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If a spacecraft is going to use solar power as its source of electricity (either for propulsion or just components or both) and it's also going to travel far out into the solar system, to quite high AU, it's going to need larger and more powerful solar arrays to generate the required power.

What's the best way of folding or somehow condensing an array of a large surface area into a small volume? I know some can operate like blinds but others work like extendable fans. Is there an outright most efficient way to do this?

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    $\begingroup$ Fancy folding of solar arrays and other large structures is an ongoing area of research - agencies are actively looking for "more efficient" ways to deploy them. $\endgroup$ – Bear Dec 8 '16 at 13:36
  • $\begingroup$ Somewhat related was a recent question about solar power on probes going to the outer solar system. Basically, it won't work as there's not enough energy. space.stackexchange.com/questions/19106/… $\endgroup$ – GdD Dec 8 '16 at 14:07
  • $\begingroup$ The arrays are usually deployed fully after launch, and incoming power is either reduced by pointing the arrays at an angle to the Sun or the excess power is dissipated as heat. Array deployment (being a mechanical action) is something you want to do as little as possible, to avoid failures. $\endgroup$ – Hobbes Dec 8 '16 at 14:08
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    $\begingroup$ You should also note there is a tradeoff between folding efficiency and deployment points of failure. For example, TV-SAT 1's solar panels failed to deploy, resulting in mission failure. $\endgroup$ – called2voyage Dec 8 '16 at 14:12
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There is another way to get more power, not to increase the area by folding but to stack several cells for different wavelengths and improving the conversion efficiency. Dawn uses triple-junction cells, see https://en.wikipedia.org/wiki/Multi-junction_solar_cell But the best efficiency was achieved with laboratory examples using concentrated sunlight. Concentrating sunlight is possible on earth, but not in a spacecraft. Hopefully multi-junction cells are developed which work well even far away from the sun.

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    $\begingroup$ Why can't concentrators be used on spacecraft? $\endgroup$ – Organic Marble Dec 13 '16 at 17:09
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    $\begingroup$ Concentrators are necessarily big, possibly heavy. Solar cells work best if you can keep them cool. Using concentrators on earth, you can allow atmospheric convection to carry waste heat away. You don't have that luxury in space. I'm guessing that, pound-for-pound, it's more effective to just put more solar cells on a spacecraft than it is to use your mass budget for a concentrator and whatever thermal mitigation you need in addition. $\endgroup$ – Tristan Dec 13 '16 at 21:20
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    $\begingroup$ If a solar panel is not perfectly aligned to the sun, the output power is slightly reduced. But with a concentrator, a small direction error will lead to a larger reduction of power and even to a total loss of power. If the concentration ratio is large, the sensitivity for a misalignment is also large. $\endgroup$ – Uwe Dec 15 '16 at 9:52
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    $\begingroup$ There is no reason in principle why concentrators shouldn't be used - see 15 years ago: space.skyrocket.de/doc_sat/hs-702.htm - the problems of that implementation aren't necessarily the end of it, it just needs more time and investment. I wouldn't be surprised if they got a bad reputation the last time around and that this has made it harder to get funding since. $\endgroup$ – Puffin Dec 15 '16 at 10:44
  • $\begingroup$ @Tristan atmospheric or aqueous The SMAP unfolding antenna for example would probably be even lighter if it didn't have to be constantly spinning. A solar reflecting surface could be the same or similar metallized foil designed and tested for solar sail applications, The PV array couldn't be too small due to the heating constraints you mention, and to allow for some pointing error, maybe 5% or 10% of the collector diameter, so a somewhat imperfect shape could be tolerable... $\endgroup$ – uhoh Jan 14 '17 at 7:02

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