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So I've got this civilization that doesn't have electricity, but does have municipal power distribution via compressed air lines.

Now, you can do a lot of useful things with compressed air. Even in the real world, compressed-air tools are fairly common. You can even use it for heating, via vortex tubes (not terribly efficient) or heat pumps (much more efficient), although suggestions for better ways of providing heat via compressed air would be appreciated.

But what I can't figure out is, is there any way practical way to produce light directly from compressed air power? Or are these people just going to be stuck using Victorian-style gas lamps, with separate municipal supplies for compressed air and natural gas?

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    $\begingroup$ This is a super interesting question. I am curious: how do they get the air compressed? $\endgroup$ – Willk Nov 18 '17 at 22:53
  • $\begingroup$ @Will Same ways we generate electrical power--windmill, watermill, or burning stuff to run a steam turbine. And I suppose they could even use solar concentrators to boil water to run a steam turbine, but that doesn't quite fit the aesthetic I have in mind. $\endgroup$ – Logan R. Kearsley Nov 18 '17 at 23:18
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    $\begingroup$ en.wikipedia.org/wiki/Sonoluminescence $\endgroup$ – Ville Niemi Nov 19 '17 at 1:52
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    $\begingroup$ Just to note that municipal power distribution networks using compressed air are a real historical thing. "As of 1896, the Paris system had 2.2 MW of generation distributed at 550 kPa in 50 km of air pipes for motors in light and heavy industry. Usage was measured by cubic meters" (Wikipedia). $\endgroup$ – AlexP Nov 19 '17 at 9:28
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    $\begingroup$ None, which is why I didn't answer. The set ups used are for tiny flashes of light and I have no idea if they can be scaled. Given that the reason for the light is still being discussed I am not sure anyone knows if it would scale up. I think it might, but I doubt it would be cheap to mass produce. $\endgroup$ – Ville Niemi Nov 20 '17 at 0:02
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Compression heats things up.

Compressed air gets hot. The compression of air in the cylinder of a Diesel engine is used to heat the fuel to ignition. The King of Random here made a clear acrylic "slam rod" fire starter. When he slams down the piston and compresses the gas, it gets hot enough to cause the piece of char cloth tinder he has in there to glow. It is really excellent.

https://www.youtube.com/watch?v=SkWJdWGdgaM slam rod firestarter by the King of Random

Compressed air piped in already got hot wherever the factory is, and so when you let it decompress it will get cold. But if you use it locally to compress room temperature atmospheric pressure air, that air will get hot.

In the screenshot, the glow is because the char cloth tinder in there is superheated by the hot compressed gas. It glows so hot it catches fire (and you will see in the video it is just then a little cinder) But it is cheating to just have light from burning stuff - that is a torch. How to use the heat of compression to make something glow without burning? Can we just keep it glowing like that?

If you heat something up in the absence of oxygen, it will glow but not burn. This is how incandescent light bulbs work: the filament inside is heated by an electrical current passing through it and it glows. Depicted: carbon filament incandescent bulb.

glowing carbon filament

Some incandescent bulbs are full of vacuum. Others are full of inert gas - like noble gases or nitrogen. The reason: without oxygen the filament will not burn up, but will just keep glowing.

What if there were a device like the King's slam rod that was repeatedly and rapidly compressed - basically imagine a clear 4 cylinder Diesel engine. The difference: the compressed gas does not contain oxygen. Nitrogen gets just as hot on compression but will not sustain combustion. A piece of carbon in the cylinder will not burn up. It will heat up and glow, just like when he hit the cylinder.

The lighting apparatus working on this principle will have multiple clear / durable cylinders in a row, driven by a compressed air powered drive shaft. The element on the piston in each is heated to glowing by the repeatedly compressed nitrogen. The gentle glow lights the room.

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  • $\begingroup$ Nice! It would need a very efficient heat exchanger to re-warm the cooled (and presumably recycled) exhaust gas, but I imagine that's solvable. On the downside, I imagine such a light fixture would be rather noisy.... $\endgroup$ – Logan R. Kearsley Nov 19 '17 at 0:15
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    $\begingroup$ It would definitely be noisy. That is part of its charm. $\endgroup$ – Willk Nov 19 '17 at 0:16
  • $\begingroup$ The compression temperature in a diesel engine is in the range of 500 C. An incandescent light bulb filament rolls more like 2500 C. Even if you could exhaust a compressed gas hot enough to glow, the problem is you would slag the compression chamber because there just aren't materials that can handle repeated pressure at that temperature. $\endgroup$ – kingledion Nov 19 '17 at 2:52
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    $\begingroup$ @kingledion - yes it would be problem, but the temperature doesn't have to be 2500 C. If you are prepared to accept a red light then 690 C should be fine hearth.com/talk/wiki/know-temperature-when-metal-glows-red also multiple cylinder could be rigged to provide hot gas in turn to a central incandescent tube. $\endgroup$ – Slarty Nov 19 '17 at 16:56
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    $\begingroup$ "the compressed gas does not contain oxygen" - so you use some gas contained in the lamp. To reuse it you will need to lower the pressure, and during this it would get cold. So you will need a big radiator to restore gas temperature. $\endgroup$ – Vashu Nov 19 '17 at 23:46
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enter image description here

Flint and steel mills introduced by Carlisle Spedding (1696-1755). A steel disk was rotated at high speed by a crank mechanism. Pressing a flint against the disk produced a shower of sparks and dim illumination. These mills were troublesome to use and were often worked by a boy, whose only task was to provide light for a group of miners.

