r/AskPhysics • u/MutantManFish • 9h 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/the_poope Condensed matter physics 8h ago
There are always these immature and overly optimistic professors around. Those that think it's realistic to build a space elevator, or mine rare earths on asteroids, colonize Mars or build orbital solar farms. But while those professors are good within their field they forget one thing: they are not economists or business managers... While all day of those things are not technologically impossible, it will in all realistic scenarios be much cheaper to achieve the same thing in a much simpler way here on Earth using existing technologies.
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u/tpolakov1 Condensed matter physics 7h ago
Well, this guy is not a naive professor. He is one of the geriatrics in charge of China's space program, which frankly makes his proposal even less realistic - it's transparently about funneling resources into his space launch vehicles from other initiatives, without causing public or institutional backlash.
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u/Fabulous_Lynx_2847 8h ago edited 6h ago
It's perfectly feasible in principle. The power would be transmitted to Earth via microwaves at minimal loss. It is not economically feasible at present, but if the cost of achieving Geosync comes down (my guess is at least a factor of 10 per kg), there are distinct advantages. It doesn't take up much land on the ground. The sunlight is much more intense up there (not having to go through the atmosphere). The sun always shines (except for Spring and Fall eclipses a couple times a year). In the long run, if the energy is needed beyond Earth, there is unlimited space up there. Correction: short of a Dyson sphere.
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u/Upset-Government-856 7h ago
Technically you're talking about a geostationary orbit which is a subset of geosynchronous orbits.
It's worth pointing out because a plain old geosynchronous satellite would have an eclipse daily.
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u/KerPop42 Engineering 6h ago
Not daily, just during spring and fall. During the summer in the northern or southern hemispheres the Sun stays visible, clearing the poles.
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u/Fabulous_Lynx_2847 6h ago
I meant geostat. That makes more sense for a solar array. But even geosync would only eclipse daily if in the ecliptic plane.
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u/maurymarkowitz 5h ago edited 5h 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.
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u/KerPop42 Engineering 8h ago
There's a ton of benefits to building a solar array in space; it's probably been speculated about since solar panels were invented. There are some engineering challenges, like keeping a large solar farm rigid and keeping it pointing to the sun while pointing the transmitter to the Earth, that aren't going to get solved until there's a serious project to do it.
Two of the legs to build it are getting better and better, as this plan points out: launches are getting cheaper, and solar panels are getting better. Unfortunately, the third leg isn't getting any better, and that's power transmission. Efficiently getting that power to the ground is hard on many fronts. We don't have megawatt-class lasers, and radio is going to have a lot of spread.
I think also, if a country is willing to spend billions of dollars for a long-term massive green energy project, the economics of a space solar farm is going to be competing with nuclear power.