Stirlingengineforum.com

This is quite interesting: a hand cranked heat pump/refrigeration system.
It doesn't look like he's having to put much effort into it at all. I wonder what kind of ∆T he's getting out of that.

https://youtu.be/Hx4tXKo8yLw

The description says "freezing" on one side and "hot" on the other.

https://youtu.be/YWUhwmmZa7A

This is a pretty long boring dull video about Peltier device refrigeration, but around 20 minutes in, relates some interesting facts.

The Peltier device, without an active cooling system, heat sink and fan on the hot side and insulation on the cold side won't get cold. It will act as a heat pump, so one side will get very hot but on the cold side, without insulation, the heat taken away will be immediately replaced by ambient heat.

It will, though, "pump" maybe 2x more heat than it uses in electricity, so each 1 watt of electricity will transfer about 3 watts heat energy total to the hot side 1+2 = 3 but only provide 0 to 2 watts of cooling.

Apparently for the same wattage, using more Peltier devices at lower voltage increases the COP.

Bottom line is these things throw a LOT of heat. The heat pumped but also the heat from the applied electrical current.

A Stirling engine is actually, as I've deduced from a number of observations and experiments, a kind of gentle heat pump in itself, but in the opposite direction of the imagined "heat flow".

In other words, a Stirling engine acts like a dam holding heat back on the hot side of the engine, forcing the heat /energy to do work on the piston.

So putting a Peltier on a Stirling engine for cooling is just about completely useless, as it will never get cold, but using it to supply heat, you might get somewhere if the Peltier is just used as a heat pump to supply heat to the engine intermittently.

A Stirling engine is too frugal in its heat consumption to keep a Peltier device cool on its hot side. As I said it's a kind of heat pump itself but working in the opposite direction. It will gradually convert the heat into work so that the heat is gradually used up but it will never use up the heat fast enough to cool the Peltier device enough to actually keep it cool

So, sandwiched between two Stirling engines, as seen in the earlier video, the engine on the hot side of the Peltier runs very fast on all the excess heat available, but the engine on the "coldish" side, it's hardly running at all.

In fact for the description for this video, posted again for convenience

https://youtu.be/vsTIWhZ-6HM

It is mentioned that the Peltier gets so hot it could damage both the little LTD engine as well as the Peltier, which is likely why the video is very brief.

From the description:

For best results I would suggest a TEC1-12715 DC12V Heatsink Thermoelectric Cooler Cooling Peltier Plate Module 40x40MM over the TEC1-12706 as the TEC1-12715 can run at a higher amperage. If you only have a TEC1-12706 then you may wish to place some form of resisters or Temperature Controller Module otherwise the heat may be too great for both the Low Heat Sterling Engine & TEC1-12706.

So, really WAY too much heat being pumped to the hot side of the Peltier and you don't want to melt your engine, which is probably part plastic and foam rubber, or burn up your Peltier.

I'm thinking the Peltier could be used in conjunction with some phase change type thermal storage, like Xylitol as an intermediary.

Use the Peltier to pump heat into the xylitol for long term storage then run the engine off the hot xylitol rather than directly on the Peltier.

I was interested in these KS90V Stirling engines mainly because of how all the friction/heat producing moving parts are elevated a distance away when running on ice.

Having ice apparently freeze or re-freeze onto a Stirling engine "running on ice" had previously seemed like an unusual and rare occurrence, but with these engines it seems to be fairly routine.




https://youtu.be/GbKdxY_jb8k

https://youtu.be/zsiMjS3Reug


The ice, in both these videos seems to be actually frozen to the bottom of the engine, not just adhering due to moisture. The ice cubes I had to rather forcefully knock loose, and the larger block of ice in the second video did not slide off when the engine was tilted, but DID easily slide around as soon as the engine stopped running.

I thought it might be possible that the ice in contact with the engine IS melting but the weight of the engine is pushing the melt water out to the sides so it only "looks like" the ice is melting from the bottom up.

To be sure I am going to try using food dye and freeze the colored water in layers.

That way there could be a thin, maybe green layer in contact with the engine with a clear layer under that.

If the thin green layer in contact with the engine melts, or not, that should be apparent.

