# Evaluation of Green Strategies for Prolonging the Lifespan of Linear Wireless Sensor Networks
## 6 Energy-Harvesting Techniques
The external (non-renewable) sources consist of solar (indoors), directed RF, thermal (waste heat), magnetic field, human (motion and temperature), and mechanical (vibrations, stress, and pressure) sources. A detailed review of energy-harvesting techniques for the WSN and IoT can be found in [48; 49; 120; 122; 125; 126]. The works of [PERSON] et al. [122] and [PERSON] et al. [49] presented a comprehensive taxonomic survey on recent energy-harvesting techniques in WSNs and a concise summary and comparative analysis of various promising techniques for energy harvesting. [PERSON] et al. [4] and [PERSON] et al. [125] presented a review of recent advances in energy-harvesting techniques for IoT. From the literature, the most popular energy-harvesting techniques used in the WSN and IoT include solar-based, thermal-based, wind-based, vibration-based, and RF-based sources [37; 124; 127]. [PERSON] et al. [124] reviewed and presented a comprehensive account of energy-harvesting sources, energy storage devices, and corresponding topologies of energy-harvesting systems, published from 2008 to 2018. In another study, [PERSON] and [PERSON] [127], in their review paper, provided a detailed overview of the existing low-power wide-area network (LPWAN) systems relying on energy harvesting for their powering. In [127], the different LPWAN technologies and protocols are discussed alongside the applicable energy-harvesting techniques and presentations of the architecture of the power management units. Figure 8: Taxonomy for classifying energy-harvesting techniques in WSN. Although EH techniques provide a viable solution to extending the lifespan of WSNs, they face some challenges, such as the stochastic nature of the energy sources [139; 140; 141]. If the source of the EH technique is abundant, then a sensor node can be powered continuously [48], thereby enabling perpetual operation. However, since most energy sources are discontinuous and provide varied levels of energy at different times, sensor nodes powered by EH must be designed to store the scavenged energy when the natural source is present for later use [120]. Table 8 presents the strengths and drawbacks of the different EH techniques presented in this subsection. It is important to note that these EH techniques are expected to have a similar impact in LWSNs as in traditional WSNs, as they are not affected by the linear topology of LWSNs. \begin{table}
\begin{tabular}{c c c c c} \hline
**Energy Source** & **Transducer** & **Conversion Mechanism** & **Typical Power Density** & **Conversion Efficiency** \\ \hline Solar (Outdoor) & Solar panels & Photovoltaic effect & 15 mW/cm\({}^{2}\)[128] & 15–25\% \\ \hline Flow (Wind) & Wind turbine & Electromagnetic induction & 7.6 mW/cm\({}^{2}\) @5 m/s [129] & 30–50\% \\ \hline Flow (Hydro) & Turbine & Electromagnetic induction & N.A. & 70-90\% [130] \\ \hline Thermal & Thermoelectric generator & Seebeck effect & 15 \(\mu\)W/cm\({}^{3}\)[125] & 5–17\% \\ \hline Mechanical & Magnetostrictive materials & Magnetostriction & 145 \(\mu\)W/cm\({}^{3}\)[131] & 10–50\% \\ \cline{2-5}  & Piezoelectric materials & Piezoelectric effect & 4.57 mW/cm\({}^{3}\)[132] & 10–50\% \\ \cline{2-5}  & Electrostatic materials & Capacitance modulation & 50 \(\mu\)W/cm\({}^{3}\)[128] & 10–50\% \\ \hline Directed RF & Antenna & Electromagnetic induction & 50 mW/cm\({}^{2}\)[131] & 5–30\% \\ \hline Ambient RF & Antenna & Electromagnetic induction & 2 \(\mu\)W/cm\({}^{2}\)[133] & 5–30\% \\ \hline Electric Field & Capacitive transducers & Electrostatic induction & 0.04 \(\mu\)W/cm\({}^{3}\)[134] & 5–30\% \\ \hline Magnetic Field & Current transformers & Electromagnetic induction & 100 mW/cm\({}^{3}\)[135] & 5–30\% \\ \hline Biomass & Microbial fuel cell &
\begin{tabular}{c} Bioelectrochemical conversion \\ \end{tabular} & 300 \(\mu\)W/cm\({}^{2}\)[136] & \textless{}1\% [137;138] \\ \hline \end{tabular}
\end{table} 
Table 7: Classification of energy harvesters based on energy source, conversion materials, and conversion mechanism.