# Energy-Aware System Design for Autonomous Wireless Sensor Nodes: A Comprehensive Review
## 3 Energy Supply for Wireless Sensor Nodes
### Wireless Power Transfer
For that, tuning resonance and avoiding interference due to multiple receivers are required during the charging process. To increase the charging distance, some designs introduce resonator coils, which are associated to each side [53]. If the distance between sender and receiver is in the range of few cm or at least smaller than the coil diameter [54], IPT can be used to realize a transmission efficiency superior to 60%. IPT systems transmit power through coils with a specific frequency in the range of hundreds of kHz [46]. Over the last years, significant investigations have been conducted to improve the performance of IPT systems, especially for wireless charging of mobile devices, but also medical applications [55] and electric vehicles [56], which can be scaled up or down for wireless sensors. Different aspects could be optimized considering coil geometry and technology, operating frequency, multi-coil systems, resonant compensation topologies and misalignment. The aspect of misalignment is very important for efficiency of power transfer. An IPT system reaches its best performance under resonance conditions and at concentric transmitter and receiver coil positions with minimal distance to avoid field losses. However, during use, misalignments between sending and receiving coils can occur, which can be lateral, vertical and angular (see Figure 5). These have a negative impact on the level of transmitted power and therefore on transmission efficiency and charging time. Increasing the tolerance to coils misalignment is one of the main aspects that should be considered in the design of a wireless sensor node or device. For that, a single transmitter-to-single receiver (SISO) coil system should be controlled to increase the transmission efficiency despite variable loads and misalignment. This can be realized by controlling the system frequency [57], the system gain [58], the compensation capacitors [59] and the DC-DC rectifier [60]. To overcome misalignment effects, multiple input-single output (MISO) coil systems can be realized [61], which are composed of multiple transmitter coils connected in series [62; 63] or parallel [64; 65]. The sending coils can be activated simultaneously [62] or one-by-one independently [65] using multiple supply circuits [66], switches [64; 67] or both [68]. The main difference between systems used to charge batteries and those to supply battery-free devices is the detection procedure. In fact, battery-free systems do not use communication for the activation of the transmitter side system due to lack of stored energy, which can be useful to activate communication procedure [65]. However, keeping the transmitter side ON to detect the presence of device for long times increases the energy losses. In [65], the authors proposed a passive peak detector circuit of the receiver coil with controls the working circuit frequency to reduce the power consumption. When the device is detected, the system increases the working frequency to reach the resonance, which helps the system to deliver energy able to supply the circuit of the receiver side to communicate with the transmitter side.