# Energy Harvesting Methods for Transmission Lines: A Comprehensive Review
## 3 Batteryless Energy Harvesting Systems  
Although many EH systems use supplementary batteries, they have a finite lifetime, require regular charging or replacement, and produce electronic waste [30]. Due to IoT devices' fast deployment, millions of batteries probably need to be replaced daily [28], so battery replacement is impractical and has significant environmental impacts [3]. By reducing power consumption and voltage supply, WSNs could be self-powered, i.e., supplied directly from the EH source without using batteries [28; 31]. The use of batteryless WSNs is directly related to the application of ULP devices and approaches [31]. Due to recent advances in low-power sensors, when combined with communication modules that operate in a discontinuous mode at low duty cycles hibernating between consecutive active states, they can, in some cases, be supplied directly from an independent source without using any battery [19]. Batteryless systems can use a variety of power sources, including vibration, solar power, temperature gradient, or wireless power transfer,among others, making them very attractive for applications where any intervention is challenging. In many cases, the energy collected by the EH unit is stored in small capacitors [3]. Due to the often stochastic nature of harvested energy and the restricted energy storage, associated WSNs typically operate in an intermittent on-off pattern [32]. However, in the case of power lines, this problem can be partially overcome due to the \"predictability\" of the energy flow, which allows better planning of the energy harvesting. Instead of batteries, capacitors can be used to store energy, being smaller than batteries and offering lifetimes of more than a decade since they withstand a larger number of charge-discharge cycles [3; 33]. However, the use of storage capacitors presents some disadvantages and challenges related to the stochastic nature of the energy transferred from the source to the energy harvester [34] and energy storage capacity, especially when using small capacitors. The WSN lacks a constant power supply so that power failure can last any duration, from seconds to hours. Further, depending on the WSN's mode of operation, the amount of power consumed can vary considerably, for example, when switching from sleep to transmission modes or during the sensing period, causing voltage changes that affect the energy stored in the capacitor. Therefore, for effective use of batteryless WSNs, applications based on such devices must have the ability to handle this intermittent behavior [32]. Another possibility is to use supercapacitors, which deliver and accept charge much faster than batteries while tolerating many more charge-discharge cycles than rechargeable batteries [35], but having much lower energy densities and exhibiting higher self-discharge currents, so once fully charged, compared to batteries they discharge at a much faster rate. Therefore, supercapacitors are used in applications that require many rapid charge-discharge cycles rather than compact, long-term energy storage. Small supercapacitors are used in WSNs because they can boot and charge batteryless WSNs in a matter of seconds when the energy is available. However, the use of small supercapacitors does not allow the WSN to perform energy-intensive operations. On the other hand, using large capacitors implies long charging times, so for start-up, the node must wait for the capacitor to reach a minimum voltage level. Moreover, the energy stored in the supercapacitor is quickly dissipated as the energy scavenged is not enough to maintain a minimum voltage across the supercapacitor, so a design trade-off is required for irregularly powered batteryless systems. However, according to [36], this topic is not well-studied in the technical literature. Communication technologies play a critical role in deploying reliable batteryless WSNs, as they have relatively high power requirements compared to the accompanying sensors.