# Energy Harvesting Methods for Transmission Lines: A Comprehensive Review
## 6 Identified Challenges and Research Needs  
WSNs are often supplied at low power and are energy constrained, resulting in low processing capabilities, reduced power storage, and limited operational lifetime, although they offer multifunctional abilities. WSNs randomly deactivate when their storage elements are drained, thus leading to reduced duty cycles and reliability or short-range transmission [125]. All of these limitations contribute to quickly draining their stored energy when operating for extended periods [22]. Therefore, different critical issues around EH circuits have to be addressed, which are listed below in no specific order. * WSNs must support hybrid EH configurations to extend communication frequency and range as well as onboard features. This area requires more research and development [124]. * Batteryless systems require a true self-starting operation; that is, to start their operation from any amount of ambient power without needing any special arrangement [124]. \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**EH Type** & **Power Density** & **Advantages** & **Disadvantages** & **References** \\ \hline Solar & \textless{}100 mW/cm\({}^{2}\) & For AC and DC grids and high output power & No constant supply and regular maintenance & [14] \\ Vibration piezoelectric & 10–200 \(\upmu\)W/cm\({}^{3}\) & No external supply required and high voltage output & Only for AC systems \& [123] \\ Vibration inductive & 1–2 \(\upmu\)W/cm\({}^{3}\) & No external supply required & Moving elements \& low & [123] \\ Thermoelectric & 50 \(\upmu\)W/cm\({}^{2}\) (\(\Delta T\) = 5 \({}^{\circ}\)C) & For AC and DC grids & Requires current flow and a heat sink & [14] \\ Magnetic field & 280 \(\upmu\)W/cm\({}^{2}\) & Easy to implement in AC & Difficult in DC grids and requires current flow & [14] \\ Electric field & 170 \(\upmu\)W/cm\({}^{2}\) & Available under no load conditions & Difficult in DC grids & [124] \\ RF & 0.0002–1 \(\upmu\)W/cm\({}^{2}\) & Abundant in urban areas & Low power density and scarce in remote areas & [14] \\ Corona & Not available & For AC and DC grids & Risk of overcurrent \& requires more research & [20] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: EH methods comparison for power lines. Further developments in low-loss rectifiers, low-power micro-controllers, mV-range DC/DC converters, and highly efficient regulators are required to maximize system efficiency, thereby increasing communication reliability and system lifetime [14]. * Efficiency and integration are key points for a widespread application of WSNs, specifically batteryless, so the harvesters, interface circuits, and sensors could be integrated into the same package and even into the same semiconductor component. These advances could lead to dramatic reductions in the size and costs of WSNs [124]. * The use of improved materials with higher efficiency, such as better materials for solar cells, thermoelectric generators, piezoelectric cantilevers, magnetic components, or capacitors with loess leakage [124]. * The minimum voltage and power levels at which the system can harvest energy must be reduced to the minimum levels possible to extract all available energy [24]. * WSN devices operate a large portion of their time in sleep mode, which largely determines total power requirements, so to increase their lifespan, technologies that minimize power consumption in sleep mode need to be further optimized [24]. * The entire system can only be turned on when the voltage on the storage capacitor is beyond a threshold level, so the system's lifetime can be significantly extended by lowering the operating voltage [24]. * Due to the energy requirements, energy-efficient wireless communication modules play a critical role in increasing the lifetime of WSNs. Therefore, to optimize power transmission efficiency, ultra-low-power (ULP) transceivers are needed, along with the development of simpler modulation techniques and optimized signal waveforms and bandwidth [14].