Elements of Radio Frequency Energy Harvesting and Wireless Power Transfer Systems

Taimoor Khan, Nasimuddin, Yahia M.M. Antar

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000257861
• Edition: 1
• Topic: Naturwissenschaften
• Subtopic: Energie

This book focuses on elementary concepts of both radio frequency energy harvesting (RFEH) and wireless power transfer (WPT), and highlights their fundamental requirements followed by recent advancements. It provides a systematic overview of the key components required for RFEH and WPT applications and also comprehensively introduces the pioneering research advancements achieved to date. The state-of-the-art circuit design topologies for the two different applications are presented mainly in terms of antenna operating frequencies, polarization characteristics, efficient matching network circuits, rectifier topologies, and overall rectenna systems. The book serves as a single point of reference for practicing engineers and researchers searching for potential sources and elements involved in the RFEH system as well as in the WPT system, and need rapid training and design guidelines in the following areas:

• Different sensing elements used in RFEH and WPT

• Inclusions of mathematical expressions and design problems

• Illustration of some design examples and performance enhancement techniques

Part B

Elements of Wireless Power
Transfer (WPT) Systems

5 Antennas for WPT Systems

5.1 Introduction

Wireless power transmission (WPT) is the technique to transfer power for short or long range without using conductor from transmitter to receiver. It gives freedom from cables and is popular for wireless charging and batteryless implantable devices [1]. The WPT systems are classified into two categories: far-field and near-field systems. Based on the technology, power can be transferred wirelessly using resonant inductive coupling, magnetic resonance coupling, and microwave power transfer [2]. An inductively coupled WPT system has a pair of coupled coils. A magnetic resonant coupling WPT system uses a pair of coupled coils with additional capacitance, which makes the transmitter and the receiver to have the same resonant frequency [3]. Antennas play a great role in WPT systems at both transmitting and receiving ends to transmit and receive signals. Various types of antennas have been developed with the aim of wirelessly transmitting and receiving signals and power. Generally, for a coupled resonant WPT system, antennas of similar type are preferred for transmitting and receiving purposes. Magnetically coupled resonators have shown the capability to transfer power over a longer distance than those of inductive coupling, with higher efficiency than those of RF radiation approach [4].

5.2 Antennas for Near-Field WPT

Near-field WPT systems, also known as non-radiative coupling-based systems, are divided into inductively coupled and magnetically coupled resonant WPT systems [5]. Non-resonant induction can only operate within a close distance (less than the device dimension), whereas resonance coupling can be utilized to considerably increase the distance to mid-range, a factor of at least twice to thrice larger than the device dimension, as the transmitter and the receiver work at same resonant frequency [6,7]. The near-field power is attenuated according to the cube of the reciprocal of the charging distance. In this case, generally coil-based antennas are utilized for power transfer. Helix-type loop antenna [8], Archimedean spiral antenna [9], helical antenna [10], and tape-wound spiral antenna [11] have been utilized for non-radiative power transfer.

Based on loop antenna theory, Fotopoulou and Flynn [12] presented an analytical model for near-field magnetic coupling incorporating misalignment of the RF coil system. They also derived formulae for the magnetic field at the receiver coil when it is laterally and angularly misaligned from the transmitter. They proposed that the presented novel analytical model for near-field magnetic coupling, incorporating misalignment effects, can be used for wireless powering of RFID and implanted biomedical sensors. The efficiency optimization of multiple antennas by transmitting circuit considering the effects of cross-coupling and the sign of mutual inductances was proposed by Imura and Hori [13]. They used the equivalent circuits of multiple antennas to calculate the parameters of the impedance matching circuit, which were used to optimize the transmitting antenna to achieve high efficiency. Antenna geometry for coupled resonant wireless power transfer was discussed by Hirayama et al. [14]. They compared open-ended self-resonant helical antennas and short-ended capacitor-loaded helical antennas from the viewpoint of undesired emission, effect of conductivity, transfer distance, and effect of human body.

The equivalent circuit of a repeater antenna was proposed by Imura [15], including the sign of the mutual inductance, which occurs when repeater antennas are used and is decided by the position of the antenna. Repeater antennas have been proposed to extend the length of the air gap. Long-distance power transfer is achieved by simply installing a repeater between the transmitting and receiving stations. Sample et al. [6] proposed a new analysis method that yields critical insight into the design of practical systems, including the introduction of key figures of merit that can be used to compare systems with vastly different geometries and operating conditions to achieve a near-constant efficiency of over 70% for a range of 0–70 cm. An efficient WPT system was proposed and verified by Park et al. [8], for which it was deduced that a class-D power amplifier (PA) has an advantage as a source when the input resistance changes with the position of the receiving antenna. They investigated a method used to achieve efficient wireless power transfer over a near-field region when the distance between the antennas varies. First, an analysis of the characteristics of the two coupled antennas was performed. Then, the conditions that achieve an efficient WPT system for the load resistance and mutual coupling between the antennas were suggested. Then, they compared several types of PAs as a source of the WPT system. It was shown that the class-D PAs have an advantage in regard to the efficiency for a varied load resistance. A WPT system composed of an unbalanced-fed, ultra-low-profile inverted-L antenna on a rectangular conducting plane was proposed and analyzed numerically by Taguchi and Hirata [16], where the input impedances of the transmitting and receiving antennas were matched to 50 ohms by adjusting the length of the horizontal element and the feed point position. An efficiency of 92.2% was achieved at the design frequency of 100 MHz. A chip-to-package wireless power transfer concept applied to MMIC and antennas on an LCP substrate was presented by Aluigi et al. [9], in which an Archimedean spiral antenna matched to a heterogeneous transformer, which couples the power received by the antenna to the chip, was simulated at the working frequency of 35.4 GHz. Lee et al. [17] presented a system that supplies power to a rotating spindle’s diagnostic sensor using wireless power transfer technology based on electromagnetic coupled resonance and gauges the strain experienced by the spindle by attaching power-receiving coils to the spindle at 120° intervals and fixing the power-transmitting coils to the ground at 60° intervals.

Open-end and short-end helical antennas for coupled resonant wireless power transfer were discussed by Hirayama et al. [10] from the viewpoint of undesired emission and biological effect, as shown in Figure 5.1. Due to the effects of human body, the resonant frequency is varied for the open-end model, whereas the loss power is increased for the short-end model. Phokhaphan et al. [18] proposed a wireless power transfer system (Figure 5.2) which used a printed circuit board as an antenna for both the transmitter and receiver. The antenna in the proposed system was driven by a high-frequency inverter that operates at the resonant frequency of the antenna.

FIGURE 5.1 Equivalent circuit considering E-field and H-field coupling: (a) open-end model; (b) short-end model. (From Hirayama, H., Amano, T., Kikuma, N., and Sakakibara, K., “Undesired emission and biological effect of open-end and short-end antennas for coupled-resonant wireless power transfer,” 2013 Asia-Pacific Symposium on Electromagnetic Compatibility (APEMC), 2013. With permission.)

FIGURE 5.2 Configuration of prototype circuit: (a) main circuit; (b) control circuit. (From Phokhaphan, N., Choeisai, K., Noguchi, K., Araki, T., Kusaka, K., Orikawa, K., and Itoh, L.I., “Wireless power transfer based on MHz inverter through PCB antenna,” 2013 1st International Future Energy Electronics Conference (IFEEC), 2013. With permission.)

In Ref. [19], the authors proposed a new design approach that uses anti-parallel resonant loops for contactless energy transfer (CET). The forward and reverse loops forming an anti-parallel resonant structure st...