Far-Field Wireless Power Transfer and Energy Harvesting 1630819123, 9781630819125

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Far-Field Wireless Power Transfer and Energy Harvesting

For a listing of recent titles in the Artech House Electromagnetic Analysis Library, turn to the back of this book.

Far-Field Wireless Power Transfer and Energy Harvesting Naoki Shinohara Jiafeng Zhou Editors

Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the U.S. Library of Congress. British Library Cataloguing in Publication Data A catalog record for this book is available from the British Library.

ISBN-13:  978-1-63081-912-5 Cover design by Andy Meaden Creative © 2023 Artech House 685 Canton Street Norwood, MA 02062 All rights reserved. Printed and bound in the United States of America. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing from the publisher. All terms mentioned in this book that are known to be trademarks or service marks have been appropriately capitalized. Artech House cannot attest to the accuracy of this information. Use of a term in this book should not be regarded as affecting the validity of any trademark or service mark. 10 9 8 7 6 5 4 3 2 1

Contents

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23 24 25 30 32 34 35 38 40

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vi

Contents

 CHAPTER 4  Wireless Power Transfer Enabled Wireless Communication

61

4.1  Introduction 4.2  WPT and Backscatter Channels 4.3  Backscatter Communication Principle and Channel Model 4.3.1  The Principle of Backscatter Communication 4.3.2  Channel Coding in Backscatter Communication 4.3.3  Dyadic Backscatter Channel and MIMO Backscatter 4.4  Demodulation of Backscatter Signal 4.4.1  Pulsewidth Measurement Demodulation 4.4.2  PSK Demodulation References

61 62 65 65 67 69 70 72 73 74

 CHAPTER 5  Medical Applications

77

5.1  Introduction 5.2  Planar Phase-Controlled Metasurface 5.2.1  Conformal Metasurfaces for Wireless Power Transfer 5.2.2  Wireless Power Transfer for Implantable Devices In Vivo 5.3  Wireless Optogenetics 5.3.1  Cavity Resonator Capable of Powering Ultrasmall Wireless Optogenetics 5.3.2  Peripheral Nerves Stimulations 5.4  Introduction to Long-Range Wireless Communication Technology 5.5  Conclusion References

77 78 79 80 84 85 86

87 89 90

 CHAPTER 6  Indoor/Outdoor-Beam WPT with Beamforming 6.1  Indoor-Beam WPT 6.2  Outdoor-Beam WPT 6.3  Beam WPT in Space References

93 93 97 102 107

 CHAPTER 7   Solar Power Satellite

109

7.1  7.2  7.3  7.4 

109 110 110 111 112 113 115

Introduction History Concepts Challenges 7.4.1  Technical 7.4.2  Economic 7.4.3  Legal

Contents

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7.4.4  Schedule

7.5  Conclusion References

115

116 116

 CHAPTER 8  Low-Power Integrated Circuit Design for Energy Harvesting

119

8.1  Introduction 8.2  RF Energy Harvesting System 8.3  RF Rectifier 8.3.1  Basic Topology of a Rectifier 8.3.2  Operating Principle 8.3.3  Internal Resistance Modeling of Multistage Rectifier 8.4  Design Challenge of Low-Power Active Rectifier IC 8.4.1  Transit Frequency 8.4.2  Structure of MOSFET Devices in n-Well Process 8.4.3  Vdrop Comparison 8.4.4  Cross-Coupled Architecture of an Active Rectifier 8.4.5  Multistage RF Active Rectifier 8.4.6  Design and Optimization of Flying Capacitance 8.5  Design Examples 8.5.1  Example No. 1 8.5.2  Example No. 2 8.5.3  Example No. 3 8.6  Conclusion References

119 119 120 120 121 122 124 124 125 125 125 127 129 130 130 131 133 136 136

 CHAPTER 9  Energy Harvesting for Smart Grid Application

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9.1  Self-Powered Wireless Sensors in Smart Grid 9.2  Magnetic Field Energy Harvesting 9.2.1  Cabled-Clamped Magnetic Field Energy Harvester 9.2.2  Free-Standing Magnetic Field Energy Harvester 9.3  Electric Field Energy Harvesting 9.4  Conclusion References

137 140 142 145 149 150 152

 CHAPTER 10  Energy Harvesting from Low-Power Density Environments

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10.1  Introduction 10.2  Wideband Antenna Design 10.3  Wide Beamwidth Antenna Design 10.3.1  Potential Modes of a Metasurface 10.3.2  Geometry of the Proposed Metasurface Antenna 10.3.3  Rectifier Design 10.3.4  Measurement Result

