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Wireless Power Transmission Technologies for Solar Power

Satellite 

Susumu Sasaki1,2), Koji Tanaka1), and Advanced Mission Research Group2) 

1)

Institute of Space and Astronautical Science(ISAS/JAXA)

 

, Sagamihara, Kanagawa, 252-5210, Japan2)Aerospace Research and Development Directorate(ARD/JAXA) , Tsukuba, Ibaraki, 305-8505, Japan

 Abstract    — Solar Power Satellite (SPS) is an energy system

which collects solar energy in space and transmits it to the

ground. It has been believed as a promising infrastructure to

resolve global environmental and energy problems for human

beings. One of the most important technologies for the SPS is the

wireless power transmission from the geostationary orbit to the

ground. Microwave power transmission has been investigated

and demonstrated for more than 40 years, but still requires

further research regarding high-efficiency power conversion and

high-accuracy beam control for SPS application. This paper

introduces the concept of SPS, and presents microwave power

transmission technologies necessary for SPS, their demonstrationexperiments both on the ground and in space in the near future,

and future prospects towards commercial SPS. 

 Index Terms  — Solar power satellite, wireless powertransmission, microwave. 

I. INTRODUCTION 

“Energy and environment” is one of the most important

global issues to be resolved to sustain our society. 80 % of

energy in our life comes from fossil fuels. If we continue to

use the fossil fuel resources at the current consumption rate,

they will be completely lost within 100-150 years.

Furthermore, the huge amount of consumption of fossil fuel

increases CO2 concentrations in the atmosphere, which raises

serious environmental concerns. If we continually depend on

the fossil fuel, we will experience substantial degradation of

life quality within this century.

The global problem in the closed earth system will be

effectively solved by a paradigm shift to the open earth-space

system. There is unlimited constant solar energy supply in

space free from the whether conditions, quite different from

that on the earth. The concept of the SPS is to tap the solar

energy using a large-scale photovoltaic array in space and to

transmit it to the ground using microwave or laser beam. It has

a great potential for a large-scale clean energy system to

replace the fossil fuel plants. Figure 1 shows the configuration

of space solar power systems consisting of SPS and associated

ground segments. The time average power per unit area in

space is 5-10 times larger than that on the ground, while the

power loss for the wireless power transmission/reception is

expected less than 50%. Thus the SPS has a competitive

advantage over the solar power plants on the ground.

The SPS was first proposed by Peter Glaser [1] in 1968,

followed by NASA/DOE studies [2] in the 1970’s. Since the

early investigations, various types of the SPS have been

proposed more than 30 in the world. They are categorized into

5 types as shown in Fig.2. Most of the SPS models proposed

so far uses microwave rather than laser for the wireless power

transmission, because the power efficiency both at the

transmitter and receiver is generally higher and attenuation

through the atmosphere is lower for microwave as compared

with laser.

II. MICROWAVE POWER TRANSMISSION FOR SPS 

Microwave frequency for SPS has been selected in a range

of 1-10 GHz, compromising between antenna size and

atmospheric attenuation. If we choose a frequency in the

industrial, scientific and medical (ISM) radio bands, 2.45 or

5.8 GHz is the potential candidate. 2.45 GHz was selected in

the early phase study, but 5.8 GHz has been recently

considered as a more desirable frequency due to recent

accelerated progress in C-band RF technologies.

As for the microwave generator, tubes such as magnetron,

klystron, and TWT have been proposed for the SPS use

because the power conversion efficiency is reasonably high

more than 70 % at low cost. Semiconductor amplifier is

Sun Light

Solar Power

Satellitein orbit 

DC 

Solar ArrayPanel

Microwave Circuit

Transmitting

Antenna(Spacetenna)

Microwave

Receiving Antenna

(Rectenna)

 DC 

DC/AC Conversion

 AC)

Power Utility

Ground

PowerFacility

Space SolarPowerSystems

 Fig.1 Configuration of space solar power systems, consisting

of solar power satellite and ground segments.

IWPT-PL-1

 

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Fig.2 Various types of SPS, categorized into 5 types.

another potential candidate as the power efficiency has beenconsiderably improved to 60-70 % with low cost expectation.

