# Application of Thermoelectric Generators for Low-Temperature-Gradient Energy Harvesting
## 1 Introduction  
Transmission and distribution lines are the highways of today's electrical systems, so it is vital to maximize their reliability, efficiency, and stability, due to the enormous economic and social impacts they have. Transmission lines are found in semi-urban areas but also in remote regions, where regular inspections can be expensive and complex. These inspections are necessary to minimize the risk of transmission line failures due to the impact of different environmental factors such as ice, rain, wind or extreme temperatures on their operational performance. Therefore, self-powered wireless sensors can be of great help for real-time monitoring of transmission lines [1]. To supply these sensors, an energy harvesting unit is required, that is, a device for converting energy from the environment or other sources into electrical energy [2]. Energy harvesters allow extending the lifetime of battery-powered sensors because in many applications they minimize or avoid battery replacement and maximize the duration of their use [3]. In some cases, such as in battery-less autonomous systems, energy harvesters allow battery usage to be eliminated [4]. Therefore, energy harvesters minimize or eliminate battery replacement and associated maintenance costs [5]. In the case of overhead power lines, the temperature gradient between the ambient and conductor is limited, because overhead line conductors (OHLC) rating is typically 75 degC for conventional aluminum-conductor steel-reinforced conductors [6], while the maximum allowable temperature is about 90 degC [7]. The operating temperature and thus thermal stress of OHLC are restricted to limit vertical sag clearance, elongation and creep, tensile loss, and maximize conductors' lifespan [6]. Instead of conductors, the substations use aluminum tubular busbars, whose maximum operating temperature is lower thanthat of OHLCs [8; 9]. Therefore, temperature gradients in the conductors and busbars of transmission systems are reduced. IoT devices allow us to monitor different devices [10; 11], thus facilitating the determination of their health condition. Today, there is an urgent need to have a better control of high-voltage transmission and distribution systems. It is accepted that by introducing low-cost wireless sensors (WSs) in this field, we can increase their stability, availability and reliability while allowing predictive maintenance strategies to be applied. Furthermore, transmission systems are found in remote areas [12], where human intervention is very difficult and expensive, so IoT-based solutions can be very useful. Thus, there is an increasing demand to monitor the health condition of such systems [13]. WSs found in IoT solutions typically include an energy harvesting unit, dedicated sensors and a communication module. It makes it possible to acquire the key parameters and send the information wirelessly to the cloud, where it can be monitored and analyzed in real time to facilitate the application of predictive maintenance plans [14]. A key element in WSs for IoT applications is the energy harvesting unit, which allows reduction of the carbon footprint by minimizing battery replacement and enabling the deployment of electronic sensors in inaccessible or hostile places [15]. Due to the lack of a time-varying magnetic or electric field, the development of energy harvesting systems for DC systems is more challenging, making the use of TEGs a necessity. Due to the ohmic resistance, conductors are heated by the action of an electric current, which generates a temperature difference between the conductor and the environment [16]. This temperature gradient produces a heat flow that allows energy to be harvested [17]. When applied to power conductors or substation busbars, TEGs convert the temperature gradient between the environment and the conductor into electric energy, which can be used to supply autonomous sensors [18].