155 155 158 159 162 163 164

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Contents

10.4  Conclusion References

167 168

 CHAPTER 11  Metamaterials and Metasurfaces for Wireless Energy Harvesting

171

11.1  Introduction 11.2  Design of Single-Mode Resonant Metasurfaces for Energy Harvesting 11.2.1  Design of Ring-Shaped Wi-Fi Band Energy Harvester 11.2.2  Complementary Split-Ring Resonator High-Frequency Wi-Fi Energy Harvester Design 11.3  Design of Multimode Resonant Metasurfaces for Energy Harvesting 11.3.1  Design of Energy Harvester with Nested Ring Structure 11.3.2  Design of Butterfly-Type Metasurfaces for Three-Band Energy Harvester 11.4  Design of Rectifying Metasurfaces 11.4.1  Metasurfaces Element and Rectifier Design 11.4.2  Array Design and Testing of RMS 11.5  An Optically Transparent Metantenna for RF Wireless Energy Harvesting 11.5.1  Design of Optically Transparent Metantenna 11.5.2  Wireless Energy Harvesting Performance 11.6  Summary and Conclusion References

171 172 172

List of Acronyms

209

About the Editors

213

List of Contributors

214

Index

215

176 179 179 181 184 184 190

195 196 198 201 203

Preface Wireless power transfer and energy harvesting technologies can have many potential applications in our daily lives. When wireless power transfer and energy harvesting are realized, people can use electricity, which has become crucial to our existence, without any worry about where it is coming from, much like air. There are already wireless charging enabled devices available in the market, in a wide range of products, such as toothbrushes, mobile phones, and drones. However, it is fair to say, these technologies are far from perfect and mature yet. This is why they are exciting research areas. In this book, we will introduce topics that are most likely to have commercial impact in the near future. We will focus in particular on far-field techniques. Chapter 1 provides a brief introduction for wireless power transfer and energy harvesting. We will also discuss the current status of commercialization and regulation for these technologies. Chapter 2 overviews wide-beam wireless power transfer for indoor applications. Two examples will be given to show how the technology can be adopted for health monitoring and infrastructure monitoring applications. Chapter 3 describes how to implement a radiative wireless power transfer system. We will demonstrate how to wirelessly power a backscattering module. The technology can be very useful for the Internet of Things (IoT) applications. Chapter 4 explains in detail how to establish wireless power transfer enabled wireless communications. The operation principle, including modulation, coding, channel, and demodulation of backscatter communications will be illustrated in this chapter. Chapter 5 focuses on medical applications of wireless power transfer, which has been and will continue to be a very hot research area. Several examples are given to show how medical devices can be wirelessly powered. Chapter 6 describes the essential technology for wireless power transfer: beamforming. The design considerations for indoor, outdoor, and space applications will be analyzed. Chapter 7 outlines the history, concept, and challenges of solar power satellites. Solar power satellites could collect the sun’s energy in space and then provide it to locations where it is needed on the Earth. The progress and challenges will be reviewed.

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Chapter 8 demonstrates how to carry out integrated circuit (IC) designs for energy harvesting. It will address how to implement IC chips that can work well under low power and low voltage conditions. Three design examples will be given. Chapter 9 shows how energy harvesting techniques can be utilized for smart grid applications. There are strong fields surrounding a power line. Both magnetic and electric energy harvesting techniques will be exemplified. Chapter 10 addresses the issue of how to effectively harvest energy from low power environments. One bottleneck of energy harvesting techniques for realworld applications is the low power level that can be scavenged. Several promising techniques will be introduced and analyzed. Chapter 11 reveals why metamaterials and metasurfaces are very useful and have a high potential for energy harvesting applications. The designs of metasurface structures, rectifying metamaterials, and metantenna will be exhibited in the final chapter. We hope that, with these independent but close-knit chapters, we will be able to provide an overview of the latest wireless power transfer and energy harvesting technologies.

