if stirling engine is driven as reversed, does it work as cooler?

[ysj11211]

I have a question for a long time.
Would the stirling engine work like a refrigerator if i add a motor to rotate the engine?

I'm not good at English but i've searched this forum for a long time and found out that at some occasions, stirling engine can be used as a refrigerater. (maybe...)

I'm wondering if it works, does it apply to any stirling engine?
I know that there is a lot types of stirling engine and i want to know if there's somebody who ran stirling engine as reversed( Not heat and see it rotates, But add it a motor and see if the temperature changes) and know the results.

there's two type i can buy: posted a image below.

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I wonder if i run those engines with motor, the change at temperature exists.


I'm sorry if i broke any rule of this forum

[Bumpkin]

Yes those engines and others would pump heat if powered. It is not “backwards” though. The direction of the flow is the same whether it is pumped or pumping — like a water turbine, for instance. Long ago I got a similar question answered by forum member Ian:

“As a stirling engine rotates, it it extracting the heat energy from the heat source, and getting rid of it from the cooling system at the cold end. If you take away the heat source, and continue to drive the motor in the same direction, it will continue to extract heat from the surounding atmosphere, and the temperature of the displacer cylinder, or hot cap of an ALPHA motor will reduce, the heat being transfered to the cold end, which will heat up, if you'r really working it, and you have water cooling, it's possible to boil it, the lowest temp I'v had is -20*C, while the ambiant temp was 19*C, the cooling water went to just over 60*C, the motor was being turned by another stirling engine of similar power, a more powerful motor, and I would have got a better temp drop. The two motors are in my gallery. The driving motor is listed as my second motor, and the driven one is the Ross Yoke one. Ian S C”

I hope that helps, and welcome to the forum.

Bumpkin

[Tom Booth]

In some types of engines, at least sometimes, the actual direction of heat flow might be opposite to what would be expected:

In some engines like this one, the "direction of rotation" is non existent as the engine operates linearly, without any rotating parts.


https://youtu.be/2CnNOY1OVhc

What might determine the "direction of flow" of heat under these circumstances is an open question, as far as I know.

Is the direction determined at start-up by either pushing or pulling first? The position of the regenerator? Length of hot chamber? Compression ratio? Why was the expected direction of heat flow apparently reversed in this particular engine, or is this a universal phenomenon with this type of engine, or is it common to ALL thermoacoustic, lamina flow, thermal lag etc. engines.

Maybe it is more common than supposed, I don't know of anyone actually looking into this.

The quote from Ian S C, above is interesting.

If you take away the heat source, and continue to drive the motor in the same direction, it will continue to extract heat from the surrounding atmosphere, and the temperature of the displacer cylinder, or hot cap of an ALPHA motor will reduce, the heat being transfered to the cold end, which will heat up

He says the "displacer CYLINDER" ... will reduce. The whole cylinder?

He also capitalizes "ALPHA". Is that to emphasize that it might be different in other types of heat engines?

Why the apparent reverse "flow" of the heat in Blade Attila's demonstration?

[Tom Booth]

A comment on Blade's channel about this video reads: (translated from Hungarian)

Blade:
It's OK, but why isn't the other side, where I've heated it, cooling down?

Comment:
Because when you compress the air, it heats up, and the compression takes place through the nozzle, and during compression, the nozzle stirs up the air at the end of the pipe, i.e. there is heat exchange with the outside world. When there is expansion, the gas is mixed at the piston, and that is why the wall cools there, because the mixed cold gas takes away the heat.

Blade:
This is only partly true. The entire volume of the gas is compressed and rarefied in a counterstroke. The fibers take up the heat generated during compression, this is also a reason, and the gas during decompression cools, as it passes to the other side, and this is more effective there. The strange thing is that this is the first machine that is as warm as a heat pump as it is as a heat engine! I'm not confused because of the cycle, don't get me wrong!

By "fibers" I assume he means the regenerator or wad of steel wool.

With compression the gas heats up and the regenerator takes in and holds some of that heat, the gas then expands and is colder without the heat that got stored in the regenerator.

The cooled gas takes in some heat from the cold side on the expansion stroke.

I imagine the gas is then in a partial vacuum absorbing heat so the return stroke then compressed some additional heat back to the regenerator side, the process continues.

If this is the "solution" to the mystery, is there any reason this heat pump cycle would work "backwards" in a running engine of this type heated on the hot side as normally operated?

I think this heat pump cycle would continue in the same way on top of the engine cycle, but being driven by the "engine" while the engine simultaneously acts as a heat pump.

Using an aluminum cylinder for the expansion/power cylinder might make the engine/heat pump more efficient as the aluminum would transfer more heat INTO the engine more quickly on the cold expansion side, which could then be transfered to the hot heat storing regenerator.

Perhaps some aluminum fins would allow heat to be drawn in more quickly.

