Regenerator Material Selection

[stirlingacount]

I'm trying to build a LTD gamma configuration Stirling engine. From my research I've seen the two most popular material selections have been aquarium filter foam and a metal foam. Do you guys have an suggestions of other materials to make this engine more efficient?

[Tom Booth]

... From my research I've seen the two most popular material selections have been aquarium filter foam and a metal foam....

Personally I haven't heard of either of those being used as displacer material, so I'm curious where you get this information.

Anyway, for small engines that run on hot water or ice, I often make due with the foam board sold at stores like Target or Walmart or an art supply, in either the office or the crafts or art supply area.

It seems identical to what many model engines are sold with, but just about anything rigid and lightweight will work. Balsa wood, foam board, styrofoam, that hard foam used for flower arrangements is recommended often.

[Tom Booth]

As you mentioned aquarium filter foam, this looks interesting. Available in several different pore sizes:

Resize_20240202_151404_4718.jpg (227.75 KiB) Viewed 1139 times

https://www.ebay.com/itm/385066437546

[MikeB]

I'm trying to build a LTD gamma configuration Stirling engine. From my research I've seen the two most popular material selections have been aquarium filter foam and a metal foam. Do you guys have an suggestions of other materials to make this engine more efficient?

Your title mentions "Regenerator" rather than "Displacer" - Tom seems to be talking about the latter, with good reason - filter foam surely isn't porous enough for a regenerator?

[Tom Booth]

...
Your title mentions "Regenerator" rather than "Displacer" - Tom seems to be talking about the latter, ...

Oh, yeah, thanks. I need to get some new glasses, sorry

An article, research paper I found, I think a few years back now, found that for a really high efficiency regenerator, like 98% efficient, the regenerator material needed to be in layers. They found that the highest efficiency required about 18 or more layers, if I remember right. I posted a link to the article here in the forum somewhere.

With a tangled mass of continuous fiber like steel wool or a solid matrix like window screen, the individual fibers conduct too much heat along their length, so the regenerator does not have a good temperature gradient. Building the regenerator up in layers creates thermal gaps between each layer so heat is not transmitted as easily across the entire regenerator, giving a much better temperature gradient for the same amount of material.

I did some experimenting with that method and it seemed to give good results, at least as far as keeping heat out of the cold side.

[Tom Booth]

This was the article/research paper:

https://www.researchgate.net/publicatio ... ass_ratios

[Tom Booth]

I have an alternative theory about how a regenerator actually works or what it does to improve performance

I don't think that it is actually to store and "reuse" heat, as that should actually be impossible.

Heat converted to work is gone out as work and cannot be reused. If there was "waste heat" that could not be stored for more than a cycle or two, since more and more "waste heat" would arrive and the regenerator would quickly become heat saturated

If the Stirling engine, however, acts similar to or the same as a heat pump, then the regenerator might act as a kind of heat accumulator that might increase the overall ∆T.

That kind of turns all former Stirling heat engine theory on its head and implies that the actual heat flow in the engine is reversed or rather duel. That is, some of the work output is used to "pump" heat from the colder (less hot) side over to the hot side.

In other words, due to added "heat of compression" the regenerator might be able to store a higher grade, higher temperature heat than supplied by the actual heat source.

In other words, though a simplification, the regenerator acts similar to a dam on a river that allows the water to accumulate, raising the water level. Heat though, is not subject to gravity, so the heat source does not necessarily need to be higher than the height of the "dam". That would make the regenerator more like a water tower that is elevated above the water source and fed by a Ram pump to offer an analogy.

[MikeB]

My own personal, but completely unproven theory is that a regenerator will improve the efficiency of some badly designed engines, but I just can't see how it could improve on a theoretically perfect engine, as it will rob pressure when it absorbs heat.

[Tom Booth]

My own personal, but completely unproven theory is that a regenerator will improve the efficiency of some badly designed engines, but I just can't see how it could improve on a theoretically perfect engine, as it will rob pressure when it absorbs heat.

I basically agree, though to some extent depends on how the term "regenerator" is defined and on its placement within the engine.

If, for example, a stainless steel wool "regenerator" were placed close to the heat input, or actually in contact with it, as in a "thermoacoustic" engine, then the "regenerator" is more a heat reservoir to be drawn from. Arguably, not a "regenerator" at all I suppose, in that instance.

However, given such an intermediary heat source, it could theoretically accumulate or absorb additional heat from "heat of compression".

This might have an advantage in the same way an electrical transformer increases voltage at the expense of amperage. Or a ram pump pumps water, basically "free" but with considerable water wasted at the source.

Not really very "efficient" but could still be useful perhaps, if your source of heat is free and unlimited but too "low grade" to be of much use.