LTD regenerative displacer

[Tom Booth]

I think I've worked out a fairly decent way to incorporate a Regenerator into an inexpensive model LTD.

It was kind of random, in that the idea came to me while putting a kit together, and after several other failed attempts.

One problem was keeping the steel wool in place. Large round holes cut in the displacer don't hold the steel wool, either it falls out, has to be packed in too tight to keep it in place, or requires some support structure that adds dead weight, which is not good for such a small engine.

I thought a STAR SHAPED port might work, because the narrow legs of the star could better grip the steel wool than a perfectly round hole.

IMG_20200807_031834138_crop_58_resize_86.jpg (34.97 KiB) Viewed 11891 times

To help hold the steel wool in place, I took the back end of a very small drill bit and poked the steel wool into the styrofoam at the points of the star to stretch it out. Taking advice from others, it seems the steel wool should be very expanded or fluffed out so that it is "see through" when held up to a light That's what I'm trying anyway.

IMG_20200807_031905879_resize_33.jpg (34.82 KiB) Viewed 11891 times

I now have this engine assembled, WITH a Regenerator, and the other WITHOUT just a styrofoam disk displacer.

I made the new displacer a tighter fit, (slightly bigger, as can be seen by the circle near the circumference, which is the size of the original)

I have a big sheet of styrofoam poster board, so I can try different things. For now I'm anxious to see if this makes a difference, or, can the engine still run at all?

I'll try running the two engines side by side.

I'm basically following this page, which claims much higher RPM with the Regenerator:

http://www.solarheatengines.com/2008/06 ... generator/

[Tom Booth]

The Regenerator really makes quite a big difference.

RPM nearly doubled 155 to 270 at least.

https://youtu.be/QqN80ZqJLoQ

The regenerative displacer is 3 grams heavier than the plain styrofoam. That may be partly because it is nearly 1/8 inch larger in diameter, so I'm not sure how much of that is steel wool or styrofoam, but regardless of the extra weight, the engine still ran much faster with the Regenerator.

The top of the engine with the Regenerator, however, felt slightly warmer to the touch. Though the tops of both engines were relatively cool. That would seem to indicate the Regenerator is letting more heat through. Possibly I made the steel wool a little too thin.

Also, if left to run, though the engine with the Regenerator ran faster most of the time, the engine with the plain styrofoam kept going about 15 minutes longer.

[Tom Booth]

I took some good advice from this video, at about 4:00, to keep the steel wool very light and transparent, not compacted.

https://youtu.be/gVU6uqn0cII

Though I think I may have gone a little overboard.

Also, possibly, instead of using a star shape, just poking the steel wool into the styrofoam would be enough to keep it in place.

I'm wondering if previously, a lot of heat was being lost through the sides of the engine as the hot air went around the displacer. Whereas now the hot air is going through the regenerator, so the TOP feels a little warmer, but overall, less heat is being lost.

I had expected that adding the regenerator would keep the sink cooler.

[Tom Booth]

I hit upon an idea for a new kind of LTD Stirling regenerator. I already installed it in the engine, so I can't exactly show a picture at the moment but it looks like this:

New_Style_LTD_Regenerator.jpg (44.83 KiB) Viewed 11186 times

Layers of steel wool, or whatever chosen regenerator material goes in the gray area between the plastic rings.

It is just two plastic rings cut from a clear plastic container. the inner ring has holes for the passage of air, and there are layers of metal wool fibers stuffed in between the plastic rings as regenerator material. The displacer needs to just barely fit inside the smaller ring so that as it rises and falls it forces air through the regenerator. I had to cut the displacer down about 1/8 inch all around, a little more than that really, but I took a little too much off the displacer which left a little bit too big of a gap. It still seems to work pretty well though. Part of the problem was that I poked holes in the plastic with a hot nail heated with a torch, instead of trying to drill holes, and that process left burrs that interfered with the displacer movement. I could have sanded the burs off, but it was easier to sand down the displacer, as the regenerator was already complete. Anyway, still some room for improvement.

I'm uploading a video to youtube.

I took readings with an infrared thermometer to check how much heat was getting through the regenerator. It didn't seem like much heat was getting through at all. More heat was radiating through the vacuum of the double walled vacuum mug, or possibly the heat was getting around the lip of the mug to the outside. Either way, it looked like less heat was getting through the regenerator than through the vacuum insulated mug.

[Tom Booth]

Here is a PDF with the results of tests describing what makes an effective regenerator.

https://faculty.ontariotechu.ca/macdona ... s-2019.pdf

Mostly it seems to boil down to having a lot of layers. Based on that, Instead of stuffing a single big wad of steel wool into the regenerator, I unrolled the steel wool and cut it into as thin strips as I could and filled the gap with one thin layer of steel wool on top of another. Apparently it does not matter too much if the layers are touching each other.

https://youtu.be/cR31i09PnNQ

[Tom Booth]

Taking some ideas from my new LTD regenerator design above, and also some ideas from TK motors diaphragm engines:

https://youtu.be/iFXiaO8iwhk

mostly motivated by JessIAm's compost heat Stirling project:

viewtopic.php?f=1&t=5234

I've been thinking about / designing an LTD style engine with internal membrane to move air back and forth through a regenerator in place of the ordinary displacer.

My plan is to do a simple test of the concept by modifying one of my model LTD Stirling engines.

