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Japan Aerospace Exploration Agency

Research and development

A day when large artificial satellites solve our energy challenges

New Research on Space Solar Power Systems

Innovative Technology Research Center
Advanced Mission Research Group

Innovative Technology Research Center
Advanced Mission Research Group

(from left)
Katsuto Kizara,Toshihiro Sezai,Hiroyuki Yoshida,
Hiroaki Suzuki
(from left)
Tatsuhito Fujita,Susumu Sasaki,
Yasuyuki Fukumuro,Fujio Nakano

Eco-friendly energy supply systems

A very large artificial satellite floating in geostationary orbit at an altitude of 36,000 kilometers. The artificial satellite sends energy to the ground without interruption. This is a new energy supply system called the Space Solar Power System (SSPS). JAXA is researching and developing SSPS with plans to construct them in 2030.
The SSPS is environmentally friendly, as it emits no carbon dioxide (CO2) or other potential contributors to global warming. Some people may associate environmental friendliness with solar cells. A solar cell is an effective, eco-friendly system that can be installed on top of a building or the roof of a house. But the solar cell cannot generate power on the ground on a cloudy day, or when deprived of sunlight during the night. An SSPS can generate power at any time in any weather (except, of course, during an ecliptic period).

SSPS studied by JAXA

JAXA is studying the development of an SSPS capable of supplying 1 million kW of electricity, the equivalent of the electric power generated by a nuclear power plant. Two system components are required to achieve this: an artificial satellite to collect sunlight in geostationary orbit and send it to the ground, and ground facilities to receive the energy sent from the satellite.
JAXA wants to convert collected sunlight into microwave or laser energy, to send it to the ground. A microwave and laser are electromagnetic waves, each with a different wavelength (refer to here) The properties of electromagnetic waves differ from one wavelength to another. Though microwaves can carry energy to the ground without being affected by cloud, the equipment to send and receive the energy must be of a reasonable size, regardless of the amount of energy handled (Figure 1). For carrying laser, the opposite conditions hold: the size of the equipment can be scaled up or down according to the amount of energy handled, but a cloud covering is likely to influence the carriage of the laser to the ground (Figure 2).
To construct the SSPS, much remains to be done. It will be important, for example, to find methods for carrying energy to the ground with high efficiency, and for dissipating loss of the heat generated by the collected sunlight. Methods to construct the SSPS will be another big challenge, as artificial structures so large have never been assembled in space.

Fig.1 Microwave SSPS (M-SSPS)
This SSPS collects sunlight into solar batteries. using two huge elliptical reflectors (mirrors), each with a major axis of 3,500 meters and a minor axis of 2,500 meters. Once these batteries generate electricity, the electric power is converted into microwave energy and carried to the ground. The microwave energy is reconverted into electricity with an antenna provided with a rectification function, a so-called rectenna, arranged in an area of a diameter of 2,000 meters. Finally, the electricity is sent to the commercial power grid.

Fig.1 Microwave SSPS (M-SSPS)
This SSPS collects sunlight into solar batteries. using two huge elliptical reflectors (mirrors), each with a major axis of 3,500 meters and a minor axis of 2,500 meters. Once these batteries generate electricity, the electric power is converted into microwave energy and carried to the ground. The microwave energy is reconverted into electricity with an antenna provided with a rectification function, a so-called rectenna, arranged in an area of a diameter of 2,000 meters. Finally, the electricity is sent to the commercial power grid.

Fig.2 Laser-SSPS (L-SSPS)
The L-SSPS has a basic unit consisting of a condenser, a laser oscillator, and a radiator arranged lengthwise. The system is very efficient, as the sunlight collected into the condenser is converted directly into laser energy by the laser oscillator and sent to the ground. JAXA wants to convert laser energy into electricity and use it to produce hydrogen, a fuel for fuel cells.

Fig.2 Laser-SSPS (L-SSPS)
The L-SSPS has a basic unit consisting of a condenser, a laser oscillator, and a radiator arranged lengthwise. The system is very efficient, as the sunlight collected into the condenser is converted directly into laser energy by the laser oscillator and sent to the ground. JAXA wants to convert laser energy into electricity and use it to produce hydrogen, a fuel for fuel cells.

Assembling the gigantic structure

The largest structure in space is the International Space Station (ISS), now orbiting Earth at an altitude of 400 kilometers. When the ISS is completed, it will be almost as large as a soccer stadium (About 108.5 m x 72 m). The efficient construction of the SSPS, a structure much larger than the ISS, will demand ingenuity.
Under current plans, the SSPS would be taken into space by rocket. Given the limits in payload capacity, only so many fairings can be launched at one time. It will thus be necessary to fold the fairings into small sizes and open them out once in space. The Engineering Test Satellite VIII (ETS-VIII.) launched in December 2006 has two large antennas made of a braided metal. Each was folded up into smaller sizes in the fairing, like a folding umbrella, and later set up in space. Another ingenious idea is an inflatable structure (Figure 3) which fills with injected air to extend its shape. Another is a "furosiki (wrapping cloth)" structure that opens up under centrifugal force. Most of these structures are still in the research phase. JAXA is collecting basic data to determine which structure is the most suitable to the SSPS.

Before opening

Before opening
Fig.3 Opening of the inflatable structure

After opening

After opening

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