# Green IoT: A Review and Future Research Directions
## 6 Energy-Efficient Radio-Frequency Identification
### Active RFID Systems  
Battery-powered RFID tags that continuously broadcast their own signal are used in an active RFID system. These tags, known as \"beacons\", are frequently used to precisely track the location of assets in real-time or in high-speed environments such as tolling. They are more expensive than passive tags but have a considerably larger scan range. Figure 13 summarizes the characteristics of the active RFID system. Figure 12: Passive RFID system. The energy storage system is the primary factor to take into account when considering an active RFID system. The two most common options are batteries and capacitors. Details are given in the subsequent paragraphs and summarized in Figure 14.
* _Batteries:_ Batteries have been a popular power source for various devices since their invention and are still widely used today. However, they have a short lifespan and must be replaced after a certain period. Even rechargeable batteries lose their energy-retaining capabilities over time. Another important factor to consider when choosing a storage device is the form factor, which refers to the shape and size of the device. However, the relationship between form factor and capacity is often conflicting, as a larger capacity often means a larger device size [111]. As a result, batteries may not be the best option for RFID environments, where a small and flexible device is needed. Research has been conducted to find ways to develop compact energy storage devices that offer more flexibility while maintaining a high capacity. The emergence of thin film technology has made it possible for batteries to have the form factor needed for RFID applications [112]. This technology allows for the creation of electronic devices that are paper-thin and recognized as well-matched for thermo-electric micro-systems. Studies have shown that thin-film batteries are an ideal choice for applications where a thermal difference is present, such as the human body [113]. Overall, thin-film technology offers versatility and is a promising option for RFID tags. Figure 14: Comparison of energy storage options: batteries versus capacitors. Figure 13: Active RFID system. * _Capacitors:_ Capacitors are highly energy-efficient storage devices and do not diminish their capacity quickly, making them an ideal component for certain applications. However, the capacity of a capacitor is largely dependent on its size, and so capacitors often sacrifice form factors for a longer lifespan. Additionally, capacitors are unreliable to some extent because they are extremely susceptible to changes in current and voltage. However, recent advancements in capacitor storage technology have led to the development of ultra-capacitors and super-capacitors, which can store large amounts of energy and combine the reliability and performance of rechargeable batteries with the longevity of regular capacitors [114]. These ultra-capacitors are selected by designers because they have high energy densities that are many orders of magnitude higher than those of conventional capacitors. Some designs deploy a combination of storage devices, creating a hybrid system that combines the best features of different devices. These models have been investigated and tested using solar-powered wireless sensors with promising results [115]. The \"Prometheus\" model, for example, uses a dual-stage storage model in conjunction with a super-capacitor in the first stage and a lithium ion rechargeable battery in the second stage [116]. This system is regarded as a buffer for powering a Berkeley Telos more extracted from a PV solar panel system, and experimental results have shown that it can run for up to 43 years at a 1% load and up to 1 year at a 100% load. In summary, batteries are more suitable for long-term, steady power supply, while capacitors are better for high-power, short-term usage. The precise requirements of the RFID application will determine which option is best between the two. On the other hand, references [117; 118; 6; 10] focused on the transmission power issue.