11
$\begingroup$

I am working on some sort of space colony many metres below the ground or underwater. Is there any way that I can work this idea toward using natural sunlight instead of artificial light source?

When it comes about habitat specifics -- to not reinvent the wheel or try to open an already opened door -- I want to take the best parts of Gateway Spaceport, Nautilus-X, Stanford torus etc. designs. Only put it 400-600 or more meters below the ground, adjusting all the issues around gravitation (and so: no need to be ring-like, no need to rotate etc.).

Many of these designs assumes huge LED-based (or not) artificial source of light in the centre of gravity, which I also want to adjust to my idea.

But, also some of these designs are based around huge mirrors idea or other means of providing natural sunlight to the colony. Is there any way I can adapt this (or any other) design to provide the same many metres below ground, seabed or underwater?

Would (current design or modified) optical fiber allow me to achieve this? Can I build some kind of sun-towers or sun-tunnels that woud stick above the ground, catch sunlight and then -- using any kind of idea of mirror tunnels etc. -- direct it to the underground or underwater colony?

Can I capture and transport natural, not artificial sunlight by any means at any distance? If yes, would would be maximum possible distance on which I can achieve this?

Or maybe I am dreaming and due to nature of sunlight, mirrors are due to any other reason, when putting humankind below the ground, seabed or underwater I am -- by design -- 100% enclosed to artificial sources of light?

Please, note that I am asking about transporting natural light at any distance (if possible) only. I am not talking about ideas or concepts (that we already know) of catching sunlight, turning it into electricity and then using that electricity to power artificial light sources.

$\endgroup$
11
  • 2
    $\begingroup$ Some oceans have clearer water than others. This is good for scuba divers, but the absence of phytoplankton suggests the sea is not healthy. I don't think anywhere on Earth has water clear enough for sunlight to penetrate a hundred meters. Are we on Earth? If we are on some water world where the oceans are many kilometres deep, the water at the surface will be pure as the heavier elements sink. Pure water is very clear. But in that case, why not be on the surface? $\endgroup$ Commented May 6 at 8:07
  • 2
    $\begingroup$ Of course you can transport light over some distance using some sort of light conductors. (The more expensive the light conducting solution, the longer the distance.) The main problem is capture area, because one square meter of capture area can only capture the sunlight which falls on that square meter. So that in the end, it all depends on how much an area you want to illuminate and for what purpose; and the question does not say. $\endgroup$
    – AlexP
    Commented May 6 at 8:55
  • 2
    $\begingroup$ You know there's a mirror on the moon for distancing measurements..... $\endgroup$ Commented May 6 at 9:32
  • 1
    $\begingroup$ @Pelinore BTW: If I would like to get into any technical discussion with you, I'd say that tiny fiber with is mm diameter has nothing to do with lightning huge areas which are disputed in my question. And no reasonable person ever thought seriously about using optical fivers as a source of natural light. Sadly, your behaviour causes that I don't want to dive into any kind of discussion with you. Would you be so kind to NOT take any more part it this discussion and generally speaking... go somewhere else? Of course assuming that they do know "nice" at your planet. $\endgroup$
    – trejder
    Commented May 6 at 11:51
  • 3
    $\begingroup$ @Pelinore: I know enough engineering to know that something that works for low-intensity at mm scale may not work for high-intensity at km scale. I do not know enough about the various kinds of fiber optics to know whether they're capable of producing the desired effects, and what the consideration should be around that. For example, with high-intensity focused in a beam of fiber optics... would you have heating issues? Like fibers melting down unless they're actively cooled? I don't know. A good answer could showcase real world solutions, and the scale at which they operate. $\endgroup$ Commented May 7 at 9:37

5 Answers 5

21
$\begingroup$

Solar fiber optic collectors. enter image description here

They consist of fresnel lens which focus some light against a fiber optic bundle and convey it considerable distance. Light in rooms without windows. Even basements for multistory buildings. enter image description hereenter image description here

