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Say you've got a space-faring civilization that needs a LOT of power. Sure, fusion is a thing, but there's really not that much helium-3 to be found in our solar system, and it's hard to get. So the best way to get gargantuan quantities of energy is to start on a Dyson swarm. We don't need the whole thing, just enough collector stations to meet current demand, whatever that might be. But if collected energy is a limiting factor to any of our endeavors, then we want to maximize our power generation for the smallest possible cost, which means we'll want to get closer to the sun than we are now, so the sunlight is more intense. The question is, how close can we get? How close is it practical to get?

EDIT: Let's assume the collectors are designed using technologies and materials we have today, though we are developed enough as a space-faring race that getting into and around in space isn't a problem. Also, assume that technologies and materials which are currently expensive cutting edge (like graphene) are now mass-producible.

Presumably, if we get too close, any station we built would need to spend an inordinate amount of mass on radiators and other systems to keep itself cool. Where do you think that point is?

At Earth's distance from the sun, we get about 1360 watts per square meter from the sun. At Mercury's orbit, we get about 9000. One suggestion for building a Dyson sphere is to mine Mercury for the raw materials and put the satellites (which would mostly be giant mirrors reflecting to central collecting stations). Could we get closer? Is Mercury already too close? Or is it just right?

EDIT2: Let's also assume that we're not restricted to solar panels; we could also be using mirrors that redirect light to a central boiler, or other means of collecting solar power.

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Your problem isn't generation; it's storage.

Ultimately, wherever you put your solar generators or collectors, your efficiency over the long term isn't going to change all that much. Modern photovoltaics actually break down a little in hot environments and you'll find that collecting the sun's energy in the long term is a tradeoff between the power you can harvest and the longevity of the collection device.

So, you could build around Mercury, but that presents its own challenges. If the collectors wear out faster, how do you maintain them? Besides, the amount of solar energy that you can collect even near the earth would be sufficient for most needs, provided you can even out the peaks and troughs in demand. This is actually your real problem; the sun is in an 'always on' mode, and you need your energy delivery to be more flexible than that for those few high-draw tasks that ramp up your energy needs, like launching ships into space.

Whatever shall we do?

With normal domestic and industrial energy consumption, it is handled with 'Just In Time' power generation; in other words, we know when power is most likely to be used, and we know that many users are generally evening out the overall needs, and further, we price electricity differently in non-peak times to even out the needs and encourage people to use power more evenly throughout the entire day. This means that we start up and shut down different generators as the needs on the grid change in real time.

We can do the same thing with launching our space ships; launch them from all over the globe at different times, say 10am local time at each location. But, the real answer is probably batteries.

As parts of the globe transition to sustainable generation techniques like solar and wind power, industrial batteries are fast becoming a part of the power mix for countries adopting sustainable techniques. This is because you can't just start up another south wind or another sun to meet demand in your country, you have to capture the energy when it is available and release it on demand. Batteries are the obvious solution to this.

Arguably, the primary limitation in your battery storage is rare earths that have become an essential ingredient in most industrial battery designs. These rare earths might be easier to come by than He3, but their extraction is a very toxic process in most cases.

Perhaps we could mine Mars for them? You need a spacesuit to live and work there already so it doubles as protection from toxins in your mine, and you should be able to pull a large number of these out of the ground to supply you with enough battery storage to support your peak draws from your constant generation from solar, wherever you put it.

Ultimately, this concept of industrial batteries is the biggest fundamental shift in energy thinking that we need to accept if we are to take sustainable generation seriously. The advantage of coal, nuclear, fusion etc. is that we can generate as needed, but we can't do that with solar power. Batteries however give us the flexibility to divorce generation from demand in a useful manner.

The downside however is that there is a word for a device that stores highly dense energy if it is capable of some form of uncontrolled release;

a Bomb.

The batteries we are talking about are not simple to build, and even harder to safely manage and maintain, but they do solve the problem of taking your generated energy and keeping it ready for direct use.

