Published online 25 November 2009 | Nature 462, 398-399 (2009) | doi:10.1038/462398b


Japan sets sights on solar power from space

Researchers aim to launch full-scale system by 2030.

Experiments this year with an airship transmitted enough power to charge a mobile phone on the ground.Experiments this year with an airship transmitted enough power to charge a mobile phone on the ground.RESEARCH INSTITUTE FOR SUSTAINABLE HUMANOSPHERE, KYOTO UNIV.

Japanese scientists are once again eyeing an off-world approach to alternative energy — collecting solar energy from satellites in orbit and beaming it down to Earth.

A space-based solar-power satellite — which could gather energy without having to worry about clouds or night-time — has been a dream for decades in both the United States and Japan. But the costs of developing it has meant that support has waxed and waned over the years. Now, however, Japan has a new sense of mission. In June, it released a national space plan calling for a programme to "lead the world in space-based solar power". And earlier this month, scientists, engineers and policy-makers met at Kyoto University to lay out development plans.

The government's commitment "is definitely a milestone and has given tremendous excitement to solar-power satellite researchers", says Hiroshi Matsumoto, a radio scientist and president of Kyoto University.

Researchers are hoping to launch a full-scale system by 2030, but costs need to come down dramatically for it to be economically viable.

Few doubt that the project is technically possible. The well-understood process starts with collecting solar energy with photovoltaic cells, transferring that energy to antennas that transmit microwaves, then receiving those microwaves with a 'rectifying antenna' that converts them to electricity. As early as 1975, scientists at the Jet Propulsion Laboratory in Pasadena, California, transferred energy by means of microwaves over a distance of 1.54 kilometres. And in May last year, scientists beamed power over a distance of 148 kilometres, between two Hawaiian islands.

Japan has been investigating solar-power satellites since the 1980s. In 1983 and again in 1993, Matsumoto, working with Kobe University's Nobuyuki Kaya, launched rockets into the ionosphere to investigate what happens to microwaves as they travel through space (H. Matsumoto Radio Sci. Bull. 273, 11–35; 1995). In March this year, a group from Kyoto University became the first to use microwaves to send power from the air to the ground when they charged a mobile phone with microwaves transmitted from a blimp-like airship hovering some 30 metres above the ground.

“I'm 100% confident this will happen. We need another stable power source.”

Current scale-up plans call for a series of tests, each with an increasingly larger capacity for power transmission. First, Japan aims to demonstrate ground-based transmission in the kilowatt range, then space-based kilowatt transmission using Japan's Kibo module on the International Space Station or small satellites. By 2020, researchers hope to have a prototype satellite that can transmit in the range of hundreds of kilowatts, and by 2030 a satellite that can transmit a gigawatt. As currently envisioned, the system to launch in 2030 would be a 2-kilometre-wide array of solar cells with an array of 1 billion transmitting antennas — each measuring 5–10 centimetres across — on the side facing Earth.

The goal is to make satellites for under ¥1 trillion (US$11 billion) each; it currently costs 100 times that. "It's exciting, but there are many problems to overcome," says Naoki Shinohara of Kyoto University. For one thing, transmission efficiency must rise to 75%, he says; the airship experiment achieved just 40% efficiency, although the technology it uses differs from what a satellite would use.

Rocket launches will also need to be cut to a hundredth of their current cost; options such as reusable rockets are being considered, according to Susumu Sasaki of the Japan Aerospace Exploration Agency (JAXA). At this month's meeting, Tokyo University's Kimiya Komurasaki discussed how a remote microwave source could power rockets. That would reduce the amount of propellant they need to carry and, in theory, mean that rockets used to build a solar-power satellite could carry more antennas and solar cells.

Matsumoto estimates that it will take ¥2 billion to ¥3 billion to demonstrate solar-power satellite technology on the ground, and ¥10 billion to ¥50 billion to demonstrate it in orbit.

The nation's space plan calls for an "all-Japan" effort to prepare for space-based demonstrations within three years. And as research budgets have been tight in many areas (see Nature 462, 258–259; 2009), the industry and science ministries have more than doubled their budget requests for solar-power satellite-related programmes, to nearly ¥1.4 billion. JAXA has pressed for a doubling of its budget for space-based solar power, from ¥250 million to ¥500 million.

"I'm 100% confident this [technology] will happen," says Shinohara. Unlike wind or Earth-based solar, solar-power satellites in space can gather energy 24 hours a day to provide a reliable source of alternative energy. "We need another stable power source," he says.


Japan looks likely to lead the way, as interest in the United States has waned, says John Mankins, who led the space solar-power programme at NASA. Most efforts in the United States are now in private companies or non-profit organizations. In April, Solaren, a company based in Manhattan Beach, California, signed a contract with San Francisco-based Pacific Gas and Electric to produce 200 megawatts of energy from a solar-power satellite starting in 2016. But Mankins, who co-founded and works at Managed Energy Technologies in Ashburn, Virginia, calls that goal "extremely challenging".

Japan's effort, he says, may lead the way: "The Japanese plan is quite well formulated." 

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