# Energy Harvesting Techniques for Wireless Sensor Networks/Radio-Frequency Identification: A Review
## 1 Introduction
### Energy Harvesting and Wireless Charging Techniques
Today, we can see that the wireless power charging concept has already been applied in numerous applications, such as power medical sensors and implantable devices [57], to restock sensors embedded in concrete wall [58], and to power a ground sensor from an unmanned aerial vehicle [59]. Generally, energy transfer techniques can be classified into (i) non-radiative coupling-based charging, which can be classified into three techniques: magnetic inductive coupling [60], magnetic resonance coupling [61], and capacitive coupling [62]; and (ii) radiative RF-based charging, which can be classified into two techniques: directive RF power beamforming and non-directive RF power transfer [63]. However, in capacitive coupling, the achievable amount of coupling capacitance is dependent on the available area of the device [64]. Nevertheless, for a typical-sized portable electronic device, it is hard to generate sufficient power density for charging, which imposes a challenging design limitation. As for directive RF power beamforming, the limitation lies in fact that the charger needs to know the exact location of the energy \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Environment**} & \multicolumn{4}{c}{**Power Harvester**} \\ \cline{2-5}  & **Solar Panel** & **Wind Generator** & **Thermoelectric** & **Electromagnetic** \\ \hline Power density of the indoor environment & 100 \(\mu\)W/cm\({}^{2}\) & 35 \(\mu\)W/cm\({}^{2}\) @ wind speed \(<\) 1m/s & 100 \(\mu\)W/cm\({}^{2}\) @ 5 \({}^{\circ}\)C motion (Hz) & 4 \(\mu\)W/cm\({}^{3}\) @ human motion (Hz) \\ Power density of the outdoor environment & 10 mW/cm\({}^{2}\) & 3.5 mW/cm\({}^{2}\) @ wind speed \(\leq\) 8.4m/s & 3.5 mW/cm\({}^{2}\) @ 30 \({}^{\circ}\)C machine (kHz) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of the indoor and outdoor energy harvesting and characteristics. receiver [65]. Due to the obvious limitation of above two techniques, wireless charging is usually realized through three other techniques: magnetic inductive coupling, magnetic resonance coupling, and non-directive RF radiation [16]. In non-directive RF radiation schemes, electric energy is sent as an electromagnetic radiation within the RF spectrum of 300 GHz and 3 kHz [66]. RF energy transfer is suitable for far-field communications. Experiences have indicated that RF power transfer has poor RF-to-DC energy conversion efficiency when confronted with RF harvested power. Detailed information on this subject is presented in [67; 68]. Using Maxwell's equation, electric current is initially generated from a magnetic coupling that is tuned to resonate at the centre frequency [69]. Electric energy is then transported through the magnetic field. Lastly, magnetic resonance coupling is generated by an evanescent field, which generates and sends electrical energy between two resonators [61]. To attain this type of resonator, a capacitance is inserted between an induction coil. Inductive and magnetic resonance coupling techniques are classified as short-range communications, also known as near-field wireless communications (NFC) [70]. Near-field wireless transmission is characterised by high power conversion efficiency, which is dependent on the coupling coefficient and the distance between two coils/resonators. The operating range (distance between transmitter and receiver) is a major challenge for near-field wireless transmissions because power rapidly decreases as the distance between devices increases (i.e., inverse square law) [71]. The advantages, disadvantages, and effective charging distance of these three techniques are summarised in Table 5.  
Wireless energy delivery to deployed sensor nodes have been investigated in several studies [35; 36; 37]. Wireless energy delivery is a new frontier that must be explored by wireless charging technologies, because it creates the environment in which sensor nodes are able to share energy between neighbours.