CHAPTER 1

General Introduction Naoki Shinohara and Jiafeng Zhou

1.1  History of Wireless Power Transfer and Energy Harvesting Radio waves were first predicted as a form of energy by James C. Maxwell in 1864. John H. Poynting derived the Poynting vector in 1884, which describes radio waves as an energy flow. In 1886, R. Heinrich Hertz transmitted and received controlled radio waves and proved Maxwell’s theory of electromagnetism was correct. Then, Guglielmo Marconi made the first public transmission of wireless signals on July 27, 1896, in the United Kingdom, and Nicola Tesla conducted the first experiment on wireless power transfer (WPT) in Colorado Springs, United States, in 1899 [1]. This marked the beginning of WPT and energy harvesting from ambient radio waves. In the twentieth century, there was a remarkable evolution of wireless communication technology, which was first developed by Marconi. Civilization could not have been transformed at such a pace without wireless technology. On the other hand, we forget Tesla’s dream of a “world wireless system,” which motivated the telecommunication and electrical power delivery systems. However, in the twentyfirst century, Tesla’s dream has been regained as far-field WPT and energy harvesting from ambient radio waves for wireless communications. Far-field WPT, as a lost technology, was rediscovered by William C. Brown in the 1960s in the United States. He applied a microwave, mainly in the 2.4-GHz band, to transmit beam wireless power to a far distance. He developed and named the first system rectenna, coined from “rectifying antenna,” which was a wireless power receiver. He had the first patent for a rectenna in 1969 (Figure 1.1) [2]. Rectenna design is the most important technology, not only for far-field WPT but also for energy harvesting from ambient radio waves. Brown performed the world’s first WPT drone experiment in 1964 [3]. He conducted a three-step WPT experiment to (1) restrain a drone in the laboratory in June 1964, (2) guide the flying drone outside in October and November 1964, and (3) freely fly drones in an anechoic chamber (Figure 1.2) [4]. In step 2, the guided drone powered by microwaves flew for approximately 10 hours. In 1975, Brown and Richard Dickinson also conducted a 1-mile beam WPT field experiment at 2.4 GHz and 450 kW in Goldstone. Without Brown, WPT research would have been delayed for 50 years.

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2 �������������������� General Introduction

Figure 1.1  Rectenna patent by W. C. Brown [2].

Brown employed mainly microwave tube technology, magnetron and Klystron, and aperture, horn, and parabolic antennas. In the 1980s, a Japanese research group studied WPT. Hiroshi Matsumoto and Kyoto University’s group focused on semiconductors for microwave amplifiers and phased-array antenna technology for beamforming [5]. In 1992, they conducted a fuel-free airplane field experiment with phased-array antennas using gallium arsenide (GaAs) amplifiers at 2.4 GHz and 1.25 kW, which was named MILAX (Microwave Lifted Airplane Experiment). The microwave beam was electrically controlled by the flying airplane whose position was monitored by two charge-coupled device (CCD) cameras. One aim of the research was to develop a solar-power satellite (SPS) (or space-based solar power, SBSP), which is a future power plant in space. Its generated power in space will be transmitted to the ground in the form of microwaves. For SPS, phased-array antenna technology is very important. WPT rocket experiments were conducted in Japan in 1983 using a magnetron and in 1993 using a phased-array antenna. Research on WPT and SPS has been ongoing in Japan in the twenty-first century. In the twentieth century, WPT research was mainly conducted using beaming WPT, in which wireless power was focused on one target with high beam efficiency. In the twenty-first century, the requirement for electricity has become lower for many applications. Recently, low power devices like Internet-of-Things (IoT) sensors have been developed. For low power devices, we do not need to focus on wireless power; it can be provided to multiuser systems, such as wireless communication systems. WPT was recently renamed as “wide-beam WPT.” Radiofrequency identification (RFID) is a wide-beam WPT system and has recently become commercialized. It employs the 920-MHz band and backscattered wave technology to identify tags or provide individual identification. Ultimate wide-beam WPT systems are energy harvesting systems for ambient radio waves that originally transmit information. They are called “energy harvesting” or “energy scavenging” systems without any specific power transmitters. In the early development of wireless communications, there was a crystal radio, which had no extra power source and therefore used received radio waves as a power source. It is one of the oldest energy harvesters. For example, in 1983, a group at the University of Arizona estimated the efficiency of an energy harvester from AM radio [6]. They named it the energy harvester. In the 1990s, there was an interesting

1.1  History of Wireless Power Transfer and Energy Harvesting

3

Figure 1.2  WPT drone experiment by W. C. Brown to (a) restrain a drone in the laboratory in June 1964, and (b) guide flying a drone outside in October and November 1964. (c) W. C. Brown and the WPT drone, and (d) free flying of a drone in an anechoic chamber (captured from YouTube Video) [4].

4 �������������������� General Introduction

Figure 1.2  (continued)

gadget for mobile phones called a lighting antenna. A lighting antenna consists of a light-emitting diode (LED), a diode, an inductor, a capacitor, and a coil, which

1.1  History of Wireless Power Transfer and Energy Harvesting

5

Figure 1.3  WPT fuel-free airplane experiment (MILAX) by H. Matsumoto: (a) field experiment in August 1992, (b) airplane with a rectenna and a car with a transmitter, and (c) phased-array antenna for WPT on top of a car.

