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I'm experimenting with a sci-fi setting where human-like mechanical androids are the dominant "lifeform." Though the setting is based on science, androids live in tribal societies. For the most part, these societies are devoid from other pierces of technology - there are no high-tech vehicles, weapons, or power stations. As such, I need a way for androids to power themselves without the use of batteries or power stations like hydroelectric or nuclear power plants.

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    $\begingroup$ You need to define "android" rather more clearly here. Are they biological, or electronic/mechanical? $\endgroup$ Feb 16 at 13:10
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    $\begingroup$ Also please clarify what is a power station for you, because a power producing unit is a power station. $\endgroup$
    – L.Dutch
    Feb 16 at 13:45
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    $\begingroup$ You would need to be specific about the setting (day length, light intensity if you want suggestions relating to solar energy), technology level (do they have fusion), power requirements (if they only need 5 watt hours per day, or 5 GW hours). Please edit to provide parameters for us to fit in with - else the question is too unrestrained. $\endgroup$ Feb 16 at 14:13
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    $\begingroup$ I would say that an android is by definition a very high-tech vehicle... (It can carry stuff, so it's a vehicle; and is very high-tech.) $\endgroup$
    – AlexP
    Feb 16 at 14:19
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    $\begingroup$ You say 'dominant lifeform' without specifying if there are, indeed, other plants and animals. Is photosynthesis, for example, still done by plants? All hydrocarbon fuels today inherently owe their existence to carbon-based life forms. In fact, if life is exclusive to Earth, no other planet in the universe has oil and coal deposits. This begs the question be asked, 'What powers the android manufacturing process?" $\endgroup$ Feb 16 at 15:54
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Futurama has a good suggestion:

Bender requesting alcohol

Alcohol fuel cells are what you need. Grow suitable crops to provide carbohydrate feedstocks, then ferment and distill to produce nice, relatively purely fuel.

Distillation can probably be done with bronze-age technology, certainly possible (but perhaps only in a limited way) with iron-age level knowledge and was clearly done by medieval societies so you have plenty of leeway in the sorts of society your androids are emulating. The resulting fuel is quite energy-dense, relatively stable and storeable, and the fuel cells can certainly be made to be powerful enough to make a human-sized thing capable of doing human-equivalent acts of physical exertion. Internal fuel tanks can tide the androids over between meals.

The nice thing is that this kind of power source makes good sense for a fairly high-tech society of the sort that would have produced your androids in the first place, removing the need for additional handwaving in the backstory.

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    $\begingroup$ A brief addendum: the link is to methanol (wood alcohol) fuel cells but ethanol (grain alcohol fuel cells exist as well: en.wikipedia.org/wiki/Direct-ethanol_fuel_cell $\endgroup$ Feb 16 at 14:47
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    $\begingroup$ +1 for this. Humorously Bender gets a hangover whenever he is sober :D $\endgroup$ Feb 16 at 14:54
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    $\begingroup$ @GrumpyYoungMan yeah, I was too lazy to include all the details ;-) ethanol is probably more desirable... its a bit easier to store, and easier to produce via biosynthesis, too. $\endgroup$ Feb 16 at 14:56
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    $\begingroup$ And in case androids ever meet humans, ethanol makes it easier to make friends. $\endgroup$ Feb 16 at 15:57
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    $\begingroup$ @TheSquare-CubeLaw But methanol is easier to subdue your would-be competitors when you invite them to dinner. $\endgroup$
    – DKNguyen
    Feb 16 at 22:51
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Food

To simplify it to an uncomfortable degree: you need material to grow or maintain the androids and energy for movement.

There are differences between humans and robots in how they gather and process both of these. For robots it's generally simple. Your parts are refined and placed/replaced according to your needs. Energy can come in the form of electricity or chemical bonds, like diesel, among other methods. A human is more complex. We eat food that is broken down and processed, stored and used for both (in addition to oxygen).

Ultimately this means that food can be a power source for any movement, as well as building material. I don't know how your androids function, but a few meals a day could power them and potentially help them grow/maintain themselves. You can enlarge/change their diet, so they can process the required materials for their android forms. You can do this in many ways you want. Examples can be it is transported and used much like blood, or maybe they have a small advanced 3D printer inside them to create the parts required, which can be used by the android or someone else for construction/maintenance.

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Solar

If you make some of them solar-powered, you can add a twist: droids which are active only by day. They can be made to hibernate for an indefinite amount of time by being pushed into a hole or cave, and going under foliage makes them slow.

Radioactivity

There is a kind of power source called Radioisotope thermoelectric generator, or RTG for short. It is used in satellites and space probes - all the five probes currently leaving the solar system are powered by it.

RTG's extract energy from the decay of certain radioactive elements. In the wiki article it mentions some isotopes with half lives over 400-years. It may be that long ago humanity left a lot of RTG powered droids around, and maybe also many extra RTG's for spare parts that the droids can use to build new ones. Over centuries though this would stop working, so the droids might be in a quest to find new power sources soon.

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    $\begingroup$ Without external solar cells, the former might imply a society that spends most of its time standing around naked on hilltops charging up, with occasional moments of other activity when their internal batteries are full. RTGs are also problematic, mostly due to their poor power density. An RTG that lasts for hundreds of years and fits within a human-sized body will have a depressingly low power output. $\endgroup$ Feb 16 at 15:22
  • $\begingroup$ @StarfishPrime I agree. Both ideas would make for a very slow society. $\endgroup$ Feb 16 at 15:23
  • $\begingroup$ More xenarthroid than android ;-) $\endgroup$ Feb 16 at 15:25
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Oxidation reactions.

Your androids are living without other tech; you explicitly rule out power stations etc. They are living in tribes like humans.

They could power themselves with chemical energy. We do that. We eat reduced carbon molecules that can be oxidized to CO2 to release energy and then couple that energy release to charging up ATP which is biologically available energy.

It is not very androidy to eat blueberries and pork. You could obtain the chemical energy through a different oxidation reaction. The oxidation of metallic iron (or aluminum or titanium) also releases energy. Lots of bacteria oxidize reduced iron for their energy. Your androids could eat reduced metals and oxidize them within their bodies, powering their internal functions.


I am not sure about the greater picture for your androids living in tribes with no tech but it seems like at one time they were part of something more technologically advanced, and have fallen to their current state. Maybe there are ruined devices around ed to be, and maybe they themselves remember.

Long ago these androids had proper fuel. Now they break little pieces off of the old machines and eat them. The androids are cannibalizing their past. It might help your story.


It is worth pointing out that the androids themselves have reduced metal titanium / aluminum skeletons. In isolated groups, some individuals might have skeletal components now replaced by wood.

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Hydrogen Fuel Cells

Basically, hydrogen fuel cells produce power constantly as long as there is oxygen and hydrogen to sustain it. Oxygen can just be extracted from the atmosphere, while the hydrogen can be purified from ingested H2O. As a result, as long as they have air to breath and water to drink, these robots can sustain themselves.

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  • $\begingroup$ The energy required to hydrolyse water into hydrogen and oxygen is always going to be less than the energy produced by turning it back into water in a fuel cell. The absolute best you could do in an ideal universe is exactly equal, and the Laws of Thermodynamics Do Not Work That Way. $\endgroup$
    – jdunlop
    Mar 10 at 23:49
  • $\begingroup$ Perhaps soar panels can be used a supplement? $\endgroup$ Mar 11 at 0:33
  • $\begingroup$ No point in supplementing it. If you're generating energy for electrolysis, you may as well use the energy directly to power the android. The only reason you use other energy sources to lyse water is if you have so much solar energy you can't use it all and have to store some for later. $\endgroup$
    – jdunlop
    Mar 11 at 1:02

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