Besides the power efficiency, beam pointing technologies to

transmit the microwave power beam precisely to the receiving

site are essential for the power transmission. They are peculiar

to the wireless power transmission, not covered by the existing

communication technologies. A beam angle 100 µrad with a

10 µrad pointing accuracy is required for the 5.8 GHz

transmission from an antenna of 2 km square in the

geosynchronous orbit to a reception site of 3.5 km diameter on

the ground. The transmitting antenna will be assembled by a

number of array antenna panels which consist of sub-array

antennas. Totally more than 1 billion antennas will be installed.

A retro-directive technology with a pilot signal from theground will be used to control the microwave beam from each

antenna panel directing to the ground station. Although each

panel is sufficiently stiff for microwave beaming, relative

motion between the panels can not be avoided for the large

antenna assembly. In order to form a microwave beam

precisely focused at the ground station, the phase of

microwave from each panel needs to be adjusted between the

panels, which requires revolutionary new technologies.

The microwave power at the receiving site is rectified to

provide dc power using arrays of rectifying antenna (rectenna)

with Schottky diode. The power conversion efficiency for

single rectenna exceeds 80 % in a power range more than 50

mW. However, further research is required to improve the

power efficiency for 1 mW class input and rectenna array as a

whole.

III. DEMONSTRATION OF MICROWAVE POWER TRANSMISSION

TOWARDS SPS 

A large-scale demonstration experiment on the microwave

power transmission for SPS application was conducted in

1975 in US using the JPL 26 m dish antenna and 3.4 m x 7.2m rectenna located at 1 mile apart from the transmitter [3].

Microwave of 450 kW at 2.388 GHz was transmitted and 30

kW was obtained at the rectenna. This demonstrated the

feasibility of the microwave power transmission very clearly.

However, the beam pointing using a single monolithic dish

antenna of 2 km scale is not feasible in orbit. After the JPL

experiment, the beam direction control using the retro-

directive method for the phased array antennas has been

studied and the associated technologies have been

demonstrated in Japan and the United States. But the power

level in the past experiments has been less than kW and the

transmission range has been limited in laboratory scale in

many cases.Japan is now planning to perform a kW-class microwave

power transmission experiment in the range around 50 m as

shown in Fig.3. It will be the first experiment in the world as a

high-power and long-range microwave transmission

experiment with a capability of retro-directive beam control.

The microwave transmitter consists of 4 individual panels,

which are movable to each other to simulate antenna motion in

Fig.3 Microwave power transmission experiment on the

ground.

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Fig.5 Sequence of microwave transmission experiment in space.

orbit. Each panel, 0.6m x 0.6m, has an array consisting of

hundreds of transmitting antennas, receiving antennas for

the pilot signal, phase controllers, and power systems. The

power level from each panel will be several hundreds of

watts, totally one kW level, at 5.8 GHz. The frequency of the

local oscillator in each panel is synchronized by a master

oscillator. The phase of local oscillator in each panel is

adjusted by a Rotating Element Electric Field Vector (REV)

method so as that the power at the receiving site getsmaximum. In the demonstration experiment, the output dc

power will be several hundreds of watts from the rectenna.

After the ground demonstration experiment, we plan to

conduct a small-scale microwave power transmission

experiment in orbit [4]. In the space experiment, power

transmission at several kW from the low earth orbit to the

ground will be studied. The major objectives of the space

experiment are;

(1) demonstration of the microwave beam control precisely

to the target on the ground from the antenna in orbit,

(2) verification of microwave power transmission (~kW/m2)

through the ionosphere,

(3) evaluation of the over-all power efficiency as an energysystem,

(4) demonstration of the electromagnetic compatibility with

the existing communication infrastructure.

Especially, item (2), the interaction between the intense

microwave and the ionospheric plasma is important because i t

can be studied only in the space environment.