The "insulation material" pictured, (below sketch) will likely be composed of refractory cement. The regenerator will be thin alternating layers of window screen, and steel wool, or "sub-regenerators" as suggested by this analysis:

https://www.sciencedirect.com/science/a ... 1919312310


The "displacer" will be a disc cut from a latex rubber glove. otherwise the crank, flywheel, power piston, air chamber body will all be the ordinary off the shelf model LTD Stirling I've been working with for most of my other experiments.

The hope is to have a Low Temperature type engine with something approaching closer to the kind of power and torque demonstrated by TK motors innovative diaphragm engine designs.

Here is a basic preliminary sketch of the idea:

IMG_20210810_121007966.jpg (42.11 KiB) Viewed 11028 times

The addition of the internal insulation, refractory cement and regenerator will add weight to the engine, reduce dead air space, theoretically increase efficiency and power output, the actual weight of the "displacer" however, that the engine actually has to lift will be reduced.

Hopefully this will operate at very low ∆T, but due to the membrane being more distant and thermally insulated from the heat source, should also be able to withstand higher temperatures than a typical LTD engine, for something like a wood stove fan as well.

[Tom Booth]

Above, I had planed on using a rubber glove or something for a diaphragm, I would rather, if possible though, use something that could withstand a wide range of temperatures, especially if this ever goes beyond a prototype, proof of concept stage.

I came across an interesting article from ten+ years ago but can't find any recent information about where, if anywhere, this development went. What happened to nanotube-rubber? Looks like it could be quite useful for a diaphragm type Stirling operating at extreme temperature differentials.

“We are currently searching for applications that could benefit from such temperature invariant properties,”

https://physicsworld.com/a/nanotube-rub ... peratures/


Not something that anyone can just go out and buy, apparently

Red RTV high temperature silicone is about the next best thing I can find, so far, but as far as I know, does not come in ready made sheets, but could be made from spread out RTV silicone caulking.

[Tom Booth]

For options I've found on making or obtaining silicone diaphragm material, see this thread:. viewtopic.php?f=1&t=5350

[Alphax]

Tom

A very interesting experiment (the foam displacer with 5 star-shaped holes carrying regenerator material).

It does look as if you've got an improvement with your novel displacer/regenerator.

However, there is a problem - why does it perform better?

I'm sure you would say it is obviously down to the regenerator in the displacer. That is certainly one explanation. But there is an equally valid alternative explanation.

Which is you have (1) altered the weight of the displacer and (2) introduced new fluid paths (the star shaped holes with porous matrix). Either (or both) of these might be having an effect - they may even interact.

Therefore the control for your (excellent) experiment isn't the original, unaltered LTD engine, it is one that has the additional weight and new fluid paths but no regenerator material. Or you could have 2 controls (one with additional weight, one with new fluid paths).


I'm not criticising. But I am saying that further work is needed to see what the explanation of your finding really is. But clearly, it does seem to work!


Excellent work, by the way!!

[Alphax]

Tom,

Talking of improvements to LTD engines, it is often asked if changing the thermal conductivity of the hot-end and cold-end plates would make improvements.

The answer is...... very counter intuitive as explained in this excellent Kontax video.

I think you can see by analogy with the Kontax findings that it is difficult to be certain what the true explanation is for your 5-star regenerator displacer improvements..... simply because you have changed multiple variables but not used matching controls (because that is rather a lot of work).


https://www.youtube.com/watch?v=0ZzPWfLDAOI

[Alphax]

Having said that..... someone else does report similar results to you (engine speeds up when regenerator inserts are added to displacer):-

https://www.youtube.com/watch?v=yeKXfF6gR-s

[Tom Booth]

Tom,

Talking of improvements to LTD engines, it is often asked if changing the thermal conductivity of the hot-end and cold-end plates would make improvements.

The answer is...... very counter intuitive as explained in this excellent Kontax video.

I think you can see by analogy with the Kontax findings that it is difficult to be certain what the true explanation is for your 5-star regenerator displacer improvements..... simply because you have changed multiple variables but not used matching controls (because that is rather a lot of work).


https://www.youtube.com/watch?v=0ZzPWfLDAOI

My guess regarding the "counter intuitive" Kotax findings that the sink side material doesn't seem to matter, is that there is nothing to "sink".

A very predictable result, IMO.

It does make a noticeable difference on the input side however.

Again, as might be expected.

[Alphax]

Tom,

- regarding the concept of the LTD regenerative displacer, it is an old idea going back at least as far as 2012.

Here is a Masters thesis dated 2012 that has an illustration of the same idea in a PT2-FBCP engine (built following guidelines in Senft, 1996):-

[Alphax]

And here is the working PT2-FBCP engine with the regenerators embedded in the displacer. That engine was instrumented and was the basis of work reported in Lemaire's thesis.

Notice how small the engine is (Canadian coin in foreground for scale).

[Tom Booth]

Thanks,

Looks like the whole thesis is publicly available:

https://escholarship.mcgill.ca/concern/theses/4b29b997z

Interesting introduction, from the opening abstract;

Abstract

Portable electronic devices have made a profound impact on our society and economy due to their widespread use for computation, communications, and
entertainment. The performance and autonomy of these devices can be greatly improved if their operation can be powered using energy that is harvested from the ambient environment. ...