receives radio waves for mobile phones. The power source of the lighting antenna is modulated radio waves only. It is an energy harvester, but it is hard to apply a lighting antenna to recent smartphones or Wi-Fi because the power of radio waves for wireless communications has become lower than it was in the 1990s, and the modulation is different from 1G/2G/3G mobiles. A revised design is needed to implement a modern lighting antenna, but it is possible. The power density of ambient radio waves is too low for energy harvesting and not too many studies on energy harvesting were recorded in the twentieth century. However, currently, there are more and more emerging applications using a very low level of power. Some interesting studies on energy harvesting have been reported. Zoya Popovic’s group at the University of Colorado reported a study on energy harvesters in 2000 [Figure 1.4(a) and (b)] [7,8]. They described energy harvesting from ambient radio waves. They developed and showed octave bandwidth radio frequency (RF) harvesting tee-shirt [Figure 1.4(c)] [9]. A bowtie rectenna array was printed on the fabric with an air layer above the body. Energy harvesting, or energy scavenging, was studied after 2005, and many studies were recorded after 2005 on multipurpose energy harvesters. The research and development of

6 �������������������� General Introduction

Figure 1.4  Energy harvester developed at the University of Colorado: (a) antenna side of the harvester (2000) [7], (b) rectifier side of the harvester (2000) [7], and (c) rectenna tee-shirt (2019) [9].

energy harvesters are progressing rapidly with the advancement of IoT and sensor technology. The main difference between far-field WPT and energy harvesting from ambient radio waves is in the WPT transmitter. Both systems require a rectenna. Hence, far-field WPT and energy harvesting should be developed hand in hand.

1.2  Technical Introduction of WPT/Harvesting

7

1.2  Technical Introduction of WPT/Harvesting The technology of far-field WPT can be divided into three parts: power conversion from dc to RF in a circuit, wireless power transmission from the transmitting to the receiving antenna, and power rectification from RF to dc in a circuit. For energy harvesting, only the rectenna (consisting of the receiving antenna the rectifier) technology is important because the energy source is ambient radio waves and a special WPT transmitter is not needed. In a typical WPT system, a transmitter (Tx) device generates a time-varying EM field and transmits power across space to a receiver (Rx) device. Since the EM field from an RF/microwave source can be divided into nonradiative near-field, radiative near-field (Fresnel) field, and radiative far-field, WPT can be roughly divided into two categories: nonradiative (near-field), and radiative (near- and farfield) techniques. For the nonradiative WPT, the energy is transmitted through electric field or magnetic field coupling, which is also called capacitive or inductive coupling, respectively. They can be further divided into resonant and nonresonant coupling. In this case, power is transferred over short distances, usually within one wavelength from the Tx. It is often operated at low frequencies (kHz up to about 30 MHz). Inductive coupling is the most common approach because it is relatively easy to implement high inductance at lower frequencies although capacitive coupling can be useful in some applications where sufficient space is available for big capacitive plates. For inductive coupling, power is transferred between coils of wire by a magnetic field. Resonant inductive coupling is usually a form of inductive coupling in which power is transferred by magnetic fields between two resonant circuits, one in the Tx and another in the Rx. By using resonance, power can be transferred at greater distances with higher flexibility. Another advantage is that resonant circuits interact with each other much more strongly than they do with other objects. Most commercially available wireless charging devices (e.g., electric vehicles) use this technique. For the radiative WPT, the energy can be transferred through the radiative near-field or the far-field of the source where the power is inversely proportional to the distance square. In this case, power is transferred through waves at high frequencies (above 30 MHz) usually. It is more suitable for long-distance (more than a wavelength) applications. The main difference between the nonradiative and radiative WPT systems is reflected by their EM energy transmitting and receiving devices: coils are normally used for the former while antennas are employed for the latter. But their Tx and Rx circuits are very similar. They all employ the same key devices, inverters in the Tx and rectennas in the Rx. A wireless energy harvesting system could be considered as the receiving part of a WPT system (i.e., the rectenna) since the WPT consists of both the Tx and Rx while wireless energy harvesting is mainly about the Rx. But there are many differences. WPT is often used for high-power applications. It is point-to-point and polarization-aligned. It operates over a narrowband, requiring antennas with good directivity. On the other hand, an energy harvester mainly harvests ambient RF

8 �������������������� General Introduction

energy and is space-to-point, normally requiring an omnidirectional antenna pattern with a wide beamwidth (or wide angle), ideally insensitive to the incident angle. Ambient RF power discretely distributes in different communication bands from hundreds of MHz to several GHz. It suggests that a wireless energy harvesting harvester should have wideband or multiband characteristics. Furthermore, ambient EM waves are often accompanied by different polarizations, requiring wireless energy-harvesting harvesters to have polarization-insensitive characteristics, also called polarization-angle independent characteristics in some literature. Additionally, wireless energy-harvesting harvesters should possess the structural characteristics of planarization, low profile, and miniaturization since the application terminals are often low-power sensors. A common problem for WPT and wireless energy harvesting is how to improve the energy conversion efficiency, which is a complex issue. It is linked to the antenna or coil performance (efficiency, bandwidth, polarization, and orientation), impedance matching, nonlinearity of the rectifier, and power management. It should be pointed out that energy conversion efficiency is generally used to assess the performance at Rx for both WPT and wireless energy harvesting. Power transfer efficiency (PTE) is often used to assess the performance of a WPT system, and it is defined as the ratio of the RF power transmitted at the Tx and the received dc power at the Rx. 1.2.1  Rectennas for WPT/Harvesting

