# Energy Harvesting Techniques for Wireless Sensor Networks/Radio-Frequency Identification: A Review
## 1 Introduction
### Energy Harvesting and Wireless Charging Techniques  
Key features of wireless sensors energy source such as sustainability and reliability, as well as reduction of greenhouse gas emissions can be met through advances in renewable energy technology [55]. Moreover, the renewable energy technology is one of the promising ways to address the EE issue of WSN located in rural and remote areas. In this terrain profile, it is difficult to replace batteries due to geographical limitations (challenging terrain), which makes access to these sites difficult [32]. The solar cells have low maintenance needs and high reliability, with an expected life span of 20-30 years. Additionally, new sensor technologies have emerged that harness power from their immediate environments, such as wind and kinetic energy [33]. The harvested energy is then converted to electrical signals, which are either consumed directly or stored for later usage. For example, using solar panels to charge a rechargeable battery during daytime. At night, nodes switch to conservative mode drawing energy from the stored power. During the protocol design stage, consideration must be given to uneven residual energy distribution, which is the difference in the quantity of energy collected [34]. In a case that there is no power to harness, battery life cycle capacity is formulated to calculate whetherits total storing capability referenced to the magnitude of charge/discharge cycles, stated as depth-of discharge (DoD), is enough for the job. The life cycle capacity is estimated as:  
\[\text{Life cycle capacity}=\text{Rated battery capacity}\times\text{Rated charge}_{-}\text{discharge cycle life}\times\text{DoD} \tag{2}\]  
For example, for the _ML1220_ rechargeable coin cell, the rated capacity is 17 mAh, charge_discharge cycles is 1000 cycles, and DoD is 10%; thus life cycle capacity = 17 mAh \(\times\) 1000 cycles \(\times\) 10% per cycle = 1.7 Ah. Renewable energy technology is associated with energy estimation schemes for astute energy management. Thus, there is the need to undertake inept energy-saving mechanisms in addition to renewable energy technology in order to attain a high reliability status. The sensors may incorporate dynamic behaviour tendencies in the face of the estimated energy not been able to sustain them in the next recharge cycle. Hence, they can optimise decisive parameters such as sampling rate, transmit power and duty cycling to adapt their power consumption according to the periodicity and magnitude of the harvestable source. On the other hand, it is justifiable to allocate sensor nodes with large residual power with bigger sleep duration and shorter RF range, whereas, those with bigger residual power are selected as the preferable routing route [56]. However, efforts have not been made to develop protocols assuming battery degradation over time (leakage, storage loss), which will impact WSN performance. The attributes and operations of the renewable energy sources available in outdoor environmental conditions are very different from those found in indoor industrial and commercial environments. Table 3 shows a summary of the indoor and outdoor energy sources and their characteristics. Given the wide spectrum of IoT device formats, applications, and use cases to choose from, it is nearly impossible to authenticate with assurance that a given device will make good a IoT energy harvesting device without prior knowledge of application specifics and system operations. This notwithstanding, some indices are available to grade some devices as likely energy harvesting devices or otherwise. Inspiration can be drawn from the viewpoint of cost-effectiveness and technical standpoint. Table 4 is a sample IoT devices/applications and their suitability for use with energy harvesting sources in South Korea. The evolution of wireless power charging technology has made it possible for energy constraint devices to maintain functionality in a more controllable manner, thus increasing the sustainability and reliability of WSNs [17].