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Comment by nanomonkey | Hacker News Reader

Can anyone point to the reason(s) why Stirling Engines appear to be unobtainable for stationary power generation? I've been wanting a reasonably sized one (say 2-5kW) for quite some time to run off of woodgas, or other high temp heat sources. Size doesn't really matter, I just want something relatively quiet, that needs much less maintenance than an internal combustion engine.

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If you're ok with using a parabolic mirror(s) as the heat source instead, then it should be doable. eg:

http://web.archive.org/web/20060508061432/http://solstice.cr...

They specifically mention systems that do 5kw, 25kw, etc (in the next pages).

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That aside, there's a kickstarter project for a 1kw Stirling engine from about 6 years ago:

https://www.kickstarter.com/projects/672465444/low-cost-ster...

The "updates" tab there seems to have potentially useful info, though the project appears to have stopped updating there.

The project itself still seems to be iterating on new developments, just slowly.

http://volodesigns-sterlingproject.blogspot.com/

More recent video's of their development:

https://www.youtube.com/watch?v=p06HDOlxDMM

Mentions here they're intending on making the design OSS:

https://www.youtube.com/watch?v=IrXltsm-Ujs

The videos don't inspire much confidence in their engineering capabilities. Very rough construction and not pressurized.

The kickstarter project has transformed into Sefton Motors, which says it has engines backordered (but in production): https://seftonmotors.com/collections/all

They've certainly been available commercially at various points. If you search for microCHP, you'll find them. I can't see that any of the big manufacturers are currently making them, though. Honda and Baxi used to have residential stirling engine CHP boiler models that seem to be discontinued.

Currently, I think the Senertec Dachs might be closest to what you're after in size, although it's based on a conventional four-stroke, not a stirling engine. The residential CHP market seems to be shifting to fuel cells.

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?

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.

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

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.

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

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

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.

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.

As I understand, the isothermal transitions in the Stirling cycle need to happen relatively slowly to preserve efficiency, which means the cycle can only run at a low frequency, which generally results in a low power density.

WhisperGen have been making silent stirling cycle generators for many years. For technical details see the user manual for one of their products here: https://www.victronenergy.com/Manuals/WhisperGen/UserManual/...

(I have no connection with this company or its products)

WhisperGen are difficult to find in the States, I looked into it in the past and was quoted $12k for a boat version that ran off of diesel. These were also only producing 800w of electricity.

Talking to the folks that worked at WhisperGen, the wobble yoke design added extra complications and tolerance issues that added to the cost.

iirc panasonic had one generator. small scale internal combustion engines are popular because power to weight ratio allows the same design to be used in other equipment and it has near instantaneous power regulation.

Sadly finding a Philips MP1002C Stirling engine generator is almost impossible. Not to mention that they are loud and only output around 200w, which is nothing compared to a Honda 2200i.

A stand alone generator need not be mobile, so it needn't be that light weight. In fact it probably would benefit being integrated with a home's heating and cooling system to utilize waste heat.

What it does need to do is be relatively maintenance free, and run off a variety of heat sources (such as pellets, woodgas or solar thermal). This is where I think Stirling engines shine...you can gasify all of your waste organic matter into dirty gas that wouldn't normally be fit for an internal combustion engine, but work fine for a swirl burner which can be used on an external combustion engine such as steam or stirling.

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