A rectenna can be designed using different types of antennas and rectifying methods, for example, dipole, monopole, microstrip, bowtie, and parabolic antennas with various rectifiers using diodes or CMOS. There are half-wave, bridge-type, single shunt, and charge pump rectifiers. The design depends on user requirements, such as mobility, the required power, and technical parameters including the antenna directivity, received radio wave power at the receiving antenna, required output voltage, and output impedance. Figure 1.5 shows a typical block diagram of a rectenna developed at the University of Liverpool [10]. Generally, a high RF-dc conversion efficiency is required. Usually, the efficiency is higher at lower frequencies. The peak efficiency of a rectenna can reach 90% at the 2.4-GHz band [11] and 5.8-GHz bands [12]. It is typically very low at terahertz frequencies because of diode characteristics [13]. Additionally, the efficiency depends on the input power and connected load impedance because of the characteristics of the diode or CMOS [14]. The maximum efficiency can be achieved using the optimum input power and connected load. It is similar to the maximum power transfer theorem of a circuit with an internal resistance [15]. It is important to know the input power at the rectenna and load impedance for a user to design a rectenna with high efficiency. For far-field WPT systems, a single shunt rectifier, whose theoretical efficiency is 100% [16], or a bridge-type rectifier is often used. The optimum input power of the rectenna for maximum efficiency ranges from a few hundred mW to a few W. This is because the breakdown voltage of a Schottky barrier diode ranges from −10V to −30V. To increase the RF-dc conversion efficiency of a rectenna, the following methods can be employed: ••

Choose a circuit that can theoretically achieve 100% efficiency;

1.2  Technical Introduction of WPT/Harvesting

9

Figure 1.5  (a) Typical block diagram of rectenna and (b) rectenna developed at the University of Liverpool [10].

••

••

Reduce the number of diodes, including their loss factor, recommending a single shunt rectifier [16]; Choose a diode with low ωR0Cj0, where R0 is the series resistance and Cj0 is the zero-biased junction capacitance of the equivalent circuit of the diode, and ω is the angular operating frequency [14];

••

Implement impedance matching at both the input and output;

••

Design rectifying circuit by considering higher harmonics (like class-F amplifier type of rectifiers), for both high efficiency and suppression of higher harmonics reradiation;

••

Realize high voltages (close to the breakdown voltage), which can be achieved with a high input power or high impedance antenna/circuit (with resonators or reuse of reflection, etc.);

••

Additionally, consider combining multiple antennas.

In contrast, for energy harvesting, the input power from radio waves is usually very weak (up to a few hundred mW/m2) [17]. The power is too weak for the realization of high RF-dc conversion efficiency for a rectenna. Based on the Carnot

10 �������������������� General Introduction

limitation, to increase the efficiency of a rectenna, the following aspects of a diode should be considered [18]: ••

••

Increase the junction resistance: •

Higher Schottky barrier;

•

Use a high Q matching circuit (high impedance antenna);

•

Increase the nonlinearity;

•

Tunnel diode;

•

Spin diode;

•

Low temperature.

Increase the voltage or input power to the diode: •

Use a high gain antenna to work with the diode;

•

Add a dc/dc converter to increase the voltage at the diode;

•

Utilize temporal energy multiplexing (multisine);

•

Adopt hybrid harvesters.

The efficiency of rectennas for energy harvesting increases with time [19, 20]. Recently, instead of a Schottky barrier diode, a tunnel diode was used in energy harvesters. The tunnel diode has good efficiency at low input power [21]. New energy harvesters (rectennas) with higher efficiency at a weak power density of ambient radio waves are expected in the future. 1.2.2  Beamforming for WPT

