Wireless Power Transfer at Higher Frequency
for SPS and for Commercial WPT
Naoki Shinohara
1
1
Research Institute for Sustainable Humanosphere, Kyoto University, GOkasho, Uji, Kyoto, Japan
shino@rish.kyoto-u.ac.jp
Keywords: Wireless Power Transfer, Microwave Power Transmission, Solar Power Satellite, Millimeter Wave
Abstract: One of the promising future power stations is a solar power satellite (SPS) station in geostationary orbit
(36,000 km above the surface of Earth) that uses wireless microwave power-transfer technology. In this
system, the power generated would be transmitted to the ground by a microwave beam. The SPS would be a
very large satellite with a large transmitting phased-array antenna that would work at 2.45 or 5.8 GHz. The
size of the transmitting antennas is theoretically determined by Maxwell’s equations. However, we must
reduce the size of the antennas to reduce the cost and to produce a small prototype satellite as a first step to
the SPS. The only way to reduce the size of the antennas is to use a higher frequency. We developed
rectennas that are optimized for 24 and 60 GHz transmission. In addition, we developed a monolithic
microwave integrated circuit (MMIC) rectenna for 24 GHz transmission and with dimensions of 1 mm × 3
mm. The maximum radio-frequency to direct-current (RF-DC) conversion efficiency is 47.9% for a 210
mW microwave input power with a 120 load. We also designed a rectenna for 60 GHz transmission
whose maximum RF-DC conversion efficiency is 46.2% for a 80 mW input power at 60 GHz with a 100
load. Finally, based on rectenna technology, we propose other satellite experiments.
1 INTRODUCTION
A solar power satellite (SPS) station is a very
suitable application for a wireless power transfer
(WPT) via radio waves, especially via microwaves
(microwave power transfer or MPT). SPSs are one
of the promising future power stations for a
sustainable power source that uses solar cells
(Mankins, 2014). Current SPS design envisions a 2
km diameter antenna that would transmit at 5.8 GHz
in space. The beam efficiency between such a
transmitting antenna and a 2 km receiving rectenna
36,000 km away is approximately 90% (Shinohara,
2014). The size of antennas is determined by
Maxwell’s equations and cannot be reduced
(Shinohara, 2014). In working toward the SPS, we
must carry out small-scale satellite experiments. The
low Earth orbit (LEO) of 400 km would be used for
such an experiment. If the small satellite system is
the same as the SPS at 5.8 GHz, the antenna and the
rectenna must each be approximately 200 m long.
However, a 200 m antenna is too large for a small
satellite; the size limit is more on the order of 10 m.
Therefore, instead of simulating a SPS, we use a
small satellite to study the other objectives of MPT.
The same problem arises with commercial
applications of MPT. Theoretically, the size of
antenna required is over 10 m in diameter for MPT
over 1 km at 5.8 GHz with 90% beam efficiency.
This size is too large for a commercial MPT system
to compete with wired power transmission. This is
the reason that no commercial MPT system exists in
the world. Therefore, we propose WPT at a higher
frequency, for example, 24 or 60 GHz. However,
there are two problems with WPT at a higher
frequency: (1) an increase in absorption by air and
(2) a decrease in circuit efficiency and power. The
latter problem can be solved by technical means.
2 DEVELOPMENT OF 24 AND 60
GHZ RECTENNA
At Kyoto University, we propose a wireless system
that simultaneously transmits information and power
in the millimeter wave range. Higher frequency
21
Shinohara N.
Wireless Power Transfer at Higher Frequency for SPS and for Commercial WPT.
DOI: 10.5220/0005420700210024
In Proceedings of the Third International Conference on Telecommunications and Remote Sensing (ICTRS 2014), pages 21-24
ISBN: 978-989-758-033-8
Copyright
c
2014 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
results in higher communication speed and lower
antenna sizes. The first application considered is a
fixed wireless access system proposed by NTT
Corp., Japan (Seki, 2011).
First, we choose 24 GHz, which is in the industrial,
science, and medical (ISM) band, and develop a
rectifying circuit in the MPT receiver. We normally
use a Schottky diode with
g
/4 distributed line and a
capacitance that is called a “single-shunt rectifier”
and with theoretical radio-frequency to direct-
current (RF-DC) conversion efficiency is 100%. For
example, the maximum efficiency of the proposed
single-shunt rectifier is over 90% at 2.45 GHz and is
80% at 5.8 GHz. For the millimeter wave system,
we consider that the capacitance is a weak point that
prevents optimizing the efficiency. Therefore, we
propose a new single-shunt rectifier with a class-F
load, which is composed of open stub resonators for
even and odd harmonics, instead of the capacitance.
The rectifying principle exploited by a conventional
single-shunt rectifier and the class-F load rectifier is
the same and both have a theoretical RF-DC
conversion efficiency (with only one diode) of 100%.
The rectifying circuit is composed of a microstrip
line and two diodes in parallel. Dimensions of the
developed 24 GHz MMIC rectenna are 1 mm × 3
mm on GaAs [Fig. 1(a)], with a maximum RF-DC
conversion efficiency of 47.9% for a 210 mW
microwave input signal at 24 GHz with a 120 load
[Fig. 1(b)] (Hatano, 2013).
(a)
(b)
Figure 1 : (a) Developed 24 GHz MMIC rectenna
and (b) RF-DC conversion efficiency (Hatano, 2013).