I just noticed there is a typo at the end of the 2nd video. That should have been 6 1/4 tbsp not tsp.

Rather than using food coloring I came up with a different plan.


https://youtu.be/FsJABF_3bMk

The food coloring was just too messy and the colored layers would bleed together before I could even get them completely frozen.

I think the dye also changed the melting point and the coloring tended to separate or settle out of the water.

Anyway the string should do the job.

What I always find striking is how it is pretty much impossible to balance a metal engine on ice. The ice is so slippery the engine just keeps sliding off to the side, when not running. in contrast to how a running engine will "freeze" to the ice and lock onto it so tight it has to be knocked loose.

The contrast is very striking when doing these experiments first hand.

I've been running into a problem with the ice experiments.

When the engine gets down below the rim of the container it stops running due to getting "cold soaked".

The engine is down in a "well" of cold and not enough ambient "hot" air circulating around the top plate to keep it going.

So I tried adding fan blades, and this works well, but then too much of the warm air gets down around the bottom cold plate, so I've added some insulation covering the bottom half of the engine to keep ambient heat away from the bottom cold plate.

I'll need to do a bit more testing and freeze some more ice then see how it goes.

So far, I've not been able to keep the engine running long enough for any conclusive results.


https://youtu.be/qRxh7YVb-M4


But with the fan, the engine has continued running for up to five minutes AFTER the ice has all melted and all the melt water has drained down, the engine keeps going for several minutes just from the residual cold left in the cold plate.

I also cut off the upper rim of the containers in order to allow more air flow across the top half of the engine.

With this setup, the bottom insulated and fan above, the engine ran on a frozen piece of damp paper for ten minutes.


https://youtu.be/iuBH92LMvKg


I would have filmed longer but my phone ran out of storage.

It kept running after 10 minutes a few minutes more but at a slower RPM.

Anyway, the paper acts like a sponge so as the ice melts it doesn't run all over, and the frozen paper can fit between two engines, one right side up and the other upside down.

After hitting the frozen paper with some air duster (difluoroethane) for a bit of extra cooling the engine ran for 30 minutes.

I still need to make a regenerator. That should help maintain the ∆T even longer.

One problem I found with the extreme cold from the air duster was, although the engine ran much longer, it did not run as well. It was slower and sounded noisy.

I think the problem there is that the engine tolerances are designed so the engine will run well on heat. As the parts expand the various gaps tend to close and make a perfect seal.

Running extremely cold, however, the parts shrink and any gaps open up more creating leaks and overall lower pressure. Parts, being a loose fit tend to rattle and make more noise.

One problem I found with the extreme cold from the air duster was, although the engine ran much longer, it did not run as well. It was slower and sounded noisy.

I have found that bringing Tmin below ambient tends to increase air mass inside the engine enough to change the way it runs/timing.

Looking forward to the back to back engine testing.

VincentG wrote: ↑Sun Sep 08, 2024 9:08 am

One problem I found with the extreme cold from the air duster was, although the engine ran much longer, it did not run as well. It was slower and sounded noisy.

I have found that bringing Tmin below ambient tends to increase air mass inside the engine enough to change the way it runs/timing.

Looking forward to the back to back engine testing.

Unfortunately the timing on these little engines is not adjustable.

bump

The two Kontax engines are on the bench for modification for the next experiment.

Fitting an effective regenerator to these very small engines has been rather challenging and time consuming.

https://youtu.be/3fF-pfgz-f8

This regenerator design has worked well and produced quite effective hot/cold separation, but getting a nearly air tight fit that does not bind or cause a lot of drag is difficult.

I don't really want to try to modify the displacer for a better fit, so I've been stumped.

I may have to make new displacers from scratch, setting the original ones aside.

Possibly, I've been thinking I could make displacers with beveled edges that narrow down to a "feather edge" on the circumference.

The solid stainless steel screen is also not ideal. The regenerator should be layered. About 20 layers if possible, but again, very difficult with such a small engine and very little room to work with.

I am continuing work on it when I have the time. Unfortunately, the house is under extensive renovation that needs to be completed before winter sets in so this project is on the back burner for a while longer.