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See Inside July 2008

Farming Solar Energy in Space

Shrugging off massive costs, Japan pursues space-based solar arrays

Kakuda, japan—In a recent spin-off of the classic Japanese animated series Mobile Suit Gundam, the depletion of fossil fuels has forced humanity to turn to space-based solar power generation as global conflicts rage over energy shortages. The sci-fi saga is set in the year 2307, but even now real Japanese scientists are working on the hardware needed to realize orbital generators as a form of clean, renewable energy, with plans to complete a prototype in about 20 years.

The concept of solar panels beaming down energy from space has long been pondered—and long been dismissed as too costly and impractical. But in Japan the seemingly far-fetched scheme has received renewed attention amid the current global energy crisis and concerns about the environment. Last year researchers at the Institute for Laser Technology in Osaka produced up to 180 watts of laser power from sunlight. In February scientists in Hokkaido began ground tests of a power transmission system designed to send energy in microwave form to Earth.

The laser and microwave research projects are two halves of a bold plan for a space solar power system (SSPS) under the aegis of Japan’s space agency, the Japan Aerospace Exploration Agency (JAXA). Specifically, by 2030 the agency aims to put into geostationary orbit a solar-power generator that will transmit one gigawatt of energy to Earth, equivalent to the output of a large nuclear power plant. The energy would be sent to the surface in microwave or laser form, where it would be converted into electricity for commercial power grids or stored in the form of hydrogen.

“We’re doing this research for commonsense reasons—as a potential solution to the challenges posed by the exhaustion of fossil fuels and global warming,” says Hiroaki Suzuki of JAXA’s Advanced Mission Research Center, one of about 180 scientists at major Japanese research institutes working on the scheme. JAXA says its potential advantages are straightforward: in space, solar irradiance is five to 10 times as strong as on the ground, so generation is more efficient; solar energy could be collected 24 hours a day; and weather would not pose a problem. The system would also be clean, generating no pollution or waste, and safe. The intensity of energy reaching Earth’s surface might be about five kilowatts per square meter—about five times that of the sun at noon on a clear summer day at midlatitudes. Although the scientists say this amount will not harm the human body, the receiving area would nonetheless be cordoned off and situated at sea.

At a facility in Miyagi, Suzuki and JAXA researchers are testing an 800-watt optical-fiber laser that fires at a receiving station 500 meters away. A mirror reflecting only 1,064-nanometer-wavelength light directs it into an experimental solar panel. (He chose that frequency of light because it easily cuts through Earth’s atmosphere, losing no more than 10 percent of its pop.) A key task will be finding a material that can convert sunlight into laser light efficiently. A leading candidate is an yttrium-aluminum-garnet ceramic material containing neodymium and chromium.

The basic science is only part of the challenge. Testing both the microwave and laser systems will require gargantuan structures in space: thin-film condenser mirrors, solar panels and a microwave transmitter stretching for kilometers and weighing 10,000 metric tons, as well as a 100-unit laser array of 5,000 metric tons that would be 10 kilometers long. The ground-based microwave antenna would have to be two kilometers long.

The total project cost would be enormous—perhaps in the tens of billions of dollars—but Suzuki and his colleagues say they are not considering the price tag. “We can’t know whether this is feasible or not if we don’t have the basic technology first,” he says. “We’re aiming to produce stable, cheap power and hydrogen at a target price of 6.5 cents per kilowatt-hour.” That would be in line with conventional power generation costs of today and might make it more economically attractive.

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