# Why aren't solar collectors in space solar power concepts placed closer to the sun?

I've been thinking about space solar power for quite some time, as to how it could be more profitable than ground based solar power. From all the concepts that I have seen the main problem I always see is the conversion of majority of the solar power to microwave energy to beam back to the ground, for re-conversion into electrical energy, and having its only benefit being above the atmosphere to get more consistent solar energy. So, here is the thought I had.

1. Why aren't the solar panels placed closer to the sun where the solar flux is MUCH higher? (at 2 million kilometers from the Sun, the solar flux is nearly 10,000X as high as the flux experienced on Earth, meaning large amounts of power can be extracted with smaller panels) Space offers the advantage of being able to place collectors much closer to the energy source, is the main reason for not doing this the issues with redirecting this energy to Earth? Or being able to build solar panels able to handle the massive amount of power?
2. Why isn't the solar energy captured from satellites in solar power satellites transferred to the ground optically? (essentially reflecting sunlight to the ground instead of converting it into microwaves) I would imagine the energy would get lost through scattering, but wouldn't the efficiency of sending optical energy down be higher than converting to microwaves? (since its only one conversion process being used)

From the calculations I've done, the main problems seem to be transmitting the power from the satellite close to the sun to the satellite near Earth and ensuring a constant link between the two. I was thinking an optical link (using the satellite close to the Sun to serve as a lens to focus the sunlight on the Earth orbiting satellite) could be used, but I don't know how practical that is since laser communications for deep space still haven't been built. Still, since the solar flux is a lot higher, the initial network could be setup using cubesats launched from the ISS using the Nanoracks system.

Has this been considered by other people? And if it was scrapped why was it scrapped?

• There is a mirror on the moon that we can bounce a laser beam off (to measure the distance). I'd always wondered how they could point the laser beam so accurately from so far away, as to hit a small mirror. Then I heard it said that the 'narrow beam' of laser light will spread out to around the size of a football field by the time it reaches the moon. And that is why I suspect it will be impractical to beam energy over great distances (even the distance between the Earth and moon would be problematic - interplanetary, out of the question). At least until we have lasers with tighter beams. – Andrew Thompson Sep 3 '15 at 5:46
• Oh, and.. "..at 2 million kilometers from the Sun.." ..just about every material we could make, would have melted. – Andrew Thompson Sep 3 '15 at 5:48
• Could a lens be used to counteract the beam divergence? – user11377 Sep 3 '15 at 6:23
• I have to be honest and say that I don't know for sure. I suspect that there are factors to the physics (or at least optics) of it that would make it not possible. (This is a topic I've thought long on, but for which I'm not trained & have had no 'expert input' on.) – Andrew Thompson Sep 3 '15 at 7:28
• A satellite in a closer orbit than the earth will only be in a position to transmit energy for a fraction of its year, it will spend most of its time on the far side of the sun, in order to send continuous power you'd need several satellites. If the satellite orbits the earth you just need one. – GdD Sep 3 '15 at 9:26

This is really two questions:

1. Where is the best place to put solar power collection on the line between the Sun and the Earth?

2. What is the best wavelength to use for power transmission to Earth?

For #2, microwaves go through clouds. Visible light does not. That is why you will almost always see microwave transmission used in space solar power concepts. Recall that the rationale to consider space solar power at all, which seems odd given that our atmosphere is transparent to most of the energy from the Sun, is that you don't get solar power at night, and it can be reduced dramatically by clouds. If you use visible light to transmit the power, then you have lost the second advantage (as well as part of the first advantage on cloudy nights).

There are other advantages to microwaves, such as the fact that the ground collectors can be much lower cost (wires and diodes vs. solar panels), and the relatively long wavelength allows a mostly transparent wire mesh antenna for the ground collector, permitting crops under the collector, or even solar panels.

For #1, if you would like more concentrated solar power on your collector, then you can do that with mirrors without having to get closer to the Sun. There are indeed concepts that do that, since photovoltaic cells can be more efficient at higher insolation, and solar thermal systems require high concentration for any kind of decent efficiency.

Now on either side we're left with the diffraction limit for how big the reflectors need to be to collect the solar energy on the input side and to focus the energy to a collector on the ground on the output side. Yes, your concentrator/collector can get smaller as you get closer to the Sun, but now your transmission antenna needs to get larger to be able to focus on the same size collector on the Earth. If you are converting light to microwaves, then you want to be closer to Earth since the microwave antennas have to be bigger at the same angular limit than for light, due to the longer wavelength. If you are converting light to light (so you don't need to do energy conversion at all -- just use mirrors), then it's a wash, so you put it near the Earth since its easier to get there.