This would LOOK LIKE the cooling fins on the power piston were helping to cool the power cylinder but in reality they would be helping to transfer heat more quickly INTO the power cylinder.

(Now I suppose Matt Brown will want to know what I've been smoking, but I think this is actually clear thinking and critical unbiased observation. )

[Tom Booth]

If this scenario is accurate then "insulating the sink" to "increase the temperature difference" might only be of some limited benefit since heat is needed by the engine when acting as a "heat pump" drawing energy from the "sink" that is not a sink but a secondary heat source.

[matt brown]

If this scenario is accurate then "insulating the sink" to "increase the temperature difference" might only be of some limited benefit since heat is needed by the engine when acting as a "heat pump" drawing energy from the "sink" that is not a sink but a secondary heat source.

Indeed, the sink can be a secondary heat source, and during my deep dive on gammas, I've found various oddities. Granted, I've done this from distinct events, but my study clearly shows that T during expansion can go below "Tlow" and (as you noted recently) that T during compression can go above "Thigh". There's so many variables at work, such as volume ratio, thermal ratio, phasing, speed (adiabatic vs isothermal) that I doubt there's any valid conclusions that can be made about engine direction.

[Tom Booth]

If this scenario is accurate then "insulating the sink" to "increase the temperature difference" might only be of some limited benefit since heat is needed by the engine when acting as a "heat pump" drawing energy from the "sink" that is not a sink but a secondary heat source.

Indeed, the sink can be a secondary heat source, and during my deep dive on gammas, I've found various oddities. Granted, I've done this from distinct events, but my study clearly shows that T during expansion can go below "Tlow" and (as you noted recently) that T during compression can go above "Thigh". There's so many variables at work, such as volume ratio, thermal ratio, phasing, speed (adiabatic vs isothermal) that I doubt there's any valid conclusions that can be made about engine direction.

Well, IMO sending heat through to the sink, or "improving" cooling to help increase ∆T is a red herring.

The object is to bring all the heat vectors together at TDC.

I don't know if that was by design or accident but that seems to be what Stirling engines, or the wide variety of heat engines labeled "Stirling" actually manage to do in one way or another.

The explanation of how the, what did Blade Attila call it? Thermal lag type engine of his he was running as a heat pump: "Lamina Flow"

I just put those comments through Google translate today and it finally came together what I've been puzzling over for years.

TDC is like the launching pad to jettison "heat" over into the "work" realm.

Having a "cold side" marginally cooler than the hot "launch pad" provides a means for cutting off the heat supply when it's not needed, (during compression), but not needed between BDC and TDC so the piston can return easily is one thing. Intentionally dumping heat that might be put to good use by "pumping" it back over to TDC seems like a potentially more productive approach.

I recall there was some device in Rudolph Vuilleumier's patents that worked very similarly. Something like a long tube similar to a fire piston with a regenerator down in the compression end.

Now in a way I'm thinking of a thermo-acoustic type engine as a kind of fire piston.

BTW, in reading through Carnot's unpublished writings again I found a passage where he references fire pistons, but calls it something like: German tinder rod, but from the description he was pretty obviously talking about a fire piston. Really quite fascinating reading.

[matt brown]

The object is to bring all the heat vectors together at TDC.

I used to favor alphas for similar reasons.

I don't know if that was by design or accident but that seems to be what Stirling engines, or the wide variety of heat engines labeled "Stirling" actually manage to do in one way or another.

However, I don't even consider alphas Stirlings any longer. Betas and gammas have no heat vector at TDC, only pressure, and this is a freak engine feature. As a long time alpha guy, I thought this was lame, but no longer, yet I can't find anyone calling out this anomaly. Output off the cold side requires regen and closed cycle inline typical Stirling mindset, but the typical bogus out-of-phase dynamics obscures the secret sauce...a gamma with distinct processes (aka "clean" phasing) does not need any coinciding ratios (compression, expansion, volume, pressure) as commonly believed. A descent gamma is possible (wink-wink) but nothing like the current crop.

[Tom Booth]

(...)
Betas and gammas have no heat vector at TDC, only pressure, and this is a freak engine feature. ...

Not really sure what you mean by that or why you say that.

The power cylinder is generally upside down on an LTD so TDC, or full compression is when the piston bottoms out, all the way in or down towards the displacer chamber, so "top dead center" is perhaps terminology that should be avoided.

But, the displacer starts to lift up, introducing heat during compression, when the piston is moving down towards "TDC".

The "heat vectors": 1) heat of compression, and 2) simultaneous heat input, do, it seems to me, come together at TDC in a gamma. I'm pretty sure this is true in a Beta as well, though I haven't ever owned any Beta type engine to actually tinker around with.

[VincentG]

Well worth a watch.

https://youtu.be/WOmjJFk8rl0

[matt brown]

Awesome find, thanks for sharing. I watched this twice and love this type of applied math vs pages of 'daunting calculus' (Allan Organ's favorite slam). However, this guy needs to slow down and include more descriptions. Nevertheless, he hit on some great stuff...

