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A Kelvin water dropper generates DC electricity with no solid moving parts directly from a stream of water--essentially, it converts the gravitational potential energy of water into electrical energy.

Given a source of pneumatic (compressed air) power rather than water power, are similar kinds of devices feasible? I.e., arrangements of conductors and insulators through which a current or air (rather than water) can be passed to convert air pressure into electrical energy?

Trivially, this could be done by, e.g., using a pneumatic motor to run a van de Graaf generator, but something more similar to a Kelvin water dropper, with no moving parts that could wear out or require maintenance, would be ideal.

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    $\begingroup$ Correct me if I'm wrong, but I think the moving fluid has to be conductive. $\endgroup$
    – Ryan_L
    Commented Mar 16, 2020 at 18:32
  • $\begingroup$ Static electricity generators cannot generate useful amounts of electric power. The voltage is impressive, but the charge and stored energy are minuscule. $\endgroup$
    – AlexP
    Commented Mar 16, 2020 at 20:04
  • $\begingroup$ @Ryan_L Yes, it does--and furthermore, it has to break into drops. Hence, an actual Kelvin water dropper will not work with air. Hence, I'm asking about alternatives. $\endgroup$
    – Logan R. Kearsley
    Commented Mar 16, 2020 at 21:29

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There is no way to do anything efficiently with compressed air.

Compressing gasses is one of the most irreversible forms of energy storage you could name. When you compress air to, say, 10 atm., it gets almost hot enough in the cylinder to ignite a fuel (diesel engines typically run between 14:1 and 20:1 compression). That hot air then loses heat into the compressor parts and tank, until it's back to ambient temperature.

Then, when you let it expand again (say, though a piston engine like a steam engine, or a vane motor or turbine) it gets very cold, cold enough to form ice if the flow rate is high enough.

Beyond that, you squeeze water out of the air as you compress it, and this water condenses inside the storage tank when the air cools, with the result that a compressor tank fills with water over multiple fill/drain cycles -- but if the tank is kept drained, the discharged air will get even drier as it expands and cools.

As noted in comments, for a Kelvin Thunderstorm Machine, you need a conductive working fluid, but if you have a way to bias the air flow with high voltage ion emitters, it ought to be possible to build something that works more or less the same way (except it probably can't be made to self-start; you have to have a piece of glass rod and a silk cloth to produce a starter charge). Air is conductive, at least for ionic flow at high voltages. If you can once get a charge on your biasing points/rings, you should be able to separate charge with ion flow induced by the movement of discharge air.

Will this be efficient? Absolutely not. The most efficient way to use compressed air to generate static electricity is to use a small air motor to run either a Van de Graff or friction-free Wimshurst machine. And this is none too efficient; steam would do a far better job here, if you have a way to boil water.

The one exception here is if your compressed air is free. Sure, There Ain't No Such Thing As A Free Lunch, but if you have a significantly hydraulic head available, you can use the power of falling water to directly compress air with a device called a "trompe" -- this works by entraining air into water flowing downward, when routing the water though a chamber where the flow slows enough for the air bubbles to separate. The head above the trompe can be close to that of the infall -- so if you have fifty meters of drop, you can get close to five atmospheres of air pressure, and once the trompe is built, it operates without further intervention. Draw too much air, and you'll get water in your air pipes, and that's really the only significant problem with the trompe (the compression and decompression dry the air, despite having been bubbled through water during compression).

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  • $\begingroup$ For my purposes, compressed air is effectively free at point of use; I am developing a setting in which compressed-air-as-a-utility, initially developed for machine shops, actually expands to the level that electrification did in the real world. $\endgroup$
    – Logan R. Kearsley
    Commented Mar 16, 2020 at 21:33
  • $\begingroup$ That still leaves you with the problems laid out by @Zeiss Ikon in their answer, though - compressed air is a hideously inefficient mechanism for power distribution prone to all sorts of problems on creation and expenditure - centralizing like electrical production just compounds those inefficiencies. I suppose it's not as important at the point of use, if that's what you care about, but using it like electrification seems extremely implausible. $\endgroup$
    – jdunlop
    Commented Mar 16, 2020 at 23:34
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    $\begingroup$ @Logan R. Kearsley: In addition to the inherent inefficiency, compressed air systems as used in e.g. auto repair shops tend to leak a good bit. $\endgroup$
    – jamesqf
    Commented Mar 17, 2020 at 6:47
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    $\begingroup$ Central compressed air does work (look at an Amish workshop, where electricity is forbidden -- they get their work done with air power, using a single, central compressor, for tasks where muscle alone won't fill the bill), but it would be much more efficient, in terms of energy, to use a neighborhood line shaft than to distribute compressed air. Losses in bearings and belts would be much less than compression losses, though maintenance might cost more. Note that machine shops in our world used line shafts as late as WWII, even when electricity was widely available. $\endgroup$
    – Zeiss Ikon
    Commented Mar 17, 2020 at 11:20
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The only way that I see to extract electricity directly from flowing air without moving parts is by magneto-hydrodynamic separation of charges (separation of charges by the Lorenz force in a magnetic field)

To get to this, one would need to make the air conductive and the ionization ratio should be (much) higher than the charges one expect to collect. Which means a quite charge-dense plasma. Theoretically feasible with a very high voltage in a corona discharge, so that cascade ionization is what multiplies the initial charge, but I feel is very hard to get an efficiency high enough to even make the process self-sustainable.

Alternatively, use the air pressure to put somehow a conductive liquid in motion and fall back on a typical MHD case.

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You can implement your Kelvin air-dropper or air-squirter if you can ionize the gases, first. Radioactivity can ionize the gases as they exit the first rings.

Everyday objects such as smoke detectors use radioactive sources — Americium — to make the air charged to sense particulates indicating smoke.

Your air-squirter will operate until the source decays too much to induce ionization.

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Old steam locomotives were run by steam engines. High-pressure steam was produced in a big boiler and directed through the steam engine. The same type of engine can be supplied with compressed air and be used to drive the electric generator.

As Zeiss Ikon commented, steam engines are not compatible with compressed air. Therefore the solution is pneumatic (compressed air) engine.

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    $\begingroup$ When running on air, the cutoff needs to be managed very differently from running the same engine on steam. Air doesn't have heat energy in addition to its pressure, the way steam does, so you can't expand it anything like as much. $\endgroup$
    – Zeiss Ikon
    Commented Mar 16, 2020 at 19:25
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Researchers team up with architects to create bladeless wind electricity generator

It takes advantage of wind moving over the inner grid—positively charged water particles are carried (via spraying) away from the small inner tube mesh, leaving negatively charged particles behind. That causes a decrease in the voltage of the system and allows for electricity to be captured.

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