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I am working on designing an underground city for my Kepler Bb story. I am now at the civilization phase and there are 100 people(Not including Robin, Lisa, Alma, and possibly 1 or more little ones) in a 1:1 gender ratio under these constraints:

  • No younger than 15
  • Not pregnant
  • Doesn't have a child younger than 10
  • Not ill

Now to prevent inbreeding, all children should mate not with family but with unrelated people.

Anyway, when I thought of what should power everything in the city here is what I was thinking.

1) Since underground is relatively close to the water table, I don't want any pollutants.

2) I don't want electrolysis since that wastes water.

So I figured pure electric is what I need and Solar would fit the bill really well. I took into consideration along with those 2 statements above that the solar system Kepler Bb is in has 4 stars in a close double binary system and the planets orbit around all 4 stars and that their daytime length, even in the darkest of winters, is longer than ours. So that means, even if they have the same solar technology as us(which I doubt very seriously since people on the very high tech planet Kepler B# send stuff to the people on Kepler Bb who aren't high tech to help them through the process of forming a civilization. Eventually, the people on Kepler Bb will be able to produce Kepler B# technology and possibly improve upon it.

As for vehicles, those won't come until later, after a civilization is established so for now all traffic is foot and hand traffic. Why foot and hand and not just foot you might ask? Well the people on Kepler Bb are equally able to be quadrupedal and bipedal though most of the time they are bipedal. When they go through underground tunnels however, if it is mostly up and down they simply climb it like they would climb a tree. If it is quite a bit horizontal though they get into their quadrupedal position. Because of this, they have injury resistant back muscles so going through an underground tunnel would rarely cause them to pull a muscle. They might be sore though and that's normal.

But when vehicles start being used, I figured that there should be solar charging stations if the battery is low and spare batteries if they can't get to a charging station in time to recharge their batteries either because of urgency(like running out of time to get to a meeting) or because of distance and battery use at their speed.

But would this work? Can I use solar power alone to power a city(all the lights, all the electric equipment(like for example the interplanetary communicator), all the vehicles, etc.)? If yes than that's great but if not, what other renewable power sources would I need?

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You're going to need some sizable solar arrays, and to expand them as the population grows. The key problems, however, are bad weather and night. Solar power is greatly reduced during cloudy weather - much more than is immediately obvious to the eye, since human eyes compensate for it rather well - and absent at night.

The high-tech solution to this is solar power satellites, collecting power in orbit and beaming it down as microwaves to collection stations on the ground. This is a pretty big and expensive project, and may not be worthwhile for a small settlement.

The lower-tech solution is to have several times as much solar capacity as your peak demand, and store energy in some way. But storing electricity is awkward. There are a lot of ways to do it, but they all have problems. They include:

  1. Chemical storage in some kind of battery. For a settlement of 100 people, this will work fine for a few years, and it might well be the best method for starting up, but you need a battery production factory, to cope with population growth and the inevitable degradation of batteries.

  2. Flywheel storage. This has its biggest advantage if you need a lot of power in a short time. It has most of the same disadvantages as batteries.

  3. Pumped storage. This is used for large electricity grids at present, as it can have very large capacity. But again, its strong point is a lot of power quickly, not a lesser supply over a long period at night.

  4. Superconducting loops. This is great, if you have cheap room-temperature superconductors. If not, not.

There are a lot more methods listed in Wikipedia: I think you need to read up on them, make some choices, and consider how they impact your story. I've assumed you don't want to build and maintain planet-wide power transmission lines for such a small settlement.

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  • $\begingroup$ The third strategy is to have collectors spread so far apart that bad weather and night never cover all the collectors simultaneously. Quite hard to do but possibly viable on a world with really short nights. $\endgroup$ – SRM Sep 11 '16 at 16:16
  • $\begingroup$ Or the colony could just go dormant over night. Seal everyone in, minimize activity to conserve air, and just wait for sunrise. $\endgroup$ – SRM Sep 11 '16 at 16:18
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Most of your question is not directly useful, except for the information that population will grow, probably fast.

If your planet supports it, I would go with geothermal.

  1. It does not depend on day-night cycle
  2. It is clean
  3. Easier to scale

For storage, electrolysis and oxygen / hydrogen tanks makes sense. Don't forget it doesn't waste any water. You get your water back once you use up your energy. Then, of course, there is a risk of explosion... But modern batteries readily explode, too! Battery in your phone probably can explode as hard as first hand grenades, and you put it next to your head without even thinking about it, right? Oxygen/hydrogen is not worse in that regard, and it is perfectly safe for water. And fuel cells make "burning" it really efficient, and reclaiming water rather easy.

Solar arrays are more delicate than geothermal, and day-night cycle dependent. Using them as supplementary source makes sense, if you want them to supply power for industry, offices etc. Of course, inherent lack of stability will be a problem, but not as big if basic needs are fulfilled by more stable system.

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While I don't know the specifics of your planet, I assume there will still be many hours a day of darkness, caused by night and the weather. I also assume that the solar power reaching the surface of your planet is on average very high, that is, not too much worse than on Earth.

Solar power is (when you don't really care about cost and the like) trivial to use exclusively during sunshine. When cost doesn't matter too much, then for times of reduced (but still existing) sunshine you can just build a larger power station, which outputs more than you need at full power, so that at 3/4 or even half its full power output, it's still enough. But for the night and really dark weather you'll need something more:

  • Just give them awesome (chemical) battery technology. We can't do it, but there is no reason it would be impossible (especially for such a small settlement. At the beginning, human technology would even be enough). You don't really need to get into specifics for your story, if you want to be realistic and have a science stuff, read a little about battery chemistry and choose some components which might work in theory.
  • Use electrolysis. You said it wastes water. But really, you don't need too much water for it. Unless you have serious water shortages, it's absolutely no problem
  • Build solar arrays on multiple sides of the planet and connect them using extremely high voltage lines to keep losses small.
  • If geography is suitable for it, you can use artificial seas at elevated levels to store electrical energy in the gravitational potential of the elevated water. Uses much, much, much more water than electrolysis. But is really easy to do, efficient too.
  • If you've got air tight caves or caves you can make airtight, then you can pump gas in there under pressure and release it through turbines.
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  • $\begingroup$ Electrolysis does not waste water. To store electricity you split water into hydrogen (stored) and oxygen (added to air). To use electricity you burn or otherwise recombine the hydrogen and some oxygen (from the air) and get back the same amount of water that you started with. Atoms are conserved by electrochemistry. $\endgroup$ – nigel222 Sep 12 '16 at 13:16
  • $\begingroup$ @nigel222 With the same argument, pumped-storage hydroelectricity doesn't waste any water either. Yet ordinarily you would say a single one uses billions of liters daily. $\endgroup$ – Nobody Sep 12 '16 at 13:38

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