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I read an experiment which demonstrated that metal filings fired at a surface using air blasts charges the particles, surface and airstream. I understand this is triboelectricity, resulting from friction.

For reference: https://royalsocietypublishing.org/doi/pdf/10.1098/rspa.1929.0004

Experiments show that the charge increases if

  • airstream is faster
  • airstream is hotter
  • particles are finer (greater surface area)
  • particles are shot at surface obliquely

What if we use concentrated sunlight to heat air (e.g. to 300° C) and blow that air at high speeds to fire fine sand on some surface and continuously do so to generate electricity? Why isn't this feasible? Is there some physical limit that comes into play which makes this idea irrelevant?

EDIT 1: I'm frustrated as to why someone is assuming I meant to make some perpetual mumbo jumbo machine. It's not! This is a conversion of heat (in solar) to electricity. The device being so simple, I think it should be useful for charging cell phones and such, with lack of efficiency being a non-issue, since solar is mostly wasted anyway!

I am using the concentrated sunlight to generate a temperature (and thus, pressure) gradient in the setup, so that airflow is generated. There is no "pump" in my modified system. I am trying to heat air to generate "chimney" effect airflow.

Note: Generally I don't like talk about efficiency when it comes to simple easy to make, use and maintain devices like this. Unless it's too low, don't bring it up.

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    $\begingroup$ "Generally I don't like talk about efficiency ..." That's sticking your head in the sand. The exercise is pointless if it consumes more energy than it produces. $\endgroup$
    – Transistor
    Nov 12, 2023 at 11:35
  • $\begingroup$ @Transistor I fail to understand. Thermodynamics laws imply total useful energy from a system is ALWAYS less than or equal to energy in? If it produces more energy than it consumes, that would be perpetual?! All I want is a way to produce electricity. I know energy out is less than energy in. Do the industrial steam turbines produce more energy than required to run them??? $\endgroup$
    – El Flea
    Nov 12, 2023 at 14:34
  • $\begingroup$ @Transistor Along with the above comment, do check the edit. $\endgroup$
    – El Flea
    Nov 12, 2023 at 15:22
  • $\begingroup$ The edit makes your question and intention much clearer. $\endgroup$
    – Transistor
    Nov 12, 2023 at 16:53
  • $\begingroup$ Why is most of solar wasted? Sunlight warms the oceans - driving the currents through the oceans. Also sunlight causes the vegetation to grow driving our food chain. I might possibly agree that sunlight on the Atacama desert is wasted since nothing lives there but that heat is probably driving some atmospheric movement. $\endgroup$
    – Solar Mike
    Nov 12, 2023 at 17:04

2 Answers 2

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This seems to be your question:

What if we use concentrated sunlight to heat air (e.g. to 300° C) and blow that air at high speeds to fire fine sand on some surface and continuously do so to generate electricity? Why isn't this feasible?

It isn't feasible because you have to generate power to blow this air around to capture some tiny electric charge potential. You are converting energy too many times to be useful: solar to heat, solar to pressurized/moving air, sand charges to electricity. You may not like talking about efficiency, but efficiency is what makes things feasible. A machine that mostly heats up air and sand can certainly be built, but these things have to make money. Make a statement to investors that you don't care about efficiency and they'll throw you out of the room.

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  • $\begingroup$ But IS the charge potential tiny? Increasing the surface area of particles and surface will increase efficiency to a good extent, don't you think? How do you determine it IS tiny, without experiment? Not challenging you, but I'm curious how you conclude that. If the contact surface area is large enough for given mass of particles, I don't think the number of conversions and efficiency is much worse than the typical heat energy in steam to mechanical rotation in turbine to electrical energy in motor. $\endgroup$
    – El Flea
    Nov 13, 2023 at 18:24
  • $\begingroup$ @ElFlea because I know that particles of sand won't hold much electrical energy. $\endgroup$
    – Tiger Guy
    Nov 13, 2023 at 19:35
  • $\begingroup$ Doesn't it all depend on the four factors I mentioned above? The higher temperatures, more surface area and angle of collision would seemingly increase the charge transfer forever? $\endgroup$
    – El Flea
    Nov 14, 2023 at 6:31
  • $\begingroup$ You can never get more power out that you put in. Converting 300 degree sand to steam is more efficient than converting it to static electricity -- as demonstrated by the steam engines and the static electricity generators. $\endgroup$
    – david
    Nov 15, 2023 at 9:42
  • $\begingroup$ @david I never I'll get more energy than put in. Just a higher efficiency than most here are calculating. $\endgroup$
    – El Flea
    Nov 16, 2023 at 5:24
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The energy required to pick up and blow the filings at the surface will exceed the truly tiny amount of charge separation that this device will generate, so it is a net losing proposition.

This is not a simple system. you need the filings, a fan driven by a speedy motor, a battery to spin that motor, a solar parabolic reflector, a feed mechanism to meter the filings into the heated airstream, a charge collection system, a power converter that will condition the voltage/current output of the tribo generator to match what the cell phone needs, and so on.

This is far, far more complex and expensive than a solar cell.

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  • $\begingroup$ Well I thought the solar concentrated heat would make temperature and pressure gradient which would cause the air to flow at high speeds through some narrow tube which is fired at particle stream falling in front of it. The motor, battery etc. is unnecessary. I want electricity without magnets :) $\endgroup$
    – El Flea
    Nov 12, 2023 at 19:26
  • $\begingroup$ As for net losing proposition, it was never meant to be a perpetual freak mumbo jumbo device. It's more of a steam turbine converting heat to electricity. Converting, NOT creating energy. $\endgroup$
    – El Flea
    Nov 13, 2023 at 14:26
  • $\begingroup$ I never said that. I only said it was impractical. $\endgroup$ Nov 13, 2023 at 19:24
  • $\begingroup$ How much electrical power do you expect from the setup input power is 500W? Some figures would help. $\endgroup$
    – El Flea
    Nov 14, 2023 at 6:29

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