A rectenna is one of the most important components for far-field WPT and energy harvesting. Its characteristics mainly depend on the power density of radio waves at the rectenna. However, while it is difficult to control the power density at the receiving side, the density can be increased in far-field WPT systems by using a specially designed transmitter. Beam-control and beamforming technologies are very important. Based on Maxwell’s equations, the propagation of radio waves after they are radiated from an antenna cannot be controlled. When a rectenna is placed in the Fresnel region (far-field) where the radio wave can be assumed as a plane wave, the power density can be calculated using the Friis equation. The power density of radiated radio waves attenuates in inverse proportion to the square of the distance. However, there is no effective real loss in the air. In the Fresnel region, the beam efficiency from the transmitting antenna to the receiving antenna is usually very poor. It can be very convenient for WPT because wireless power can be received everywhere. This is called a wide-beam WPT system. The energy harvesting system is almost always applied in the Fresnel region. For wide-beam WPT systems, it is very important to create an environment where the power density is evenly distributed even when the system is in a multipath circumstance with unexpected objects. When a rectenna is put in the Fraunhofer region (radiative near-field) where the radio wave is spherical and the power density at the receiving area is not uniform,

1.2  Technical Introduction of WPT/Harvesting

11

the theoretical beam efficiency reaches 100% [22–24]. The power density of a rectenna can be controlled using a narrow-beam WPT system. If we can use a kilometer-sized transmitting antenna at the microwave frequency, even a-few-thousandkilometer distance between antennas would be in the Fraunhofer region, and the theoretical beam efficiency reaches 100%. The 100% theoretical beam efficiency in the Fraunhofer region is realizable only when we know the exact position of the receiving antenna and focus of the beam. If the receiver moves, the beam efficiency would decrease abruptly. If the receiver is put in the reactive near-field, the antennas are electromagnetically coupled and the system becomes a coupled WPT via the magnetic or electric field. In this case, the change of the position of a receiver can be known because the circuit parameters, such as the impedance and resonant frequency, change with the coupling coefficient, which depends on the relationship between the positions of the antennas. However, in the Fraunhofer region (radiative near-field), the change of circuit parameters could not be easily detected even when the antenna position changes. Thus, the far-field WPT is called uncoupled WPT. Target-detection technology and beamforming or beam-direction control technology are very important to maintain high beam efficiency in narrow-beam WPT systems. For beamforming or beam-direction control, a phased-array antenna is most suitable. It is a useful technology for electrically controlled beam direction and beamforming. The phased array comprises several antennas (Figure 1.3(c)). The phase and amplitude of radio waves transmitted at each antenna can be controlled using phase shifters or beamforming network circuits. The beamforming is controlled by the interference of radio waves (Figure 1.6). Various phased-array antennas have been developed in Japan for narrow-beam WPT systems at 2.4- and 5.8-GHz bands [25]. In 2019, phased-array antennas with newly developed gallium nitrate (GaN) high-electron mobility transistors (HEMTs) were employed in a field WPT experiment on a flying drone at the 5.8-GHz band in Japan [26]. Even if the narrow beam can be controlled using phased-array antennas, the beam cannot be accurately directed to a receiver without determining the position of the receiver. We can apply any target-detection method, such as the optical method, Global Positioning System (GPS), radar, or direction-of-arrival (DOA) method with a pilot (beacon) signal from the receiver. For example, the optical target-detecting method was adopted in MILAX in Japan. There is another simple and important method for detecting target positions by detecting the shape of the phased-array antenna plane. It is called the retrodirective method with a pilot signal from a target. Unlike the DOA method, a phase conjugation circuit can be used instead of a DOA algorithm and phase shifters. When the phase conjugation in each antenna element in the phased-array antenna is created, the beam of the radio wave can be directed to a target that transmits the pilot signal. The concept of the retrodirective method is depicted in Figure 1.7(a), and the developed retrodirective system is shown in Figure 1.7(b) [27]. The retrodirective method was based on a corner reflector and Van Atta array [28]. The advantages of retrodirective systems include: ••

Both detection of the target and recognition of the shape of antenna planes;

12 �������������������� General Introduction

Figure 1.6  (a) Concept of beamforming by phased-array antenna, and (b) example of a beam formed by a phased-array antenna.

••

Very high-speed beam control (with analog phase conjugation circuits only);

••

Low cost (without expensive phase shifters).

1.2  Technical Introduction of WPT/Harvesting

13

Figure 1.7  (a) Concept of the retrodirective method, and (b) example of developed retrodirective systems [27].

However, the disadvantages of retrodirective systems include: ••

Beamforming for the user only (without phase shifters);

••

Interference between pilot signal and the WPT beam (theoretical requirement of the same frequency) [25];

••

Performance affected by the fluctuation in the pilot signal and WPT sources (two independent sources) [25, 29];

••

An extra power source is needed in the receiver for pilot signal radiation [30].

