# Energy Harvesting Techniques for Wireless Sensor Networks/Radio-Frequency Identification: A Review
## 1 Introduction
### Energy Harvesting and Wireless Charging Techniques
Therefore, in the nearest future, wireless networks nodes are envisioned to incorporate energy sharing scheme by harvesting energy from the environment and transfer some of these energy to other sensor nodes, making a self-sustaining network [72]. To realise this paradigm, multi-hop energy harvesting techniques have been studied [68], thus opening a new dimension in the development of wireless charging protocols and energy cooperative systems, as well as energy-efficient routing. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{**IoT Applications**} & \multicolumn{5}{c}{**Energy Harvesting Source**} \\ \cline{2-6}  & \multicolumn{2}{c}{**Solar Panel**} & \multicolumn{2}{c}{**Wind Generator**} & \multicolumn{2}{c}{**Electromagnetic**} & \multicolumn{1}{c}{**Thermoelectric**} \\ \hline \multirow{5}{*}{Smart Home} & _Outdoor sensor_ & & & & \\  & _Smart thermostat_ & & & & \\  & _Air quality monitor_ & & & & \\  & _Lightting_ & & & & \\  & _Security monitor_ & & & & \\  & _Smart door lock_ & & & & \\ \hline \multirow{2}{*}{Wearables} & _Smartwatch_ & & & & \\  & _Monitoring and tracking_ & & & & \\ \hline \multirow{2}{*}{Health} & _Medical patch_ & & & & \\  & _Fitness band/monitor_ & & & & \\ \hline \multirow{2}{*}{Industrial} & _Factory automation_ & & & & \\  & _Machine monitor_ & & & & \\ \hline \multicolumn{2}{c}{Vehicles} & _Wireless parking meter_ & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: IoT devices/applications and their suitability for use with energy harvesting sources. Simultaneous Wireless Information and Power Transfer (SWIPT) is a technique that can reduce the total recharging cost to enhance the lifespan of WSNs. The concept of SWIPT was first introduced in [73]. SWIPT has recently attracted attention in the field of wireless communication networks [66; 74; 75]. In the era of 5G communication, SWIPT technology could be fundamentally important for energy and information transmissions within numerous types of modern communications networks. In WSNs, power usage is usually divided into three parts [76]: sensing, data processing and communication. Among the three, communication is the most costly function in a typical sensor node [77]. Hence, local data processing is important to minimise the power utilisation of WSNs because the failure of any node can re-route the structure. In addition, robust communication protocols are critical to efficiently exploit energy sources. [PERSON] et al. [78] proposed a hybrid TS and PS spectrum sharing protocol for EH wireless sensor nodes. However, communication protocols must be further investigated to offer an optimal self-sustainable solution. Moreover, clustered WSNs with SWIPT that re-charge low energy relay nodes in cooperative clustered WSNs have been considered in [79], in which the authors considered a WSN consisting of multiple clusters of sensors and a sink node that collects data from the sensors in the clusters. To enable SWIPT, the head node of each cluster works as an information and power transferred. Relay node receiver harvests the RF energy from the head node of the cluster and recharges the batteries. [PERSON] et al. [79] proposed a distributed iteration algorithm with closed-form transmission power, PS ratio and relay selection by utilising decomposition. [PERSON] et al. [80] focused on the development of network deployment and its routing strategy. The idea is to reduce the total recharging cost to enhance the lifespan of WSNs. An optimisation problem is formulated using joint network deployment and routing in [81] by assuming that the sensors can be continuously recharged before their power source is exhausted and by obtaining complete knowledge of CSI. A few prototype implementations of sensor nodes are investigated in [81] and [82]. By reaping the benefits of SWIPT, low power WSNs deployed in the health care sector can recharge power sources while in operation, thereby improving the QoE [83].