With a few exceptions, sources of renewable energy tend to be inconveniently unreliable. Things like wind and wave energy are subject to the whims of weather, and even if you manage to find something that isn’t weather dependent (like tidal flow), you still can’t rely on a constant output of power. Strictly speaking, solar power is the worst: it's heavily weather dependent, and half the time, it doesn't work at all. (Even the moon gets in the way sometimes.)
Some think the way to make solar power the backbone of a renewable energy economy is to avoid the problematic Earth entirely and head out into space, where the sun is always shining and weather means something entirely different. Solar power satellites (SPS) are more than a concept: it’s an area of active research and development, led by the Japan Aerospace Exploration Agency (JAXA). JAXA explained its 25 year technology development roadmap that culminates in a 1 gigawatt SPS sending solar power back to Earth in the 2030s in IEEE Spectrum last year. Last week, JAXA and Mitsubishi demonstrated their progress on one of the most difficult components of that system: long range wireless power transmission.
Space-based solar power on a commercially viable scale will be an enormous undertaking. For an output of 1 gigawatt, Japan is planning on deploying a solar collector weighing over 10,000 metric tons and measuring several kilometers across. It would live in geosynchronous orbit, some 36,000 kilometers from Earth.
Arguably, the most difficult part of this whole business (from a technological perspective) is getting the power from the satellite back down to Earth where we can actually use it, and until we can find a long enough extension cord, there's only one way to make it work: wirelessly.
The only efficient ways to transmit power wirelessly over a very long distance, according to JAXA researchers, is with either lasers or microwaves. Lasers are impractical because they’d run into the same problems that solar power does on Earth: they don't work through clouds. Microwaves, though, work even if the weather is bad, so they're what JAXA has been planning on using to transmit power.
On Thursday, JAXA was able to deliver 1.8 kilowatts “with pinpoint accuracy” to a receiving antenna (rectenna) 55 meters away using carefully directed microwaves. According to JAXA, this is the first time that anyone’s been able to send such a high power output with this level of direction control. Also on Thursday, Mitsubishi (in partnership with JAXA) managed to send 10 kilowatts of power over a distance of 500 meters, using larger antennas with more of an emphasis on power over precision.
The obvious question here is one of efficiency: being able to transmit power is great, but if you lose most of it along the way, will the overall system ever reach commercial viability? At this point, the conversion system (solar to DC to microwave to DC to AC) is about 80 percent efficient, but that excludes loss of energy in transit. Neither JAXA nor Mitsubishi are commenting on the efficiency of these specific tests (which, to be fair, weren't’t optimized for efficiency), but we do know that as of last year, JAXA expected a 1.6 kilowatt microwave beam to yield a rectenna output of about 350 watts from a 50 meter test.
Within the next five years or so, Mitsubishi is hoping that they’ll be able to use this system for short range high power delivery (like electric car charging), and medium range delivery of small amounts of power (like powering warning lights on transmission towers). Meanwhile, JAXA is planning on testing the technology in space by 2018, with a small satellite transmitting several kilowatts from low Earth orbit to a microwave receiver on the ground. JAXA hopes to have a 100 kW satellite in orbit by 2021, and a 200 MW version by 2028. By 2031, if everything goes well, a 1 gigawatt commercial pilot plant will be in operation, with a full on commercial space-based power industry to kick off with one launch per year starting in 2037.