Some effective solutions to the disadvantages of retrodirective systems have been proposed [25, 29, 30]. A retrodirective system can be used for not only one target without multipath effect but also multiple targets in multipath circumstances [31, 32]. For phased-array antenna systems or simple power transmitters, a good power amplifier or generator with high dc-RF conversion efficiency is required. There are some differences between those for WPT and those for wireless communication systems. The transmitter for wireless communication requires linearity and modulation of the radio waves, and that of WPT does not need modulation but requires high dc-RF conversion efficiency. A class-F amplifier, whose efficiency has already reached over 80% at 5.8 GHz [33], is often developed for WPT. The advancement of power amplifiers for wireless communication is also expected. If more power

14 �������������������� General Introduction

with high efficiency and low cost is needed, vacuum-tube technology can be employed, especially a magnetron at the microwave frequency, even for phased-array systems [34].

1.3  Current Status of Commercialization/Regulation/Research on WPT/Harvesting Currently, WPT and energy harvesting devices are expected to be commercialized and grown worldwide. The far-field WPT market in Japan in 2025 is estimated to be $5.52 billion, and the details are as follows: ••

••

••

Factory automation (FA)/IoT: Market perspective is $3.75 billion. IoT sensor systems would be $2.5 million and $125 billion. The estimated installed WPT is 30%, and the cost of systems will be 10%. Sensor for nursing and monitoring: Market perspective is $500 million. Market will be $5 billion, and WPT will be 10%. Mobiles: Market perspective is $1.27 billion. Smartphones will account for $34 million. The estimated installed WPT will be 47% ($550 million). Tablet will be $24 million, and WPT will be 30% ($230 million).

To exploit the future market, there is a need for novel WPT and energy harvesting technology. Additionally, the health and safety aspects of radio waves need to be considered in harmony with conventional wireless application systems. Therefore, we need new radio regulations for WPT after sufficient discussion with all stakeholders of the technology. The first discussion for the new radio regulation of WPT started in Comité Consultatif International pour la Radio (CCIR) in 1978. CCIR is presently the International Telecommunication Union Radiocommunication (ITU-R) sector. ITU-R manages international RF spectra and satellite orbit resources and develops standards for radiocommunication systems to ensure the effective use of the spectra. Figure 1.8 shows CCIR Report 679: Characteristics and Effects of Radio Techniques for the Transmission of Energy from Space. It is the first report on far-field WPT for SPS. It has been revised twice (in 1982 and 1986), and almost all reports, including this one, were abolished in the 1993 Radiocommunication Assembly in ITU-R. After reorganizing ITU-R, the U.S. National Aeronautics and Space Administration (NASA) submitted a document of far-field WPT for SPS, Question ITUR210/1 (wireless power transmission), in ITU-R, 1997. The main contents of the document are summarized below. It demands the following information be gathered: 1. What applications have been developed for the use of WPT technologies? 2. What are the technical characteristics of the emission employed in or incidental to the applications of WPT technologies? 3. What is the WPT’s standardization situation in the world?

1.3  Current Status of Commercialization/Regulation/Research on WPT/Harvesting

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Figure 1.8  CCIR Report 679, Characteristics and Effects of Radio Techniques for the Transmission of Energy from Space.

16 �������������������� General Introduction

It demands that the following questions be answered: 1. Under what category of spectrum use should administrations consider WPT: industrial, scientific, and medical (ISM), or others? 2. What RF bands are most suitable for WPT? 3. What steps are required to ensure that radiocommunication services, including the radio astronomy service, are protected from WPT operations? The question regarding the category of spectrum use is an important one. The ISM frequency bands are regulated for industrial, scientific, and medical applications. WPT is not a wireless communication or remote sensing system. Therefore, ISM frequency bands, for example, the 2.4- or 5.8-GHz band, were and are used for research, development, and demonstration. ISM bands are defined by ITU Radio Regulations (RR) (article 5) in footnotes 5.138, 5.150, and 5.280 of RR as follows [35, 36]: RR Section 1, 1.15: ISM applications (of RF energy): Operation of equipment or appliances designed to generate and use local RF energy for industrial, scientific, medical, domestic, or similar purposes, excluding applications in telecommunications. RR Article 5, footnotes 5.138: The bands (6765–6795 kHz, 433.05–434.79 MHz in Region 1, 61–61.5 GHz, 122–123 GHz, and 244–246 GHz) are designated for ISM applications. The use of these frequency bands for ISM applications shall be subject to special authorization by the administration concerned in agreement with other administrations whose radiocommunication services might be affected. In applying this provision, administrations shall have due regard to the latest relevant ITU-R Recommendations. RR Article 5, footnotes 5.150: The bands (13553–13567 kHz, 26957–27283 kHz, 40.66–40.77 MHz, 902–928 MHz in Region 2, 2400–2500 MHz, 5725–5875 MHz, and 24–24.25 GHz) are also designated for ISM applications. Radiocommunication services operating within these bands must accept harmful interference that may be caused by these applications. ISM equipment operating in these bands is subject to the provisions of No. 15.13.

