# Green IoT: A Review and Future Research Directions
## 5 Energy-Efficient and Eco-Sustainable Wireless Sensor Networks
### Energy Harvesting, Batteries, and Wireless Charging Techniques
#### 5.3.3 Wireless Charging Techniques  
Wireless power charging technology allows for more controlled recharging of network elements, resulting in increased sustainability and reliability in wireless sensor networks (WSNs). This concept has been implemented in various applications, including medical sensors, implantable devices, wireless sensor replenishment in concrete, and powering a ground sensor from an unmanned aerial vehicle. Energy transfer methods can be divided into two main categories: non-radiative coupling-based charging, such as magnetic inductive coupling, magnetic resonance coupling, and capacitive coupling; and radiative RF-based charging, which encompasses directive RF power beamforming and non-directive RF power transfer. Capacitive coupling is limited by the available area of the device, making it difficult to generate enough power density for charging, which poses a design challenge. Similarly, directive RF power beamforming requires knowledge of the energy receiver's exact location, which can be a limitation. Therefore, wireless charging is often achieved  
Figure 7: A selection of battery types commonly used in IoT/sensor devices. through alternative techniques such as magnetic inductive coupling, magnetic resonance coupling and non-directive RF radiation, which are less restrictive [61]. * The non-directive RF radiation technique uses electromagnetic radiation in the radio frequency range between 300 GHz and 3 kHz to transmit electric energy. This method is well suited for far-field communication; however, it has low efficiency in converting RF energy to DC energy when the harvested RF power is low. For further details on this technique and other RF power transfer methods, readers can refer to [62; 63]. * Magnetic inductive coupling involves generating electrical energy between two coils that are tuned to resonate at the same frequency through magnetic coupling [64]. The magnetic field is used to transfer the required electric power. An illustration of the operating concept is depicted in Figure 8. * Magnetic resonance coupling involves the utilization of an evanescent field that generates and transmits electrical energy between two resonators [65]. To create this type of resonator, a capacitance is inserted between an induction coil. An illustration of the operating concept is depicted in Figure 8. * The inductive coupling and magnetic resonance coupling techniques both fall under the category of short-range communications, also known as near-field wireless communications (NFC). For more information on these, readers can refer to [66]. The high power conversion efficiency of near-field wireless transmission is well-known, and it is strongly influenced by the coupling coefficient and distance between the two coils/resonators. Nonetheless, the operational range (i.e., the distance between the transmitter and receiver) poses a significant challenge for near-field wireless transmission since the power diminishes rapidly as the distance between the devices increases [67]. Table 5 provides a summary of the advantages, disadvantages, and effective charging distance of three techniques: the non-directive RF radiation technique, magnetic inductive coupling, and magnetic reso  
Figure 8: The operating concept of wireless charging by both magnetic inductive and magnetic resonance coupling. nance coupling. Based on information in existing reviews or research in the field [68; 69], these factors were determined by the authors of this article. Several studies have examined the method of delivering energy wirelessly to sensor nodes that have been deployed in the field [70; 71; 72]. However, new challenges are introduced such as energy cooperation, as nodes can now share energy with their neighbors. In future wireless networks, nodes are expected to be able to both gather energy from their environment and transfer energy to other nodes, making the network self-sufficient [73].