Stirlingengineforum.com

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VincentG wrote:even automotive air conditioners routinely ice over on the low pressure lines and can go quite a bit below freezing even in a hot engine bay,

If your automotive cold-side line is freezing over on the outside, most likely there is something wrong with the system. Often it is low refrigerant pressure. It allows boiling at a lower temperature.

Automobile AC, does often drip liquid water from the evaporator coils..

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Fool wrote: ↑Tue Nov 05, 2024 10:09 pm .
... Classical theory is not just ideal gas law.

Weak, LOL

Fool wrote: ↑Tue Nov 05, 2024 10:20 pm .

VincentG wrote:even automotive air conditioners routinely ice over on the low pressure lines and can go quite a bit below freezing even in a hot engine bay,

If your automotive cold-side line is freezing over on the outside, most likely there is something wrong with the system.
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Yeah, like it's working, LOL Ha! Ha!

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https://www.thespruce.com/why-window-ai ... o%20freeze.

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Fool wrote: ↑Wed Nov 06, 2024 2:35 am .

https://www.thespruce.com/why-window-ai ... o%20freeze.

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Your Google highlighted text from that link:

Your air conditioner depends on the free flow of air past the coils to properly work, and if dust, debris, or other obstacles build up on the coils themselves, the chilled refrigerant in the coils can't properly absorb heat from the room and instead may absorb it from moisture on the coils, causing it to freeze.

Like I said, if the heat flow is reduced, by being blocked by debris, or just because less is available, the heat source temperature (the Stirling engine/room) is lower or whatever, the evaporator temperature will naturally get colder and colder and colder. If the air is humid the evaporator will get covered with frost and ice.

In VincentG's application, that is not a bad thing. First of all the air in the Stirling in contact with the coils should not be humid.

The Stirling engine is the "debris" if you will on the evaporator coil. The heat pump evaporator gets progressively colder and colder. With no frost and ice to block the tubing, much colder than freezing.

Likely, at that point, the cold side of the Stirling engine is now quite cold enough and the heat pump can be shut down while the Stirling engine continues to run and run and run... converting heat from the room into useable power, not only more efficiently, due to the greater ∆T but without the parasitic load from the AC.

Now, with the supposed "delivery" of heat to the heat pump greatly restricted due to the engine converting that heat, the engine will run for a good long time until tbe evaporator on the AC warms back up.

Then the AC can run again briefly and the system repeats the on/off cycle.

Of course, when the building is cold enough, the system can turn off entirely.

But all the time the Stirling is running by itself, no power is being used. The system is generating power.

So the evaporator temperature getting colder to the point of potentially icing up is a good thing. The system can simply generate "free energy" for a while, but it is still working just fine.

Thanks Tom, well said. Tc could easily get below 0F with a modern heat pump and as I have found, these extreme low temperatures greatly benefit the Stirling cycle when the chamber is designed right.

VincentG wrote: ↑Thu Nov 07, 2024 8:25 am Thanks Tom, well said. Tc could easily get below 0F with a modern heat pump and as I have found, these extreme low temperatures greatly benefit the Stirling cycle when the chamber is designed right.

I don't disagree that Stirling engines benefit from cold or increased temperature difference, "designed right" (not really sure what you mean by that) or just a conventional design.

What I don't agree with is the conventional explanation for why this happens: i.e. : the "gradient" is "steeper" so the heat runs through the engine faster, more rapidly or in greater volume.

Such an explanation contradicts conservation of energy. Heat "running through" in greater volume would reduce efficiency. Heat "running through" FASTER would reduce efficiency.

More heat going through the engine to the "sink" more quickly and in greater volume improving efficiency or power output would make sense if heat were a fluid (Caloric theory) and if a Stirling engine were actually some kind of heat turbine or paddle wheel intercepting the "flow" of heat.

But if heat is energy, then more heat flowing to the "sink" more rapidly would mean more energy was being wasted. More waste heat "flowing through" would lower efficiency.

