# Energy Harvesting Techniques for Wireless Sensor Networks/Radio-Frequency Identification: A Review
## 3 Green Radio-Frequency Identification
### Active RFID System
The operating period of a battery is determined by its power requirements and energy density. Energy density is the amount of electrical energy stored in a battery in a given weight (Wh/kg). Moreover, the selection of energy storage devices depends on the type of applications. Recent research has revealed that hybrid models that use both components can achieve good results, considering their own capabilities. On the other hand, references [115; 116; 6; 10] considered the transmission power issue. The sleep mode technique, when the reader is not interrogating, to achieve energy saving was proposed by [10]. The authors in [6] proposed an algorithm and protocol to avoid overheating when the reader is not interrogating, in order to achieve energy saving. Meanwhile, the authors in [115; 116] proposed an algorithm and protocol to avoid tag collision during the transmission. Meanwhile, reference [22; 48] proposed adjusting the transmission power level dynamically, which could achieve energy saving, but less than a sleep mode technique. Recall that the major driver of green technology is ecological and energy fears. Reduction in RFID tags size should be exploited, as there is a direct correlation between quantity of non-degradable substance deployed in their engineering (e.g., biodegradable RFID tags, printable RFID tags, paper-based RFID tags), since the RFID tags themselves are generally hard to reuse [97]. Security and privacy are major concerns for IoT deployment. Implementing security algorithms requires a substantial device processing. Along with security, we must investigate appropriate mechanisms that consider energy consumption and the required QoS. The potentials of energy-efficient and secure mechanisms remain in their infancy, thus encouraging extensive research and development in this area. IoT involves resource-constrained devices, such as RFID, sensor nodes and high-end data servers. Therefore, finding and exploiting trade-offs is important to provide security among heterogeneous devices in the green IoT paradigm. Tags are typically energy-constrained, thus limiting the implementation of complex security algorithms. Energy-depleted RFID tags are a source of potential attacks from intruders with malicious intent. Moreover, modifying the data in RFID tags is also a security threat. Therefore, a natural solution is to password-protect the memory in RFID tags. Doing so is a challenge from an energy consumption perspective, because cryptographic algorithms \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Device** & **Power** & **Battery Type** & **Operating Period** & **Weight (g)** &
\begin{tabular}{c} **Size** \\ **(\(\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{ \mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{ \mathbf{ }}}}}}}}{}}}{{}}}}}}\) \\ \end{tabular}) \\ \hline Watches & 3–10 \(\mu\)W & Silver oxide button & 1–2 years & 2.4 & \(11.6\times 5.4\) \\ Pacemakers & 25–80 \(\mu\)W & Lithium button & 7–10 years & 2.83 & \(20\times 3.2\) \\ Hearing aids & N/A & Zinc-mercury & 25–30 days & 0.3–1.85 & (5.8–11.6)\(\times\) (3.6–5.4) \\ Digital clocks & 13 mW & Silver oxide button & 6–10 months & 15–25 & (5.8–11.8)\(\times\) (1.65-5.4) \\ LEDs & 25–100 mW & Silver oxide button & (depends on the usage & 15–25 & (5.8–11.8)\(\times\) (1.65-5.4) \\  & & & frequency) & \\ Pedometers & 250 mW & Silver oxide button & (depends on the usage & 15–25 & (5.8–11.8)\(\times\) (1.65-5.4) \\  & & & frequency) & \\ Portable radio & 500 mW & AAA & (depends on the usage & 8.5–11 & \(10.5\times 44.5\) \\  & & & frequency) & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Examples of wearable devices and the basic characteristics of their batteries. require a large amount of energy and battery (active tag). In this regard, several methods with low energy requirements have been recently proposed for these cryptographic solutions [117; 118]. The authors in [119] developed a communication strategy where a tag periodically communicates its security status to the reader and is authenticated.