$\endgroup$
5
  • 4
    $\begingroup$ Is any distance given? There's quite a difference between 60m (which a manufacturer could already consider long-distance) and 600m. And since any loss probably ends up as heat... would those things need cooling down? $\endgroup$ Commented May 7 at 9:31
  • 8
    $\begingroup$ fibre optics are used for long-range telecommunication these days, so distance isn't an issue. Most of the loss is at the input and output pieces. The main issue is focussing the sunlight into the cable because it has a limited angle where it'll be transmitted. $\endgroup$
    – Tom
    Commented May 7 at 13:22
  • $\begingroup$ @Tom You can get really good internal reflection for very narrow frequency bands, but I don't think there's anything ≈100% for the whole visible spectrum. $\endgroup$
    – wizzwizz4
    Commented May 8 at 17:39
  • $\begingroup$ @Tom, given that today's fiber optic grade glass is able to achieve attenuations rates ~2dB / km. We are talking about an approximately 25% losses. That isn't bad. But optical fiber grade glass is really expensive... $\endgroup$
    – Aron
    Commented May 9 at 4:11
  • 3
    $\begingroup$ You can't transmit sunlight through telecom fibers, those 2dB/km losses are for narrow-band deep IR wavelengths. For sunlight, you'd need something like this thorlabs.com/newgrouppage9.cfm?objectgroup_id=362 , which is at least 100 dB/km at 400nm, so the light that gets down will be very red and probably not useable for either regulating circadian rhythms or photosynthesis $\endgroup$ Commented May 9 at 18:11
21
$\begingroup$

In real life there are devices, called solar pipes, which are used to bring light inside buildings without using windows. [DISCLAIMER: I am not affiliated in any way with the company producing them]

These devices boil down to a sun exposed end closed by a transparent optical element, a reflecting pipe and an open end into the environment to be shone: the solar light captured by the optical element bounces in the pipe and shines at the other end.

enter image description here

enter image description here

In real life these devices have a maximum advised length of 15 m: even with a good reflecting inner pipe, after too many reflections too much light is lost. Consider that after n reflections on a surface with 90% reflectivity, the available light is $0.9^n$.

I can imagine that economic considerations would be different in your scenario, prompting the use of higher reflectance pipe, which would stretch the length a bit further, to 100 meter and beyond.

This table tabulates how much light is available after n reflection on a surface with different reflectivity.

n 90.0% 99.0% 99.9%
1 90.0% 99.0% 99.9%
2 81.0% 98.0% 99.8%
3 72.9% 97.0% 99.7%
4 65.6% 96.1% 99.6%
5 59.0% 95.1% 99.5%
6 53.1% 94.1% 99.4%
7 47.8% 93.2% 99.3%
8 43.0% 92.3% 99.2%
9 38.7% 91.4% 99.1%
10 34.9% 90.4% 99.0%
$\endgroup$
10
  • 2
    $\begingroup$ Hm, I wonder if installing a bundle of fiber optic cables inside (for total internal reflection) would not overcome the issue of length...? Probably at the cost of reduced light captured at the upper opening... $\endgroup$
    – Qami
    Commented May 6 at 18:39
  • 3
    $\begingroup$ @Qami: Fiber optic cables have the same issue, to an extent. You also need repeaters to strengthen the signal after a certain distance. It may work, it may not scale to the amount of light needed, I don't know... $\endgroup$ Commented May 7 at 9:28
  • 3
    $\begingroup$ Would it be possible to reduce the number of internal reflections by better guidance at the top and a mostly straight conduit? $\endgroup$ Commented May 7 at 9:38
  • 3
    $\begingroup$ @MatthieuM. : With "better guidance" being a movable mirror, yes. The problem is that a low angle of incidence causes the beam to bounce dozens of times from side to side. You solve that by a mirror at 45 degrees, reflecting the sunlight straight down. $\endgroup$
    – MSalters
    Commented May 7 at 12:35
  • 3
    $\begingroup$ A much larger diameter pipe would reduce the number of reflections for a given depth. Build the settlement around the pipe rather than squeezing a narrow pipe into an existing building or architectural design. $\endgroup$
    – nigel222
    Commented May 8 at 9:10
10
$\begingroup$

Heliostat mounted mirrors.

Normally used with solar furnaces (or telescopes), these are used to track the Sun and direct the light along a specific path without intermediary reflections needed.

