# Energy Harvesting Methods for Transmission Lines: A Comprehensive Review
## 4 State-of-the-Art Energy Harvesting Methods for Transmission Lines
### Thermoelectric Energy Harvesting  
The action of an electric current heats any conductor due to the ohmic resistance, which produces a temperature gradient between the conductor and the environment [4] so that objects at different temperatures allow energy to be harvested through heat transfer [11]. Thermoelectric energy harvesting techniques are typically based on thermoelectric generators (TEGs); that is, devices that convert the temperature difference between their two sides into electrical energy [5,64], according to the Seebeck, Peltier, and Thomson effects. Thermoelectric energy harvesting converts the temperature difference between the  
Figure 3: Hybrid magnetic–vibration EH using a permanent vibrating magnet due to the action of the AC magnetic field of the power line. (**a**) Piezoelectric energy harvesting adapted from [62]. (**b**) Inductive energy harvesting adapted from [63]. (**c**) Electrostatic energy harvesting.  
environment and the power line into electric energy [2]. However, the efficiency of thermal EH is governed by the Carnot cycle, so for small temperature gradients, low efficiencies are expected [11]. In addition, similar to solar and wind, thermoelectric EH often cannot generate stable electric power since it depends on environmental variables [14], and in the case of power lines, it depends on the intensity of the line current, which largely determines the temperature of the conductor. When applied to transmission lines, the wide temperature swings of both ambient and line conductors make it necessary to take special care and apply specific energy management approaches to ensure reliable and stable thermoelectric generation [38]. In the case of low temperature gradients, thermoelectric energy harvesting is increasingly applied to supply wearable devices [65]. According to [65], a 900 mm\({}^{2}\) TEG allows the extraction of between 5 and 50 \(\upmu\)W with a few \({}^{\circ}\)C temperature difference, whereas in [66], it is shown that a 4-layer TEG with 5000 thermocouples can generate up to 200 \(\upmu\)W with a temperature gradient of approximately 8 \({}^{\circ}\)C.  
It is possible to harvest the energy of the heat flux between the conductor and the environment by wrapping a thermoelectric generator around it. A heat-transmitting paste is usually placed between the conductor and the thermoelectric generator to maximize the heat flux. A heat exchanger is often attached to the cold outer surface of the thermoelectric generator to maximize the temperature difference between the cold and hot surfaces of the thermoelectric generator [4].  
In DC systems, due to the lack of a time-varying variable such as the electric or magnetic field, the development of EH systems becomes more difficult. Thermoelectric EH is especially appealing for DC systems since Joule losses are characteristic of both AC and DC systems [4]. Since AC transmission lines are much more common than DC transmission lines, few works apply thermoelectric generators for EH in DC transmission systems. In [67], thermoelectric EH was applied to supply a WSN that includes several sensors (temperature, current, and voltage drop) mounted on a cylindrical busbar for a substation connector. It was shown that the temperature gradient is a key parameter to determine the output power of the thermoelectric generator, which generated 155 \(\upmu\)W for a temperature difference between the busbar and the ambient of 19 \({}^{\circ}\)C.