• The diffraction problem has to do with the Sun being large, not a point source. But if a kilometer sized reflector is put near it, then from some modest distance, the reflector will be a point source, right? So a secondary reflector could focus that light to an arbitrary dot. – LocalFluff Sep 6 '15 at 0:38

This actually is a fairly popular concept in Science Fiction, although they usually don't go quite so close to the Sun. They usually conveniently ignore the elephant in the room, which is how to get said power back to Earth. You are correct, the only way to reasonably do this is to have it be transmitted via some kind of a laser. It is theoretically possible to have a very focused laser, but it would have to be very large, much larger than anything we have now. I've seen that for a Mars to Earth communication link, which is about the same distance as Mercury to Earth, for instance, the size of the laser beam would be on the order of a US Region, such as the Southwest Region, or perhaps the size of European countries (The larger ones).

Still, I suspect that this could eventually be considered, but isn't really practical in the short term. Microwave transmission might be possible from Earth orbit, which is why that concept is discussed far more than a system very close to the Sun.

• I thought beam divergence is a problem only when the beam has traveled very far away from the focus. What if the focus is placed near earth? Granted to do that, the lens would need to be made very accurately, which may not be possible. Does anyone know what the tightest tolerance is on lens curvature? – user11377 Sep 3 '15 at 17:51
• If you look in to lasers, there is actually lenses in them, or at least the same thing. A lens can't correct the beam any better than increasing the Laser chamber size would do, it's an effect related to diffraction limits. – PearsonArtPhoto Sep 3 '15 at 17:52
• A more reasonable approach is to use a huge fleet of satellites with mirrors that would create segments of a mirror lens focusing the sunlight near Earth, where another set of satellites would reflect/beam it down to Earth. One problem: the mirrors would be quite efficient and rather undesired solar sails; keeping the satellites in a fixed orbit would be somewhat tricky. – SF. Nov 13 '15 at 9:07

Money

...that is the reason. That and the fact that you will need to put more energy into getting this stuff into orbit than you will ever get back from it.

Getting 1 kg to Low Earth Orbit costs between \$3 800 and \$13 000.

Getting 1 kg to Geostationary Transfer Orbit costs between \$12 500 and \$25 000.

Getting anything to a close orbit around the Sun will be much more expensive than that.

The energy needed to get this supposed energy collector into that orbit will be a lot more than it can ever gather.

And then - as people have pointed out - we cannot get the energy it collects back to us because no matter what kind of radiation you use, it will spread out and become very faint.

In the end it is a lot more efficient to just use all that money to build the energy collectors down here on Earth.

• That is an answer to a different question. – Mark Adler Sep 5 '15 at 22:38
• Actually the question starts with "I've been thinking about space solar power for quite some time, as to how it could be more profitable than ground based solar power". In short: it cannot because a) it costs a heap-load of cash just to get the stuff into orbit b) the energy you need to put out on that will not be returned to you and c) you cannot get the energy to Earth, so you cannot get revenue from it. So the entire premise of the question - that orbital energy collectors would be more profitable - is entirely moot because in a lifecycle perspective they cannot be that . – MichaelK Sep 5 '15 at 22:45
• a) At current prices, yes. These studies always have as part of their calculation at what \$/kg LEO they could break even. Reusable launch vehicles offer the prospect of much lower launch costs. b) Not true. The energy invested can be returned. It is the money that is in question. c) Not true. The energy transmission is the one part that is entirely feasible with existing technology. Your statements are far too sweeping. I personally have serious doubts that space solar power will ever become profitable, but your absolute statements are simply unsupportable. – Mark Adler Sep 5 '15 at 23:49
• I'm sorry but you don't have me to contend with.... it's physics you need to try to prove wrong here. Physics and economics. At present prices for lifting stuff to space, and with present energy transfer technology, space-generated power if just a waste of money. Build it on Earth instead and you can build so much more energy collectors for the same money that it is entirely pointless to lift them to space. In increased influx does not make up for the losses you take in both money and energy in getting them there. – MichaelK Sep 6 '15 at 0:53
• sigh Let me point out again what the original question is: "how [space solar power] could be more profitable than ground based solar power". It cannot. Sure, you can get more power to reach the collector and if you could harvest that, it means more revenue per unit. However(!)... that is completely annihilated by immensely increased cost per unit of getting it into space, and the losses in transmission. To make an analogy: it doesn't matter if you could get a can of coca cola for free in Liberia. The cost of shipping yourself to Liberia, or the cola to you, will negate any such gain. – MichaelK Nov 13 '15 at 13:05

An aspect not yet mentioned, is that I don't think solar panels work too close to the Sun. Solar Probe Plus, at 8.5 solar radii or 6 million km at the closest, will actually fold back its solar panels into the shadow of the spacecraft while near the Sun. Also, optically concentrating photovoltaic cells are among the most efficient today, and I think that overheating is a limit.

• Solar panels do indeed work close to the sun; the reason Parker Solar Probe (its new name) folds its back into its shadow when close to the sun is to keep them from being vapourised. – Sean May 22 '19 at 2:58