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.

Thinking outside the square

It was just over 40 years ago that the concept of a solar power satellite (SPS) first emerged. American scientist and aerospace engineer Dr. Peter Glaser won a patent for a broadcast system using a one-square kilometer antenna to channel power via microwaves to a receiver on the ground. The advantage of such a system, and space-based solar power in general, is that it harnesses the unobstructed output of the sun, unlike land-based solar systems which are affected by the weather and Earth's day/night cycle.

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 JAXA approach

JAXA is working on two concepts. The simpler one involves a huge square panel that measures 2 km (1.24 mi) per side. The top surface would be covered with photovoltaic elements, with transmission antennas on the bottom side. A small bus housing controls and communication systems would be tethered to the panel via 10 km (6.2 mi) long wires. A limitation with this design is that the orientation of the panel is fixed, meaning that as the Earth and the satellite spin, the amount of sunlight the panel receives will vary, impacting its ability to generate power.

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.

Generating solar power in space is one thing ...

The mirror-based system would require more than 100 million 10-watt semiconductor amplifiers and be capable of producing 1 GW of power, according to Sasaki. Transmitting this power back to Earth is where the proposal gets tricky. And more speculative. JAXA has earmarked 5.8 GHz as an appropriate frequency for transmission, claiming that it would allow effective penetration of the atmosphere without the need for exceedingly large transmitting antennas. There would however, need to be a huge number of antennas – more than one billion in fact. They would also need to form a single focused beam, meaning their phases must be synchronized, a process made more difficult by the fact that the panels move in relation to one another.

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.

What's next?

JAXA is planning to conduct tests by the end of this year demonstrating its retrodirective beam control system. In 2018 it hopes to perform the first microwave power transmission in space, channeling several kilowatts from low Earth orbit to the ground while ensuring that the microwave beam doesn't interfere with existing communications infrastructure. Following these initial steps, JAXA is aiming to commence a 100 kW SPS demonstration in 2020, whereby engineers could verify the technologies required to scale up the system. From here, it claims an international consortium involving experts from around the globe would be required to construct a 1 GW commercial SPS throughout the 2030s.

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.

Sources: IEEE Spectrum, JAXA