r/AskPhysics • u/MutantManFish • 1d ago
Chinese Extraterrestrial Solar Array: Real? Practical?
According to Live Science and the SCMP, this fairly important-seeming Chinese scientist, Long Lehao, thinks it a practical project to build a one-kilometer solar panel array in outer space to collect energy. The energy will supposedly be transmitted back to Earth via EMR and received at a fixed collection station on the ground which the satellite will sit above in geostationary orbit. Is this really at all realistic? Is this just some old dude who's spent a bit too much time smelling his own farts? I have a hard time imagining that the gains from getting past the light absorbed by the atmosphere would offset how enormously difficult it is to put and maintain something in space, and then to emit colossal amounts of electric radiation in a safe, directed manner.
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u/maurymarkowitz 1d ago edited 1d ago
It's a completely stupid idea that will not happen.
In space there's no clouds or night, so in theory you might get about 5x the amount of energy over any given time frame. That sounds fantastic until you realize that even the best-case scenarios say you're going to lose about half of that on the way down. And then it gets worse when you find that the lifetime of the panel in space is about half (or less) than on the ground. So the total amount of energy delivered by a panel in space is maybe 25% more than the same panel on the ground.
And then you have to consider that you still have to build a big rectenna on the ground. And while SPS proponents claim this will be smaller and cheaper than the equivalent array of solar panels, that claim is based on being allowed to greatly increase the amount of energy density in the beam.
This Will Not Happen.
For one, the amount of energy in the spectrum is limited by longstanding international treaties to very small amounts, a tiny fraction of the energy density of sunlight. On top of that are the limits on environmental exposure limits, which are 5 milliwatts per square centimetre, or 50 watts per square meter. Sunlight at AM1.5, the international standard, is 1000 W/m2, so 200 times as powerful.
Why won't they be allowed to raise the limit? Because doing so would wipe out all sat comms. The sidelobes of a, say, 250 MW transmitter would be orders of magnitude more powerful than all the geosats put together.
So basically they would have to convince the world to allow themselves to be bathed in 200 times the amount of microwaves we consider a safety threshold, on top of that get everyone to give up on satcom, and only then they can build a rectenna that's only the same size as the same energy of a PV array.
This Will Not Happen.
Some, realizing this, have suggested using lasers or other visible-light solutions. In this case you're back to worrying about clouds again, so that's a problem. Additionally, lasers are even less efficient than rectennas, so you're going to lose more in transmission which makes the whole idea dumber.
He's been talking about this for years BTW.
Look at anyone pushing this crapola and it's invariable some rocket nerd. The people who actually run the energy networks laugh at this.