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Comment by Izkata | Hacker News Reader

Heh, I actually came here to take advantage of this post to ask something similar: Could a Stirling engine be used to, say, charge a battery using the exhaust heat from an AC unit?

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The exhaust temperature of the AC will be as low as is practically possible to remove a given amount of heat (assuming the AC is properly engineered). To extract energy from this exhaust you have the problem to remove heat from the cold end of your stirling engine while maintaining a lower temperature than the exhaust, which will prove difficult, given the assumption.

OTOH, if the exhaust of your AC is hotter than necessary (the unit wastes energy), then the best you could hope for is to recover some part of (depends on temperature differential) the wasted energy. Better get a better AC unit.

Unlikely that you could get much power the heat from an AC unit as the temps aren't that high. Perhaps you could run the AC's exhaust fans off of a low temp stirling.

You can uses a Stirling engine as a cryocooler, by running it in reverse, and cool your house itself directly. This is only really useful if you're trying to get to low temperatures.

I’m interested in reading about this

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

I have two of the Twinbird FPSC Portable Freezers which utilize this technology, nice and quiet and sips electricity.

The energy generated is related to the difference in temperature. So if you're using AC out as hot and ambient as cold, the generation will be small, and it's necessarily much less than the energy spent to run the AC.

What about "Moves more energy than uses"? I mean those heat pumps which have COP over 2?

Yes, although obviously you can't have a perpetual motion machine; the Stirling engine also moves more energy from hot to cold than it can emit as useful work.

Both the Stirling engine and the AC unit are using the Carnot cycle (simplification), and going around the expansion-heat-transfer cycle in opposite directions.

Now you've got me wondering if you could design a reverse-stirling engine that runs off an intensely endothermic reaction of some sort.

It would still be a normal Stirling engine and not reverse. The engine only needs a large enough temperature differential. Where that comes from does not matter.

A reverse stirling engine takes mechanical energy and produces a temperature differential. These kinf of setups are actually being used for quite efficient cooling.

You could certainly run such an engine, very efficiently at that, from vaporisation of LNG.

The engine only requires a "hot" side and a "cold" side, so there's no reason why the cold side can't be way below ambient temperature, such as an endothermic reaction or a tank of liquid nitrogen. Here's a video of a guy running one of those coffee-cup-top toy engines off liquid nitrogen: https://www.youtube.com/watch?v=O6eEvU5piQg

Some people have proposed this as yet another bulk energy storage system for the grid; giant underground cryotanks of liquid nitrogen. With careful design the same gear could be used in "forwards" or "reverse" mode. The limit is that the Carnot efficiency isn't great.

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