# Energy Harvesting Techniques for Wireless Sensor Networks/Radio-Frequency Identification: A Review
## 3 Green Radio-Frequency Identification
### Active RFID System
This affords a deeper analysis and evidence in support of the notion that it is ideal for applications involving settings where a thermal difference is evidently accessible, e.g., human body. The popularity of the thin film approach is again supported in [108]. The aforementioned paper provides a design of rechargeable battery in an RFID tag environment in great detail. Capacitors are very effective components for storing energy. They are equipped with an inherent ability to hold energy instantaneously, thus making them the preferred device for energy storage purposes. Meanwhile, there is a direct proportionality between capacitor size and energy storing capacity; the capacity to hold energy greatly depends on the size of the capacitor. Evidently, bigger sized capacitors store more current than smaller sized devices. Capacitors are also prone to damage in the face of high current and voltage fluctuations, making them dependent on the deployed environment. There is an evolution towards the development of super-capacitors and ultra-capacitors that have the capacity to store greater amounts of current. When ultra-capacitors are integrated into rechargeable batteries, they provide both longevity and solidity to the rechargeable batteries. Ultra-capacitors can store current in many times the amount of conventional capacitors, and as a result have become the design choice of many researchers and industry players. These ultra-capacitors are optimally appropriate in power conversion electronics circuits, as shown in [109]. Hybrid models have also emerged in which two models are utilized rather than a single model. Such models have found application and deployment in solar-powered WSN nodes [110]. In [111], the authors proposed a model, named \"_Prometheus_\", involving the deployment of a hybrid method. In this configuration, a dual phase storage scheme is discussed, consisting of a super-capacitor in one phase and a lithium rechargeable battery in the second phase. This scheme is deployed as a safeguard for running a Berkley's Telos Mote from a PV solar panel system. The experimental results are encouraging, suggesting that this system can run for 43 years in an application having 1% load, and can last for 1 year with a 100% load value. Powering the electronics embedded into a smart garment is essential for e-textiles. Regarding energy storage devices, batteries are accepted as one of the most important and efficient ways to establish electricity networks. However, other technologies can be used, such as super-capacitors, which can now achieve energy density levels that are comparable to lead-acid batteries. In addition, super-capacitors are environmentally friendly and offer high power density, fast charging/discharging speed, and a long lifecycle [112]. The requirements of batteries change significantly depending on the wearable energy consumption. Three different types of devices can be distinguished, depending on their power needs [35; 113; 114]:  
* Low-end devices. These devices are similar to traditional watches or pacemakers that can be powered for a long time (years) by button type batteries, given their low energy consumption (usually under 100 \(\upmu\)W). * Mid-range devices. These devices consume an average of 500 mW, considering their use of wireless communications transceivers. They usually last less than a day (often just several hours) when transmitting continuously, although certain technologies make use of sleep modes or periodic transmissions (e.g., BLE beacons) to last long. This kind of device requires bulkier batteries than low-end devices (e.g., AA or AAA batteries), making them less appropriate to be embedded into smart clothing. * High-end devices. These are devices similar to smartphones or laptops that consume up to 50 W. They usually utilise Li-ion batteries, which can be bulky and add weight to a garment. Table 7 presents certain examples of wearable devices with their power consumption and the basic characteristics of the battery that they usually carry.