In your case, the mirror would track the Sun, the light would shine down a shaft, then be reflected upwards in your underground caverns to the ceiling or directly where it's needed.

This would cut down the total number of reflections needed before the light got to where it's useful, thus boosting efficiency.

This can be done with multiple shafts to give the desired light levels and cover the required area.

Needless to say, the apparatus would need electronics, sensors and actuators to do it's job, or in the case of a primitive society (or an emergency) the cranking could be done by hand - either at the mirror's mount or remotely from way below.

$\endgroup$
3
  • $\begingroup$ Was just about to write something similar. With "smart" materials that could change shape into a parabolic mirror the light could be collimated, further improving efficiency. A large orbital mirror of smart material would work too. The key point being aiming the light where you want it to go, not just trying to collect it from any odd angle $\endgroup$ Commented May 6 at 23:40
  • $\begingroup$ There are (surface) precedents: Viganella in Italy and Rjukan in Norway use heliostatic mirrors to reflect sunlight into towns which are deep in valleys $\endgroup$
    – Chris H
    Commented May 9 at 15:14
  • $\begingroup$ @thegreatemu rather than going for collimation, which would require curved surfaces, a segmented concentrator mirror with moving segments could direct plenty of light into a shaft. But the light that goes straight down without touching the sides (many times) would be the most useful. A primary and secondary mirror rather like a reflecting telescope might be an effective way to engineer this, with the secondary directly on top of the shaft $\endgroup$
    – Chris H
    Commented May 9 at 15:18
7
$\begingroup$

Optical fiber can transmit light for several kilometers losing only a fraction of the optical energy passed into them (30% at 10km for some types of fiber, according to Wikipedia). It should just be a matter of creating, installing, and maintaining a sufficient volume of fibers to transport sufficient light for your needs.

You'll probably also need light collectors with a much larger surface area than your total fiber cross-section, to gather a useful amount of light. And then you're talking about concentrating sunglight to an unnaturally high intensity, which has implications e.g. overheating your fibers. Or you could just use more fibers, but that would probably be more difficult to manage.

$\endgroup$
3
$\begingroup$

The question is, WHY is the habitat underground? In most cases, this is done to shield it from deadly radiation, too much heat, or too little heat.

  • if the planet your habitat is on gets too much heat or radiation, then your habitat needs to be very deep, and thus your solar pipes long. They also need some way to filter out harmless sunlight from deadly radiation/heat that you do not want to pipe down. Decide how deadly the surface radiation is, because as some level the whole idea of solar pipes would be more of a problem than a benefit.
  • if the habitat is underground because the surface is too cold, then it very likely receives little to no sunlight, which means you have nothing to pipe down.

The fundamental problem is, sunlight comes entangled with several other forms of radiation, some of them useful, most of them damaging or deadly. If you are hiding from the radiation, you are effectively hiding from sunlight as well. If your planet gets a lot of sunlight but no deadly levels of heat and other radiation, why are you hiding underground?

$\endgroup$
3
  • $\begingroup$ You answer provides valuable arguments to consider, but (in my case) in some other story (thus upvote only). In this story habitat is underground, because it is not habitat at all, but rather an underground office / data center. Their owners want to stay there for security and secrecy reasons and to avoid traffic jams. $\endgroup$
    – trejder
    Commented May 8 at 15:45
  • $\begingroup$ Radiation protection only requires a few meters or so of dirt/stone IIRC, and either way none can get into the fiber optic, because its either a particle or a gamma ray that doesn't reflect. So no, sunlight doesn't get "entangled" with other forms of radiation (besides maybe UV, though that can get filtered out with normal glass). $\endgroup$
    – Seggan
    Commented May 9 at 13:42
  • $\begingroup$ Let's say there's too much infrared radiation on the surface - too hot: dichroic mirrors can be used to deal with that today, either reflecting it away, or not reflecting it towards your light shaft. Similar coatings can be already applied to glass on a scale of several metres (UV is similarly easy). And a cold planet could get plenty of sunlight and dangerous quantities of UV by not having an atmosphere. However that could be dealt with by shallow digging. Deeper could access geothermal heat if the surface was really cold. $\endgroup$
    – Chris H
    Commented May 9 at 15:28

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .