100 mW/cm\({}^{3}\), 50 times higher than using conventional cores. In [77], the material role of magnetic toroidal cores was evaluated, showing that nanocrystalline alloy cores increase the power density about four times compared to ferrite cores. In [78], the role of core size was analyzed, showing that it is essential to optimize the power output of the harvester.  
A miniature wound multicore structure made of mu-metal was proposed in [79], showing that it is able to deliver 10 mW to a 50 Ohm load. A magnetic field EH device based on a double-ring core was proposed in [80], showing that 32.8 mW can be harvested when the line current is as low as 10 A. In [11], a wound flux concentrator core made of silicon steel stuck to the conductor was proposed, which allows the concentration of the nearby flux very efficiently. The proposed design was shown to generate up to 257 mW when 1000 A flowed through the conductor. The design proposed in [11] does not have to be clamped around the conductor, which limits practical applications. In [81], an annular cored current transformer with high power density was presented, showing that it can generate 350 mW for a line current of 10 A. In [75], a toroidal core design methodology considering the saturation effect was proposed.