They were used because inventor hoped that flint sparks would not ignite mine gas (he was wrong).

Use a small compressed air powered engine to rotate this and you would get light.

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  • $\begingroup$ (he was wrong) and how! $\endgroup$ – Draco18s Nov 20 '17 at 2:40
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I do not like the premise that light requires heat. Must it be so?

Consider neon light bulbs. neon bulbs from http://www.instructables.com/id/Static-discharge-toy/

A neon bulb contains low pressure neon and is lit by an electrical discharge across it. A static charge caused by persons shuffling their feet across dry carpet will suffice.

Yes, this society lacks electricity. I take this to mean they lack generators, dynamos, motors, electric lights and electrical transmission: current electricity.

Static electricity, however, is a different animal.
Thales of Miletus discovered and described the generation of static charges, which he produced by rubbing amber on fur - this in 585 BC. Here is a stamp celebrating Thales and his discovery. Thales of Miletus and static charged amber

So a light without heat: This society with experience using pressurized gas should not have trouble making a bulb which can contain low pressure neon (or other handy noble gas). A turning wheel powered by compressed air or at the site of the bulbs generates a static charge (an electrostatic generator), which is allowed to go to ground through the bulbs.

I see that a static electricity generator (like rubbing the bulb with a sweater) can also be used to light fluorescent bulbs: this too without current electricity and without an excess of heat.

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Light requires high temperatures

The problem with generating light is that you need really high temperatures to do so. The excited air that is light-emitting in a candle or campfire is around 1000 C. Light bulb filaments are in the range of 2500 C.

The issue with these kinds of temperatures is that they are hard to contain. For a fire there are obviously some precautions that must be taken to keep things that are flammable away form the fire. But neither torch nor candle nor lantern require any complex mechanisms for handling the 'fuel' the way that any sort of mechanism involving compressed air will.

1000 C is pretty hot. For example, stainless steels (except the very expensive types) are not rated to 1000 C for intermittent use. Surely there are materials that can handle such temperatures routinely, but they are expensive and hard to come by; commonplace metals simply can't handle repeated cycling at that temperature.

Grid distribution of compressed air has a high cost

You simply can't send compressed air that far without losing most of the pressure. When I imagine high pressure air, I am thinking in Navy terms with 3000 psi systems. This is equivalent to 20 MPA or aboout 200 atms. At this rate, with a 1" (25 mm) piping and 10 m$^3$/min of flow, you will lose about 0.04 psi to the meter. Run a kilometer of pipe and you lose 40 psi. That will quickly add up; and note that 1 " piping is huge; much larger than anything we see in the Navy.

I strongly doubt that a compressed air generating station would be able to provide compressed air to even a medium sized city. A big city would need dozens of stations to provide satisfactory pressure.

Conclusion

Using compressed air to run a factory seems legitimate. Residential distribution of compressed air does not seem practical, and using that to generate light has significant materials hurdles. Any civilization able to create the necessary materials should probably already have discovered electricity.

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  • $\begingroup$ maybe they're making use of some kind of plant that grows pneumatic tubes naturally? $\endgroup$ – Daniel Bensen Nov 19 '17 at 15:38
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    $\begingroup$ England had a few utility companies that sold compressed air for powering local machinery. It was at that point a viable alternative to an industrial plant owning and running their own steam engine. 20 minutes searching has failed to locate the link. Their main line was some 4" in diameter and several miles long. $\endgroup$ – Sherwood Botsford Nov 19 '17 at 16:24
  • $\begingroup$ Try red light instead hearth.com/talk/wiki/know-temperature-when-metal-glows-red $\endgroup$ – Slarty Nov 19 '17 at 16:57
  • $\begingroup$ The lighting bit aside, I figure the air grid is structured much like our electrical grid to minimize transmission loss. When the system is not actually being used, in the static case, it should be isobaric (modulo differences in altitude); you only get a drop when it's actually flowing, exactly analogous to electrical transmission. So you have a main plant that supplies extreme high pressure (/voltage) via high-capacity trunk lines to local substations. The substations have their own storage tanks and pressure regulators (/transformers) to smooth out the erratic loads in their service area. $\endgroup$ – Logan R. Kearsley Nov 19 '17 at 21:34
  • $\begingroup$ And then each house would have it's own smaller storage tank and regulator to smooth the loads transmitted back to the servicing substation. And if it's not being used for lighting, the total power that needs to be transmitted via compressed air would end up being much less than what we transmit by electricity anyway. $\endgroup$ – Logan R. Kearsley Nov 19 '17 at 21:36
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Well in looking for an answer I found this very applicable Wikipedia page:

https://en.wikipedia.org/wiki/List_of_light_sources

Some useful ones:

Biological or chemical reactions, which could be controlled by altering the supply of air into a sealed container. Basically something like a glow stick.

enter image description here

Triboluminescence - mechanical action breaking chemical bonds can release light, usually involving breaking crystals. Sugar responds this way, which is what makes biting winter green life savers in the dark spark. Similarly quartz crystal will do the same thing, the Ute Indians made leather shakers filled with quarts pieces that when shaken would show sparking visible at night. Most of these reactions are not very bright.

