Space-based solar power seems like an idea from a Star Trek script, but given the uncertain future of its power generation industry, Japan stands to gain as much as anyone by exploring this potential source of renewable energy. The disaster at Fukushima, limited access to fossil fuels and advances in technology has, at least in the eyes of the Japan Aerospace Exploration Agency (JAXA), added further weight to the notion of a space-based solar power system. The agency is developing a complex roadmap involving a 1 GW extraterrestrial solar farm, a microwave beam and a man-made island in the Tokyo harbor which could be used collect solar energy in space and supply power to Earth by 2040.
While Glaser's proposal never got off the ground, it did inspire further investigation of the potential of space-based solar power by various government departments and institutions. In 2008, a company called Space Energy conducted a long-range wireless power transmission test using a microwave beam between two Hawaiian islands, a distance of 148 km (91.96 mi). The result was a power yield of 1/1000th of one percent on the receiving end, raising questions over whether the technique could be employed over the much larger distance between a satellite in geosynchronous Earth orbit (GEO) and a ground station.
Writing in IEEE Spectrum last week, Professor Emeritus at JAXA, Susumi Sasaki, argues that this experiment failed largely due to the dense atmosphere disturbing the microwaves' phases as a result of the horizontal transmission. In detailing the agency's proposal he emphasized that in a space-based system the microwaves only need to pass through this dense atmosphere for the last few kilometers of their journey. This, along with new designs for the solar power satellites and anticipated advances in technology over the coming decades, gives JAXA confidence that it can eventually achieve an effective wireless transmission of solar energy over the necessary 36,000 km (22,500 miles) from GEO.
The more complex solution seeks to address this problem by positioning two huge mirrors alongside two photovoltaic panels. These mirrors would reflect sunlight onto the panels 24 hours a day and would be free-flying, meaning that they are not tied to the panel nor the transmission unit. While the technology that enables formation flying in space continues to develop, Sasaki says considerable advances would need to be made to coordinate formation flying with kilometer-long structures. Other challenges in building this type of SPS include developing light materials for the mirror and high-voltage power cables to transmit the power from the two solar panels to the unit, technologies that Sasaki says are still years away.
This is a problem that has not been solved with current technologies. JAXA hopes to overcome it with a so-called retrodirective system. This involves sending a pilot signal from a rectifying antenna, or rectenna, on the ground to the antennas on the satellite. As each antenna receives the pilot signal, it would calculate the necessary phases for its microwaves and adjust to form a solid beam back to the rectenna on the ground. A rectenna array would be located on a man-made island in the Tokyo harbor and measure 3 km (1.86 miles) in diameter, a size that JAXA says would require limiting the divergence of the beam to 100 microradians. From there, the array of rectennas would convert the microwave power to DC power at an efficiency of more than 80 percent, with the DC power then converted to AC and directed into the grid.
The microwave beam itself would have a power density of one kilowatt per square meter. With the typical regulatory limit for human exposure to microwave radiation set at 10 watts per square meter, such a beam would come with its own set of problems. Sasaki says that the site of the rectenna, an area of around 2 km (1.24 miles) in diameter, would need to be restricted and the staff would require protective clothing, but it is unclear what safeguards would be implemented to prevent such a powerful instrument being misdirected or falling into the wrong hands ... say, someone with a hairless cat and penchant for world domination.
With a finite supply of fossil fuels and increasing pressure to shift to clean, renewable energy, a constant stream of power drawn from an unlimited source could have environmental and economic impacts across the globe. While it's a concept that seems out of this world in more ways than one, and a 1 GW solar farm would hardly register a blip on Japan's energy consumption radar, demonstrating progress toward a functioning SPS could constitute an effective proof-of-concept and garner further private and public sector interest in space-based solar.
See the stories that matter in your inbox every morning