Solar energy experts have long known that the best place to collect the sun's rays is in space. A solar farm in orbit could collect energy all the time, whereas ground-based arrays sit idle during the night. And huge chunks of real estate are easier to come by in space, where solar collectors can be as enormous as they need to be. But the problems with turning solar energy in space into useable energy on Earth have kept space solar stuck in the land of science fiction since the 1960s.
Microwaves are key to JAXA's plans. Some space solar concepts have proposed using lasers to beam the energy in orbit down to collectors on the surface, but the water molecules in the clouds can scatter laser light. That means you'd lose some of the energy on anything less than a perfectly clear day. Microwaves don't have that problem. So JAXA has designed multiple concepts in which the DC (direct current) power generated in orbit would be transformed into microwaves and then beamed to Earth's surface, where a farm of antennas would collect the microwaves and transform them back into DC electricity. JAXA says it can now perform these transformations with at least 80 percent efficiency on each end.
Another major hurdle for space solar is keeping the collectors pointed at the sun at all times so they can collect energy continuously. JAXA released one design that features a square panel measuring 2 kilometers (1.2 miles) on each side. But the panel's orientation is fixed, meaning the amount of energy it can produce varies. Another JAXA design solves this problem by incorporating two enormous mirrors that reflect sunlight onto photovoltaic panels positioned between them.
More hurdles: If a space solar array had to burn fuel to adjust its position, for example, that would add millions of dollars to the cost. So Japan is trying to design its components in such a way that they naturally counterbalance Earth's gravity and stay in position without adjustment. In the case of the two-mirror design, all those pieces would need to fly in careful, precise formation, something that has not been tried on so grand a scale.
In the transmission phase, more than a billion tiny antennas affixed to moveable panels would be required to receive the microwave energy coming from space. Those panels must constantly adjust their orientation to maximize how much energy they receive. JAXA plans to help them by sending a pilot signal from the ground to the satellite that would tell the satellite how to adjust the beam.
Despite these and more challenges, JAXA has rolled out an ambitious timeline: It plans to unveil a ground-based demonstration this year, then reveal progressively larger experimental satellites in 2017, 2021, and 2024. The major goal would come to fruition in the following decade: A 1-GW power station in 2031, and then one power station launched per year by the late 2030s.
Within a quarter-century, then, perhaps Japan's energy will come not from nuclear plants -- which are vulnerable to earthquakes, tsunamis, and planetary outbursts -- but from solar arrays that aren't even on the planet.
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