Argon Flash - Similar to heating causing things to emit light, heated noble gases, like Argon, will release a very intense light. Rapidly compressing it can be used to heat it, normally this is done with a small explosive charge. A well designed compression device could possibly do this repeatedly.

Any of the methods that produce a short intense light source could be made more useful by surrounding it with a material to absorb the light and reemit it over a longer period of time or a different wavelength, something like Florescence or Phosphorescence.

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There may be another possibility, albeit very complicated and inefficient: Gas dynamic Lasers. With the right gases, an expanding nozzle, and a resonant cavity and optics you can actually attain population inversion by the expansion of those gases. The efficiency is about 30%, (good for a laser actually), but the light obtained is typically in the infrared range for common gas mixtures like CO2 and nitrogen, so you ought to use that light to heat up a filament or excite some chemical substance or crystal to produce visible light, so there is likely going to be a loss of energy in this step too. Considering incandescent lightbulbs are averagely 5% efficient, maybe there is some way to make this system feasible.

https://en.wikipedia.org/wiki/Gas_dynamic_laser

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  • $\begingroup$ Physicists help: could it be possible to use the laser beam to excite a gas to plasma state and so create a sort of neon light? $\endgroup$ – Damiano A. Ferrario May 19 '18 at 15:12
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    $\begingroup$ If you are interested in asking a question, you should post it as a separate question. But make sure it is on topic! Ensure that you aren't asking a stright physics question, since there is a different site for that. Regarding your question, a fluorescent light uses electricity to excite a gas to a plasma state and creates a 'neon' light. Florescent lights work well; why use a laser? $\endgroup$ – kingledion May 19 '18 at 16:30
  • $\begingroup$ because gas Dynamic lasers don't necessarily use electricity. A burner could achieve the same effect or even a canister of compressed gas. they work by letting a gas espand into a nozzle that is the right dimensions and length and the gas passes bertween two mirrors to obtain resonance. Since we were talking about a no electricity civilization, this mechanism would create light from the expansion of a gas withouth using electricity. It could be used as a weapon as well. a misxture of co2, nitogen and helium would emit in the range of the infrared, for example. $\endgroup$ – Damiano A. Ferrario Sep 11 '18 at 17:03
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Here a "compressed-air-powered-bio-light" which may be possible but I don't think that anyone has done it yet.

  • The compressed air is used to pump water and make water pressure.
  • The water pressure is used on a semipermeable biomembrane.
  • The membrance contains ion channels which are powered by the ions following the pressed water through the membrane.
  • The ion channel produces some energy container (ATP?)
  • The energy container powers bioluminescence and there will be light.
  • Water and Ions may be "recycled".
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I like this question and I think I have a smart solution utilising the above mentioned flint mill. Imagine if you will a small glass jar around 6 inches in height and roughly 6 inches across, within it is a small paddle with steel fingers, let’s say there are 4 paddles in a cross pattern and each paddle has 6 fingers but are staggered so the gaps of one paddle are where the fingers are on the next one, alternating on each paddle. 2 strips of flint are in the jar on each side, perhaps glued to the side or mounted on some sort of cushioning to keep tension on the fingers when they are passed over it. The jar could be screwed into a socket on the ceiling which has a small turbine in the housing so as the air is pushed past the fins the steel paddles are spun against the 2 flint rods creating a shower of sparks contained within the glass jar, they could be easily changed by simply unscrewing the jar, they could be dimmed by spinning it slower and would create a neat way for people in your town to make an income by coming and emptying these jars of the blackened flint remnants. Admittedly they wouldn’t last long but they would be cheap to replace or they could be expensive to replace and a house with bright lights could be seen as wealthy across the city.

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The slam rod must contain some combustible material that starts the fire. The air generates heat only upon compression. All heat will dissipate through the pipes once compressed air is generated and transported. A possible way is that a compressor will compress air locally inside a metal pipe. The compression ratio must be high enough to heat the pipe until it glows like an incadescant lamp. There are two drawbacks:

1- The compressor runs on compressed air, so in reality it must compress air into a tube with a thin inner diameter. So, it uses lts source of air to power a lamp by compressing a smaller volume of air.

2- The tube must be sturdy enough to withstand high pressure even when hot, so it must have a thick wall...

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  • $\begingroup$ There is no particular need for the same material to provide both incandescence and structural support. You could have a thing metal or carbon mantle supported by a thick glass tube, for example. $\endgroup$ – Logan R. Kearsley Nov 19 '17 at 21:29

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