# Energy Harvesting Techniques for Wireless Sensor Networks/Radio-Frequency Identification: A Review
## 3 Green Radio-Frequency Identification
### Active RFID System  
A battery-assisted RFID tag that uninterruptedly broadcasts its signal is known as Active RFID. Active RFID tags are usually deployed as \"beacons\", which precisely trail the real-time position of targets or in high-speed settings such as tolling. Active tags which are battery powered have a greater reading range when compared to passive tags; however, they are also much costlier. A summary of the active RFID system is given in Figure 4.  
The most important issue when considering an active RFID system is the energy storage device. The batteries and capacitors are the two most common storage devices. A summary comparison for these two commonly storage devices are provided in Table 6.  
Batteries have been the most notable energy sources for countless devices since their invention, and are still the most used portable power source in the world [105]. However, the problem with \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Issues** & **Battery** & **Capacitor** \\ \hline \multirow{4}{*}{Advantages} & • High power density. & • High power density. & • High power density. \\  & • High energy density. & • Does not loose ability to retain power \\  & • Easily replaceable in most cases. & • with time. & • with time. \\ \hline \multirow{4}{*}{Challenges} & • Poor to “form factor”. & • Low energy density. \\  & • Rechargeable form loses ability to retain & • Susceptible to damage with \\  & • Power with time. & current fluctuations. \\  & • Have to be replaced periodically, and & • Capacity is highly dependent on size. \\  & difficult to replace in larger numbers & • Harder to replace compared to batteries. \\  & and some applications in remote areas. & • \\ \hline \multirow{4}{*}{Improvements} & • Introduction of thin film technology has & • Introduction of ultra-capacitors have \\  & removed the issues related to & enables the best of both components \\  & • form factor. & with high power and energy densities. \\  & • There are a range of new technologies & • Size of ultra-caps are also significantly \\  & which could have been tested, from & smaller than regular capacitors. \\  & lithium-ion varieties to & \\  & redox-flow batteries. & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison of the two most common storage devices. Figure 4: Active RFID system. batteries is that they have a limited lifespan. After this certain period, they have to be replaced. Even with rechargeable batteries, there is a certain period after which the energy retaining capabilities of the battery diminish considerably. Additionally, form factor morphology is another key parameter that is considered when choosing the right storage device for a particular use-case. However, the relationship between these two characteristics (capacity and form factor) is most often conflicting. As the size of the storage device rises, normally, the device size will become larger, and vice versa [106]. Taking the capacity and form factor issue into consideration, batteries are slowly becoming a less viable option for RFID environments. Consequently, a compromise is needed to maintain desirable battery design in terms of both practicality and size, in order to retain batteries as options as storage devices for use in RFID environments. Extensive research has been carried out by academic institutes and the industrial sector on how to make energy storage devices as small and flexible as possible while having ample capacity and lasting usability in an RFID tag environment. With the introduction of thin film technology, batteries are starting to adapt to form factors appropriate for usage in wireless sensors. The end-goal of this process is a system which will enable electronic devices fabricated in a paper-thin width range. [PERSON] et al. [107] describes a thin film battery scheme in conjunction with suitable option for thermo-electric micro-systems. They recommend a strategy for fabricating thin film solid-state rechargeable batteries.