(1) gamma for calculating anticipated change in P and T per known volume change
(2) how diameter:surface area varies (cube stack diagram)
(3) how flow resistance varies by diameter vs length
(4) argon being monatomic trumps air and is cheap (welding supply shop)
(5) alpha allows higher volume ratio

[matt brown]

The "heat vectors": 1) heat of compression, and 2) simultaneous heat input, do, it seems to me, come together at TDC in a gamma. I'm pretty sure this is true in a Beta as well, though I haven't ever owned any Beta type engine to actually tinker around with.

I'm thinking of a classic gamma like Solar 1 model, not LTD where everything is more obscure. I'm also thinking of an ideal cycle with distinct processes (aka clean phasing) where for Solar 1 type gamma (1) heating in limited to displacer (2) displacer moves gas to hot space with PP at TDC (3) PP descends towards BDC while source supplies input. In this manner, yes, Tmax is when PP is at TDC, but Tmax remains in hot space thruout PP output (heck, ideally Tmax remains in hot space continually). And since (ideally) PP always is Tlow, the only (again ideal) distinction for PP is Pmax at TDC and Plow at BDC. There's also a "Pmid" after PP reaches both TDC and BDC while displacer is moving (during regen).

[Tom Booth]

The "heat vectors": 1) heat of compression, and 2) simultaneous heat input, do, it seems to me, come together at TDC in a gamma. I'm pretty sure this is true in a Beta as well, though I haven't ever owned any Beta type engine to actually tinker around with.

I'm thinking of a classic gamma like Solar 1 model, not LTD where everything is more obscure. I'm also thinking of an ideal cycle with distinct processes (aka clean phasing) where for Solar 1 type gamma (1) heating in limited to displacer (2) displacer moves gas to hot space with PP at TDC (3) PP descends towards BDC while source supplies input. In this manner, yes, Tmax is when PP is at TDC, but Tmax remains in hot space thruout PP output (heck, ideally Tmax remains in hot space continually). And since (ideally) PP always is Tlow, the only (again ideal) distinction for PP is Pmax at TDC and Plow at BDC. There's also a "Pmid" after PP reaches both TDC and BDC while displacer is moving (during regen).

Just to be clear, I think this is the Solar 1, the old Andy Ross book model (?)

Resize_20230808_105442_2384.jpg (54.47 KiB) Viewed 11928 times

The phasing, I would assume, is the same as numerous other similar models of the same type. Compression ratio is higher than an LTD but same 90° "advance". Or offset.

(heck, ideally Tmax remains in hot space continually)

If you say so.

"Ideal" (as far as I'm concerned) means only theoretical, existing in some academics imagination having virtually no correspondence with any real engine, real forces, real physical laws or valid mathematics.

You avoid mentioning it but pretty obviously your talking about "isothermal" expansion which is no more real in a Solar 1 than in an LTD.

The theory that these "ideal" cycles are actually "perfect" or have "maximum efficiency", in my book is also imaginary, and mathematically flawed.

In a real engine heat input does not continue through to BDC and if it did the engine would not be more efficient, it wouldn't run at all.

If a bucket of water stays full in a water wheel where it enters at top and none spills out, all the way down to the bottom, sure, you have a point, but in the real world heat is not a fluid. The heat is converted to work during the power stroke (a good thing IMO) and much beyond TDC the temperature and pressure fall off (also a good thing).

[VincentG]

Awesome find, thanks for sharing. I watched this twice and love this type of applied math vs pages of 'daunting calculus' (Allan Organ's favorite slam). However, this guy needs to slow down and include more descriptions. Nevertheless, he hit on some great stuff...

I was surprised that he did not mention any form of active cooling beyond the aluminum heat exchangers. You would think they get very hot.

It was very informative on how difficult it really is to get the temperature of the heat source into(or out of) the working gas.

These pulse bases system definitely work, but a uniflow design(turbine) would be so much easier to get into the sweet spot, assuming the turbine is capable of sufficient vacuum.

I'm curious to use a car engine at idle to pull a vacuum(easy 20 in/hg) and see what kind of cooling performance can be had dry or evaporating water or a water alcohol mixture in a long tube after a venturi.

[matt brown]

In a real engine heat input does not continue through to BDC and if it did the engine would not be more efficient, it wouldn't run at all.

gamma engine.png (46.09 KiB) Viewed 11882 times

consider DP=displacer piston and PP= power piston

If this was an ideal engine with clean phasing and distinct processes, PP would remain at TDC until DP reaches BDC. Then PP would move downwards expanding gas while input continues in DP hot space. This is a much better Stirling visual to study than any LTD or beta. If you want to complain about Stirlings, don't worry about their lame "isothermal" input, worry about their lame out-of-phase gas dynamics.