Japan aims for space solar power
JUNYA IWAI, Nikkei staff writer
TOKYO -- Japan thinks outer space might offer a solution to its dire energy issue.
It is looking to use the abundance of solar power in space to generate electricity by the 2040s. A number of technical and financial challenges to doing this lie ahead.
In March, the Japan Aerospace Exploration Agency, known as JAXA, Japan Space Systems and other entities successfully transmitted electricity via wireless power transfer to a target 55-meters away.
Researchers from the group used a Mitsubishi Electric facility in Hyogo Prefecture to carry out the experiment. They first converted electric energy into microwaves and then sent it using an antenna. Microwaves are often used in cellphones. The receiving antenna received the microwaves and converted them into 300 watts of electricity, enough to power three PCs.
Wireless power transfer systems either use magnetic fields to send electricity between transmitting and receiving coils or send electricity through radio waves. Some electric shavers and mobile phone batteries can be recharged wirelessly. Space solar power generation would use the radio wave technology.
The concept of space solar energy was first presented in the 1960s in the U.S. Since then, scientists around the world have been studying ways to realize the technology.
Currently, the idea is to generate electricity by receiving sunlight with 2-km by 2-km solar panels in space and convert that power into radio waves for transmission to Earth. On the ground, antennas receive the radio waves, which are turned back into electricity.
One advantage of space solar power generation is that it would not be affected by weather conditions. As such, Japan's Ministry of Economy, Trade and Industry estimates it could generate power of around 1 million kilowatts, equal to that of a nuclear reactor. If this new source of energy becomes widely available, a resources-poor country like Japan will be able to turn itself into a power exporter.
But now comes the difficult part. Japan needs to develop the technology to efficiently convert electricity into radio waves and vice versa, as well as for sending radio waves accurately to targets on the ground from 36,000km away. The test in March was a step in that direction.
To generate power in space, an array of 1-meter square panels will be formed to make a 2km solar antenna in radius. Numerous solar panels will be lined up on this giant power generation system. The test used a panel 60-cm in radius, 2.5-cm thick and 16 grams in weight.
If the location of individual antenna elements shifts, that could disrupt radio waves and reduce power transmission efficiency. Mitsubishi Electric has developed the technology for fine-tuning the timing for each antenna element to send radio waves. In the test, the company shifted antenna elements by a few centimeters on purpose, but it succeeded in transmitting radio waves without any problems.
"Our technology is coming up to the level where it can be used in space," said Yukihiro Honma, an official at Mitsubishi Electric's Communication Systems Center.
IHI Aerospace created a receiving antenna board -- 2.6-meter high and 2.3-meter long -- aligned with about 2,300 round patch antennas. The company verified that this antenna successfully received radio waves.
In February, Mitsubishi Heavy Industries also succeeded in wireless power transmission. The company plans to use the technology to send electricity to isolated villages suffering blackouts due to natural disasters, as opposed to space solar power generation.
Companies still face a number of technical challenges, including how to improve the efficiency for power transmission and reception.
Experts believe that space solar power generation will become commercially viable if 50% of the power generated in space can be transmitted to Earth. However, JAXA and other tests show that only 5-10% of the power was actually transmissible. To boost this ratio, these companies need to develop new semiconductor chips, among other things.
Koji Tanaka, an associate professor at JAXA's Institute of Space and Astronautical Science, is working on a project that sends radio waves from a satellite 300km above the ground. He uses the data to examine transmission efficiency, safety and other technological concerns.
At this altitude, molecules are broken up and ionized with solar radiation. Therefore, radio waves at this height could be diverted or absorbed. "We would like to launch the project in a few years' time and verify how it actually goes," said Tanaka.
Another huge hurdle is the exorbitant costs needed to fund space solar power generation. The equipment for such a space-based system weighs about 10,000 tons in total. It would take more than 1,000 launches by Japan's H2A rocket to bring all the equipment to the space. In theory, Japan needs to spend 1.2436 trillion yen ($10.1 billion) to construct a 1 million kilowatts facility using a space solar power generation system, according to JAXA estimates.
Furthermore, Japan needs to put in place a more efficient management system. Currently, the science ministry is responsible for space research, METI supervises the business aspects and the internal affairs ministry is in charge of radio waves.
"To secure energy, the important thing is to build a cooperative system for realizing this technology," said Naoki Shinohara, a professor at Kyoto University who researches space solar power generation.