Either a small satellite or Japanese Experiment Module

(JEM) on the International Space Station will be used for the

initial demonstration experiment as shown in Fig.4. There are

two configurations in the experiment; microwave transmission

to the ground perpendicularly to the spacecraft velocity vector

(Mode A) and microwave transmission parallel to the velocity

vector (Mode B). The beam direction controlled by the pilot

signal from the ground is verified only in mode A. Mode B is

a favorable configuration to study the microwave/plasma

interaction because the plasma segment is irradiated and

heated by the microwave longer than in Mode A.

The weight of payload instruments will be 200 kg for the

small satellite and 500 kg for the Space Station, including

diagnostic instruments such as plasma probes, particle energy

analyzers and plasma wave receivers. For the small satellite

experiment, a power transmission panel consisting of 4

modular panels, 1.6m x1.6m totally, will be used to transmit a

3.8 kW power beam. The experiment on the International

Space Station will have 9 modular panels, which are capable

of transmitting a 8.6 kW power beam from a 2.4 m x 2.4 m

antenna.

In order to study the nonlinear interaction of the microwave

power beam with the ambient plasma, power density more

than 100 W/m2  is required. In case of the JEM experiment,

maximum beam intensity more than 1000 W/m2  will be

realized for 130 m from spacecraft and that more than 100

W/m2 for 410 m.

Figure 5 shows the experiment sequence near the ground

station. The microwave beam at 10 % of the full power is

transmitted to the ground for the first 2 minutes. The onboard

Fig.4 Microwave power transmission experiment in space on a smallsatellite or on the International Space Station.

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Fig.6 Technology roadmap towards commercial SPS.

computer controls the beam direction without the pilot signal

from the ground. When the experiment system passes over the

ground station, the microwave beam at the full power is

transmitted to the ground for 16 sec guided by the pilot signal

from the receiving site. The beam direction is changed in ± 10

degrees from the normal line of the panel to target the

receiving site. After the full power operation, the power

transmission at 10 % of the full power is performed for

another 2 minutes.

IV. ROADMAP FOR COMMERCIAL SPS

A technology roadmap from research phase to commercial

phase is shown in Fig.6. Based on the results from the small-

scale demonstration experiments in space, together with the

results from the ground experiments, we will make a decision

on the technology option, microwave or laser, for the wireless

power transmission. With the selected transmission medium,

we will make a 100 kW-class SPS demonstration experiment

in orbit before 2020. All basic technologies required for the

commercial SPS will be verified at this stage. This approach isin accordance with the basic plan on space development by the

government's space development strategy headquarter in

Japan. After completion of these demonstrations we will select

a configuration for the initial target of the commercial SPS,

one of the models shown in Fig.2 or other model. The

expected power cost depending heavily on the development of

space transportation and public acceptance will be the major

trade off factors for selection. For the selected configuration, 2

MW and 200 MW class plant will be constructed and tested

before 2030. This scenario guaranties the start of construction

of the 1 GW class commercial SPS in 2030’s.

V. CONCLUSION 

One of the most critical technologies for the SPS is

microwave power transmission from the geosynchronous orbit

to the ground. Evolutionary microwave technologies are

required for a high power conversion efficiency more than

80 % from/to DC and an extremely high-precise beam controlwith 10 µrad accuracy. These technologies will be partially

verified in the ground demonstration experiment within

several years and will be fully verified in the space

experiments within 10 years. Although the required

technologies are quite challenging, continuing research

activities along with the proposed roadmap will lead to

opening the new SPS era in 2030‘s. 

REFERENCES 

[1] P.E.Glaser, “Power from the Sun: Its Future”, Science, vol.162,

pp.867-886, 1968.[2] DOE/NASA, “Program Assessment Report Statement of

Finding - Satellite Power Systems, Concept Development and

Evaluation Program”, DOE/ER-0085, 1980.

[3] R.M.Dickinson, “Performance of a High-Power, 2.388-GHzReceiving Array in Wireless Power Transmission Over 1.54km”, 1976 IEEE MTT-S International Microwave SymposiumDigest of Technical Papers, pp.139-141, 1976.

[4] S.Sasaki, K.Tanaka and Advanced Mission Research Group,“SSPS Technologies Demonstration in Space”, IAC-10.C3.4.1,61st International Astronautical Congress, Prague, Sep.-Oct.2010