# Energy-Aware System Design for Autonomous Wireless Sensor Nodes: A Comprehensive Review
## 3 Energy Supply for Wireless Sensor Nodes
### Wireless Power Transfer  
Wireless power transfer (WPT) provides several benefits such as safety, flexibility and the possibilities of range extension and battery-free realization. It has a great potential in various applications, such as portable systems, medical systems, non-accessible systems and mobile systems and robots. In energy-aware wireless sensors, various combinations with energy harvesting and battery-free systems can be realized, where the WPT is the main source of energy supply. It can also be used on-demand or permanently as a supplement  
Figure 4: Selected examples of hybrid converters including: (**a**) photovoltaic/thermoelectric generator (see [41]); (**b**) triboelectric/thermoelectric generator (see [42]); (**c**) piezoelectric/electromagnetic generator (see [43]); and (**d**) photovoltaic/triboelectric generator (see [44]). to energy harvesting. In applications where a high reliability is required, the WPT can provide the functional guarantee. It can therefore promote the spread of sensor systems that are supplied with energy harvesting. The WPT can be classified by two major types of techniques: far-field transmission and near-field transmission [46]. Far-field transmission realizes, due to safety issues, a low-power transmission level and is therefore interesting in cases where transmission efficiency is not the highest priority. One example for far-field transmission is laser-based transmission through a direct line-of-sight path or microwave transmission at high frequencies. More interesting is the radio frequency transfer [47; 48] and capacitive power transfer [49]. Charging via radio frequency offers flexibility of positioning and possibilities of charging in several frequency bands in the range of MHz to GHz. Nevertheless, the main challenge of RF energy transmission is the low power level and the RF-DC power conversion efficiency (PCE). In [48], a passive RF-DC converter is designed for energy harvesting at ultra-low input power at 868 MHz. The converter consists of a reactive matching circuit to minimize discontinuities and signal reflections, a passive Dickson voltage multiplier to increase the voltage level and a rectifier [50]. It stores energy in an input inductor and capacitance during the negative wave and conveys it to the output capacitance during the positive wave. The realized PCE of the proposed RF-DC converter outperforms [48] state-of-the-art RF converters especially at a low input power of \(-40\) dBm. Near-field WPT realizes a high efficiency and is more practicable considering safety and power level. It can be realized by various methods such as capacitive power transfer (CPT) and inductive power transfer (IPT). In CPT, the sender and the receiver are the plates of an air capacitor [49]. The level of transmitted power is strongly dependent on the distance between the plates. The efficiency becomes extremely low at higher distance. Generally, big plates, having an ideal alignment at extremely low distances, are used to reach an acceptable energy level for supplying a wireless sensor node. Additionally, the dielectric properties of the materials between the two plates, as well as in the environment corresponding to the stray field, have an important influence on the transmission efficiency. This method is therefore not suitable for use in dirty or wet environments [46]. Due to the use of large plates, this technique is not suitable for small devices or ultra-high-power applications [46]. Magnetic Resonance (MR) is a method of wireless power transfer suitable for a medium distance range between sender and receiver, from centimeters to few meters, realizing a medium efficiency [46] and able to transmit power in the range of mW to few W. It is characterized by a high-quality factor due to the self-resonating of the coil between their internal capacitor and inductor in the range of MHz [47; 51]. Besides, it can supply multiple devices [51; 52] at the same time on different positions.