Edit, or Reflecting on Changes to the Question;

The question of whether or not a reflection based heat collector would fare better closer in is an interesting one so let me add to my answer to address it.

In theory, the closer you get to the sun, the more 'dense' the energy that can be reflected back into a central heat reservoir, therefore the smaller the reflection array you need to reach the optimum heat levels.

In practice, visible light is an infinitesimally small percentage of the entire EM spectrum, and most materials are not reflective to it all. What that means is that while your 'mirrors' can reflect more of that spectrum into your heat reservoir, there is ALSO more that is absorbed by the mirror in the ranges that are not reflected, heating it up. So, arguably your maintenance periods contract as well.

Whether or not the maintenance period halves as the density doubles is the big question and I don't have any figures on that although I do plan to capture them next time I'm building heat reservoir generators at varying distances from the sun.

But; ultimately it comes down to a simple variable; if the lifetime of the array is reduced by LESS than the increase in generation, move your arrays closer. If the lifetime is reduced by MORE, them move them back. Because the density of energy reduces by some exponential measure as distance from the sun increases, my guess is there is a 'sweet spot' at which you gain energy at the highest density to degradation ratio, but I'll admit without detailed schematics and two year's research in material sciences, I have no idea where that sweet spot is.

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  • $\begingroup$ My idea was that the collectors were using some nebulous high-energy process to produce industrial quantities of helium-3, a process that requires at least as much energy put into producing the helium-3 as the actual fusion will produce. The helium-3 is then used in starship fusion rockets, which require ridiculous amounts of energy. So the vast amounts of energy collected from the sun are stored in fusion fuel form, and used to power interplanetary (or interstellar) spaceships. $\endgroup$ – FlyingLemmingSoup Aug 29 at 2:31
  • $\begingroup$ @FlyingLemmingSoup well then that solves your battery issues to be sure, but ultimately your panels are still going to suffer from a rate of degradation that increases as you get closer to the sun. I still would say that your tradeoff is how much energy you want a single collector to gather versus its operational lifetime. $\endgroup$ – Tim B II Aug 29 at 2:33
  • $\begingroup$ @B II That seems reasonable, as it would imply that a single solar collector will probably collect about the same amount of energy over its lifetime wherever it is placed, it's just that the rate will vary. But what about energy collectors other than solar panels? We have arrays on Earth that consist of giant mirrors redirecting light to a central boiler. In space, such an array wouldn't need moving parts and could be absolutely enormous. $\endgroup$ – FlyingLemmingSoup Aug 29 at 2:37
  • $\begingroup$ @FlyingLemmingSoup yes, you're right, that's a different matter and you can scale up that kind of generator, but only to a certain point; there is a limitation to the amount of energy that (say) a molten salt system can store as heat before the medium, the salt, is overwhelmed. In such a case, the closer you go in, the smaller the reflection array will have to be in order to make the heat reservoir work to capacity. So in that sense, you might actually be able to get such systems to generate the same amount of power with a smaller array, but I don't have any figures to hand on maintenance. $\endgroup$ – Tim B II Aug 29 at 2:48
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Instead of solar cells that

  • transform the Sun's energy
  • have to somehow move the stored energy from the Sun to wherever it is needed
  • have to be cooled (because they absorb the light)
  • degrade with time

I would use mirrors that:

  • collect the solar radiation ("light")
  • easily direct the radiation to where the energy is needed
  • do not have to be cooled (because they reflect the light)
  • are made of glass and/or metal and technically simple and therefore longer lived
  • and then transform the solar radiation into energy on site, avoiding the energy transportation problem

You know how solar cooking works:

solar cooking

So you build a huge mirror array. You can build it to completely sourround the Sun, if you want to collect all the Sun's energy. All you need to do is leave a small hole in the direction of Earth for the sunlight that you want to continue to shine on our planet. The mirror array rotates around the Sun at the same speed as the Earth, so the small hole always faces Earth. The mirrors direct the sunlight to wherever you need energy, focussing all of the emitted energy of the Sun into one single tiny spot, giving you vast amounts of energy that you can either use directly as heat or transform into electricity.

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