It has not been agreed whether WPT is ISM or not. Some countries consider WPT as ISM, whereas other countries consider it as others. The topic is kept for future discussion. In 2012, the question was revised to 210-3/1, “Wireless power transmission,” which involves not only far-field WPT but also coupled WPT. In ITU-R, far-field WPT is currently called “beam WPT,” and coupled WPT is called “nonbeam WPT.” After 2013, “beam WPT” and “nonbeam WPT” were discussed simultaneously in ITU-R. ITR-R documents for all WPT are linked to the IEEE website [37]. After a long discussion in ITU-R Study Group 1 Working Party 1A (ITU-R SG1 WP1A) for beam WPT [38] in 2016, ITU-R published the first ITU-R report SM.2392 on beam WPT [39]. It covers all applications of beam WPT, wide-beam

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WPT to IoT sensors, narrow-beam WPT to fly a target, and SPS. In 2019, reports on specified WPT applications, such as wide-beam WPT and low-power applications named SM, [WPT.BEAM.IMPACTS] and frequency recommendation, named SM [WPT.BEAM.FRQ], were discussed. As a result, “Working Document Toward a Preliminary Draft New Report ITU-R SM, [WPT.WIDE-BEAM.IMPACTS] rev” was accepted by ITU-R. System specifications for beam WPT for the first step of commercialization (2020) are shown in the working document (Table 1.1). In the next ITU-R meeting, discussions will be extended based on the working document. WPT systems in Table 1.1 were supported by Japan and the United States, where the Federal Communications Commission (FCC) approved wide-beam WPT systems for different WPT venture companies (e.g., Energous Corp., Ossia Corp., and PowerCAST Corp.) in December 2017, June 2019, and October 2019. In the United States, WPT approvals are mainly based on ISM band systems in the 920MHz and 2.4-GHz bands, which are governed by Part 18 of the FCC rules, whereas Part 15 contains the rules for unlicensed communication devices, even those that share ISM frequencies. Simultaneously with ITU-R discussions, the discussion toward new domestic radio regulation (RR) for far-field WPT started in Japan in 2019. The discussion is based on future WPT system scenarios in three-frequency bands, including 920MHz, 2.4-GHz, and 5.7-GHz bands, as shown in Table 1.2. The parameters for electromagnetic interference evaluation are shown in Table 1.3. After a tough negotiation with several affected parties, wide-beam WPT was agreed as a new domestic RR in Japan on July 14, 2020 [40]. The agreed technical conditions for wide-beam WPT are shown in Table 1.4. It is based on neither the ISM band nor the same wireless application as other wireless communications. It is a great first step for WPT in Japan. However, further discussion is needed to enact the new RR, which is expected to be achieved in Japan before the end of 2021. Considering RR for WPT, there are still differences worldwide. However, they will be discussed in ITU-R in the future. A new world, where electricity is wireless and like air, will be realized.

Table 1.1  System Specifications for Beam WPT for the First Step of Commercialization (2020) in “Working Document Toward a Preliminary Draft New Report ITU-R SM” [WPT.WIDE-BEAM. IMPACTS] rev in ITU-R System System 1 System 2 System 3 Spec Frequency 920-MHz band 2.45-GHz band 5.7-GHz band (915–930 MHz) (2.40–2.499 GHz) (5.470–5.770 GHz) Output Power 1W 15W 32W Antenna Gain 6 dBi 24 dBi 30 dBi Equivalent Isoto- 4W (36 dBm) 65 dBm 70 dBm pically Radiated Power (EIRP) Modulation TBD TBD TBD Place of Use Indoor Indoor Indoor

18 �������������������� General Introduction Table 1.2  Future WPT System Scenarios for New WPT RR in Japan, 2019–20 920-MHz Band 2.4-GHz Band 5.7-GHz Band Circumstance Factory (indoor), nursing Factory (indoor), plant, Factory (indoor), plant, home, etc. garage, etc. garage, etc. Purpose Wireless power supply for Wireless power supply for Wireless power supply for sensor network sensors, small display, etc. sensors, small display, etc. Method Wide areas, including out-of- One-to-one WPT with a Equivalently one-to-one sight and simultaneous WPT cheap receiver by beacon WPT of high power and long for multiusers by omnidirec- signal that uses wireless LAN, duration by change of beam tional antenna or by wide which is collaborated with a direction from the transmitbeam conventional system ter, which is finely controlled by a special receiver Number of Re- 5–10 (simultaneous) 1 to several dozens 1 to several dozens ceivers per One (sequential) (sequential) Transmitter Required Power A few mW to a few hundred Approximately 50 mW to A few mW to a few hundred from User mW app. 2W mW Tx-Rx Distance