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Inutilisable t1_j4n7mm4 wrote

Reply to comment by SacredRose in How do non electric heat operated fans work? by ranman12953

I designed lab equipments with precision pistons made of graphite in glass tubes. It’s really good but it is expensive, especially in low quantities, something like >40$ for 1/2” diameter piston, a few inches long. There was no other way to get low friction. I imagine that other constraints gets involved when you want to get any useful energy from it, at large scale.

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joalheagney t1_j4pb3u8 wrote

The other big issue is the driving gas. You want something with a really low molecular mass for maximum thermal expansion. Hydrogen gas would be ideal if it didn't have a distressing tendency to diffuse into and through metals. And. You know. Explode in contact with heat and Oxygen.

Helium is half as good (twice the MM) ... but incredibly expensive and almost as hard to contain. Doesn't explode though which is good.

And then you're into N2, O2 and you may as well use air for obvious reasons. At about 14 times the MW of hydrogen gas. :/

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