Next, we designed a rectifying circuit that operates
at 60 GHz. The rectifying circuit is composed of a
microstrip line on a Teflon substrate and two diodes
connected in parallel. For the 60 GHz rectifying
circuit, we focused on three points to increase the
RF-DC conversion efficiency: (1) First is the length
of the microstrip line between each diode with a
through hall, which in turn is connected to ground
plane. Upon changing the length of this microstrip
line, the efficiency goes through a maximum and a
minimum. (2) Second is the number of
corresponding harmonics of the class-F load. We
estimated a relationship between the number of
corresponding harmonics of the class-F load and the
efficiency, and concluded that to increase the
efficiency, it is sufficient to use only one stub
resonator for a fundamental wave. (3) Finally, the
impedance of the class-F load. We increased the
impedance of the class-F load. The rectifying circuit
designed is shown in Fig. 2. An ADS simulation of
the maximum RF-DC conversion gives an efficiency
of 46.2% for 80 mW input power at 60 GHz with a
100 load.
Fig.ure 2 : Designed and simulated 60 GHz
rectifying circuit.
Third International Conference on Telecommunications and Remote Sensing
22
Figure 3 shows the frequency dependence of the
RF-DC conversion efficiency of rectifying circuits
for rectennas developed since the 1960s. The star
marks are our contributions, which are at 24 and 60
GHz. The RF-DC conversion efficiency in the
millimeter-wave frequency range is sufficient to use
millimeter waves for WPT.
Figure 3 : Frequency dependence of RF-DC
conversion efficiency of rectifying circuits for
rectennas developed since the 1960s.
3 PROPOSED SATELLITE MPT
EXPERIMENT FOR SPS
In 2009 in Japan, the Basic Plan for Space Policy
was published, which states As a program that
corresponds to following major social needs and
goals for the next 10 years, a Space Solar Power
Program will be targeted for the promotion of the 5-
year development and utilization plan.” We thus
need both a technical advance and a “surprise” in the
next space experiment based on the Basic Plan for
Space Policy. In 2013, the Basic Plan for Space
Policy was revised, but it still promoted the SPS for
Japan. In addition, the SPS figures in the Japanese
“Basic Plan for Energy Policy” from April, 2014.
As a first step to the SPS, a WPT experiment in
space or from space to ground is very important.
Only three MPT rocket experiments have been done
in the world, and they were done in Japan. In 1983,
Professor Matsumoto of Kyoto University conducted
the first MPT rocket experiment, which was called
the Microwave Ionosphere Nonlinear Interaction
Experiment (MINIX). This experiment was in
collaboration with Kobe University and the Institute
of Space and Astronautical Science (ISAS)
(Matsumoto, 1986). In the MINIX experiment, they
used a 2.45 GHz cooker-type magnetron and
waveguide antenna as microwave transmitter. In
1993, Professor Matsumoto’s group carried out their
second rocket experiment, which was called the
International Space Year Microwave Energy
Transmission in Space (ISY-METS) experiment
(Kaya, 1993). This experiment used a phased array
at 2.411 GHz. The MINIX and ISY-METS were
space-to-space MPT experiments. The third and last
WPT rocket experiment was carried out in 2006, by
Professor Kaya of Kobe University, ISAS, and the
European Space Agency (Kaya, 2006). This is the
only a rocket MPT experiment whose microwave
was transmitted from the rocket back to the ground.
However, the microwave was diffused and did not
qualify as a power beam.
The difficulty of the MPT experiment from space
to ground is caused by the low frequency of
microwave radiation. A small satellite must orbit at
300 to 400 km. A distance of several hundred
kilometers is too far to create a microwave beam at
2.45 or 5.8 GHz (these frequencies are too low).
Therefore, we propose an MPT space experiment at
24 GHz that is based on the technologies described
in Section 2. In the early 1990s, a 24 GHz MPT
satellite experiment was proposed and studied in
Japan (Matsumoto, 1993). However, this was a
space-to-space MPT experiment. Herein, we propose
a space-to-ground MPT experiment.
Using the microwave frequency of 24 GHz has
the following advantages and disadvantages:
Advantages
(1) The antenna size of the MPT can be decreased
to a tenth of the size of Tx × Rx antenna.
(2) 24 GHz is in the ISM band so there are very
few users.
Disadvantages
(1) Efficiency and power are lower than at 2.45
GHz.
(2) Absorption in air is greater.
(3) Technical obstacles are greater.
Wireless Power Transfer at Higher Frequency for SPS and for Commercial WPT
23
Orbital Velocity
7.5km/s
Satellite
Antennas for Beam
Shape Detection
Ground
Microwave Beam
Pilot Signal
Antenna
Frequency : 24GHz
Diameter of Tx Antenna : 20m
Power : 10kW
400km above
Peak Power Density
: 126mW/m
2
Half Power Distance
@ 400km : 240m
ex. Beam Efficiency 75%
with 400m Rectenna
Fig. 4 Parameters of proposed 24 GHz satellite MPT
experiment.
However, we can transmit microwave power from
a 400 km orbit to the ground with higher beam
collection efficiency if appropriately calibrate the
values for experimental parameters. Estimated
values for the parameters are shown in Fig. 4. With
these parameters, we can receive sufficient
microwave power from the space. Therefore,
provided the requisite technical advance and
surprise,” the 24 GHz space experiment can be
conducted.
4 CONCLUSIONS
In working toward the SPS, an MPT satellite
experiment is desired as soon as possible. However,
Maxwell’s equations and the required satellite orbit
render an effective MPT experiment difficult.
Therefore, to perform the MPT satellite experiment,
we consider using frequencies of 24 GHz or higher.
We have already developed 24 and 60 GHz
rectifying circuits for a rectenna with sufficient RF-
DC conversion efficiency.
There are still technical problems preventing an
MPT experiment with higher frequency. For
example, the experiment would require a high-power
transmitter and amplifier with high DC-RF
conversion efficiency. We hope the advance of
radio-wave technologies will support the MPT
satellite experiment and realize the SPS.
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