## 4 Different Application Models of Wireless Power Transfer Technologies
### Application with Static Wireless Charging Station (WCS): Mobile Nodes vs. Static WCS  
In this scenario, a WCS is deployed across a sensor network to wirelessly transfer power to IoT nodes within its coverage area or vicinity. The WCS transfers energy to the mobile nodes wirelessly as the nodes move within the coverage distance of the charger. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref** & **Year** & **IoT Energy Harvesting Mode** & **Characteristics of the Paper** \\ \hline
[2] & 2018 & RF-based WPT & The paper considered a wireless RF powered IoT system with a central node and multiple RF powered sensors acting as both power receivers and data transmitters \\
[12] & 2018 & EM radiation & The paper revealed a case of device-to-device (D2D) communication seen in the data exchange between the nodes and the WCS \\
[18] & 2017 & RF-based WPT & The paper provided a high-level abstract system model of WPSN based on real-life experiments. \\
[25] & 2016 & RF-based WPT & The study provided the vulnerabilities associated with WPT networks \\
[23] & 2017 & EM wave energy & The study provided a concise review on some recent advancements specific to near-field WPT techniques \\
[6; 30] & 2018 & RF-based WPT & The study provided health and safety requirements necessary for the permissible exposure level to EM radiation as well as the charging performance \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Different modes of IoT energy harvesting for WPT technology. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref.** & & **Wireless Power Transfer Techniques** \\ \hline
[15; 23; 41; 42; 43] & **WPT via Radio Waves** & **Resonant Coupling** & **Inductive Coupling** \\ Field & Electromagnetic (EM) & Electric (E), Magnetic (H), or EM & Magnetic (H) \\ Method & Antenna & Resonator & Coil \\ Efficiency & Low to High & High & High \\ Distance & Short to long & Medium & Short \\ Power & Low to high & High & High \\ Safety & EM & None (Evanescent) & Magnetic \\ Regulation & Radio wave & Under discussion & Under discussion \\ Applications & Electric vehicles & Electric vehicles, mobile phones & EVs, mobile phones \\ Frequency & - & 5.92–12.5 MHz & 125–150 kHz \\ Output Power (W) & - & 4.2 & Up to 5 \\ Efficiency (\%) & - & 40–60 & 70–90 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Characteristics of wireless power transmission. This application model addresses the energy provisioning problem of the energy-hungry opportunistic IoT networks [12]. Because of the pervasive nature of wireless mobile devices, their energy-supply requirements over time have not been subdued with the use of plugs and wires, as the wired charging technologies cannot cope with them [12]. This brought into light the emergence of WPT which serves as a viable solution that enables battery-recharge without the use of plugs or wires. For the current scenario, an apparatus which performs this function is known as the WCS. Despite the obvious usefulness of using a WCS, this technique is limited in capability due to some limitations associated with WPT technology, which revolve around hardware capability [12], human security and safety considerations [12; 25; 27; 28], thus limiting the use of WCS from radiating EM waves of higher power thereby further restricting the charging coverage of WCS. An alternative idea will be to install multiple copies of WCS to cover a larger percentage of the network area but this also comes with huge deployment costs. In essence, in spite of the limited charging coverage of WCS, energy is still supplied to the IoT nodes operating within the charging coverage. This is possible because of the random mobility-patterns of the sensor nodes.