Stirlingengineforum.com

To start with, I'm using the term "cryocooler" very broadly to include any substantial refrigeration.

Also, what I'm interested in is a true HEAT DRIVEN Stirling refrigerator. Not something mechanically or electrically (or acoustically) driven.

To start out with, here is a screenshot of a section of a Wikipedia article on Stirling cryocooler:

I've seen the type of "cold finger" here illustrated in a number of reciprocating compressor driven type cryocoolers.

It works by changes in air pressure without any mechanical linkage.

As pressure increases, the temperature of the working gas increases, The increase in internal pressure causes the regenerator in the cold finger to move due to the pressure difference. This movement results in the regenerator absorbing heat from the working fluid.

When the internal pressure drops, the opposite takes place. The regenerator moves back releasing the heat.

All that is required is a rise and fall of internal pressure, which in a Cryocooler, is most often implemented by mechanical compression or work input.

But we know that the same kind of pressure increase and decrease is possible in a Stirling engine with heat input alone, no external mechanical work input being necessary.

I've recently been looking over the various tin can walking beam Stirling engines built from the plans available on this forum: stirling/

I think it might be possible to adapt this kind of cryo-cold-finger to a Boyd House type walking beam tin can engine.

I've made a few design modifications, like using a stationary regenerator with a diaphragm type displacer and also a diaphragm type piston. There is also the addition of the pressure driven cold finger. Other than that, my first preliminary, and rather messy sketch of the idea is based on the Boyd House walking beam engine plans available here.
In this instance, the cold finger is only being used to help chill the engines water bath

Probably there should be some thermal isolation between the heat input and chiller ends, maybe a small section of PVC pipe to divide the main body in half.

The cold water bath might also be insulated to exclude ambient heat. An exposed top on the water bath might (or might not) be beneficial for evaporative cooling, depending on how well the cold finger might, or might not work.

The piston air port was moved to the top of the can to avoid any conflict with the regenerator on the side, and connected with a flexible tube.

Here is a patent of a conventional piston type compressor driven Cryocooler that describes in some detail how these pressure driven cold fingers work.

https://patents.google.com/patent/EP0046585A1/en

This is interesting, mostly because a few degrees of cooling are actually achieved simply by shaking a rather crude looking "test tube" type Stirling engine by hand, and without any insulation.

If that is possible, it should also be possible to produce some measurable cooling with just about any kind of actual heat driven Stirling engine.


https://youtu.be/P0LRZEEqRpI

Another can cooler from Blade Attila, The engine pretty quickly comes apart, but still manages to cool down a few degrees before the displacer comes lose inside. Again, with no insulation.

I think it is pretty hopeless to try to get any cooling while the "cold finger" is fully exposed to the ambient heat, like trying to keep the refrigerator cold with the doors off. Nevertheless:


https://youtu.be/yf9H-KuwA5w

Looking at the above compressor (Fig. 1) with the cold finger, this "compressor" only compresses the same volume of air over and over, apparently.

Compress - release - compress - release etc. etc.

Which gives me an idea.

For a long, long time, I've been looking at fire Pistons, wondering if there wasn't some way to use that heat, such as in this video:


https://youtu.be/iOSH6D5L-dk


How much air, cold winter air no less, does it take to compress to generate enough heat to operate an LTD engine, like the p-90 for example, runs on a 0.5° temperature difference.

Take a fire piston type cylinder, add a cold finger, like in the above.

Hit the plunger repeatedly. The base of the fire piston gets hot, the cold finger gets cold.

Couple that up with a Stirling engine?


I added fins to the cylinder, not to cool the cylinder but to add heat with each cycle.

That is, the piston goes down, generating, or rather concentrating the heat in the air, the heat, with any luck, dissipates into the top hot plate of the engine. The gas therefore has lost energy. So...

When the fire piston- piston rises, expanding the gas, it might just be colder than ambient, so the fins allow heat to be quickly conducted to the walls of the fire piston to put some heat back into the air for the next plunge.

In the meantime, what's happening with the cold finger?

The lower tip of the finger should drop down to absolute zero almost immediately right?

Well, no. But it might cool down a bit. A degree or two?

The heat from the fire piston and cold from the cold finger could create enough of a ∆T to power the LTD engine, which of course could never generate enough of a temperature difference to run the engine to run the fire piston contraption continually.

But, in this video, it is obvious the plunger is propelled out of the fire piston.


https://youtu.be/Bjy6m6MR-PQ


That thing really comes flying out, though the first few tries, the cotton did not ignite. So, it wasn't due to combustion, just the high pressure returning the piston.

When a compressor pumps air into a tank, all the work of compression is lost to the air outside the tank, but the compressed air is still in the tank ready to do some work. Or so I've been lead to believe by Atlas Copco etc.

Alert, 2.28 MB PDF at this link:

https://www.ien.eu/uploads/tx_etim/3572 ... _Copco.pdf


Up to 105% heat recovery?

With a tank full of compressed air to boot?

Not possible!

But, suppose it just is possible, the compressed air in the fire piston, after giving up heat to the engine then expands "isothermally" re-absorbing heat in the process and so delivering torque to the crankshaft on the way up.

If the compression could be adiabatic (fast) to retain heat, and the expansion could be isothermal (slow) to absorb heat, this might work better, though making the bottom of the cylinder something like copper and the cylinder acrylic. Maybe copper on the bottom, acrylic in the middle and aluminium at top?

At the bottom the copper conducts heat to the engine, at the top the aluminium delivers heat, during compression, in the middle, the acrylic helps assure adiabatic (no heat transfer out) compression.

This is, essentially a heat pump, we are only moving the heat, not generating heat, so...

With all this information, I'm not convinced it is entirely impossible.

The engine is not using its own energy to generate heat, it is only moving the heat already in the air.

The engine removing heat as compression takes place could make compression easier.

Too bad it's impossible. Why? Because. Because why? It just is.

Well, I'll give him an A for effort.


https://youtu.be/FYbCdL_pJOc


Reflectix has an R value of about 1.5

Hand heat? Hand resting on the foil.

The foil itself, contacting the engine is conductive.

Still, there was a little bit of difference on the top plate. Maybe. Or maybe his hand was not in contact as much.

I may try this. I have some aerogel blankets on the way. The R value is a little better and no foil. Foil would be OK on the outside but probably not in direct contact with the engine, like in the video.

Could an LTD engine actually compress any air at all, even in a tiny tiny fire piston?

I must admit, that doesn't seem very likely.

How about a Stirling cryocooler / even a refrigerator, driven by a bigger Stirling Engine?

It will be great fun to build one !!