So, the water wheel analogy IMO is very misleading and should be discarded.

A greater temperature difference allows "room" for more expansion so more heat can be converted to work, but heat is still not "flowing through" to the sink.

By designed right I mean that unless cold space volume can go to zero, extreme low temperatures hurt more than help as it will act like a severe thermal short.

It helps not because of the temperature spread but because the internal air density goes up, along with power, since we are so close to a total vacuum at STP.

VincentG wrote: ↑Thu Nov 07, 2024 7:14 pm By designed right I mean that unless cold space volume can go to zero, extreme low temperatures hurt more than help as it will act like a severe thermal short.

It helps not because of the temperature spread but because the internal air density goes up, along with power, since we are so close to a total vacuum at STP.

Still not sure what you mean.

Do you see this as a problem?

Or would "cold space volume going to zero" be different from conventional Stirling engine design? The displacer, for example, occupying the cold space effectively renders the cold space non-existent.

Also a "thermal short" implies a circuit or "flow" of heat.

If gas particles are their own force carriers then no such circuit exists.

In other words, if gas expands rapidly near the hot plate and the expanding hot air volume does not actually leave the vicinity but only pushes against the cold volume, which in turn pushed against the piston, then the work output causes the hot volume to loose energy and collapse or shrink back, then the dense cold volume just acts as a kind of inert air piston. Kind of like the central part of a Newton's Cradle that merely acts as a force transmitter between the two end points.

Anyway, my ramblings aside, what design changes would you propose, or have you already planned or implemented?

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VincentG wrote: ↑Thu Nov 07, 2024 8:25 am Thanks Tom, well said. Tc could easily get below 0F with a modern heat pump and as I have found, these extreme low temperatures greatly benefit the Stirling cycle when the chamber is designed right.

Yes but they are equally bad a hurting the heat pump.

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Fool wrote: ↑Thu Nov 07, 2024 10:28 pm .

VincentG wrote: ↑Thu Nov 07, 2024 8:25 am Thanks Tom, well said. Tc could easily get below 0F with a modern heat pump and as I have found, these extreme low temperatures greatly benefit the Stirling cycle when the chamber is designed right.

Yes but they are equally bad a hurting the heat pump.

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There is no harm to a heat pump by going below zero fahrenheit.

Every household refrigerator is a heat pump and has a freezer compartment. Many dedicated freezers exist in households and the frozen food section of supermarkets. Even many a mini-fridge has a freezer compartment, and as already discussed, heat pumps designed for cold climates must operate with the condenser well below freezing to draw heat from the sub-Zero air.

You're an incurable moron. Your just wasting time and delaying progress with your endless idiotic interjections.

Fool you are confusing the outside temperatures that a heat pump needs to compete with and the internal temperatures it can reach. If the engine reduces the heat going to the the evaporator, it can reach lower temperatures. This is better for the engine and means less heat needs to be removed from the refrigerant in the condenser.

Tom what I meant is that every Stirling engine I’ve seen including the Essex does not bring the cold space to true zero. This means that if the displacer is at the cold end of the displacer, there is energy flowing through the gas from the hot space. This is a waste of heat but also limits the max pressure potential for the power stroke.

Matt has clearly demonstrated that a cold connected piston is losing energy. Well a hot connected piston is also losing energy if the cold space can’t be eliminated for the expansion stroke.

VincentG wrote: ↑Fri Nov 08, 2024 6:31 am
Tom what I meant is that every Stirling engine I’ve seen including the Essex does not bring the cold space to true zero. This means that if the displacer is at the cold end of the displacer, there is energy flowing through the gas from the hot space. This is a waste of heat but also limits the max pressure potential for the power stroke.

Matt has clearly demonstrated that a cold connected piston is losing energy. Well a hot connected piston is also losing energy if the cold space can’t be eliminated for the expansion stroke.

That all may be correct. I'm still not sure what you mean or what changes would be required.

Not to be contrary, but I think I have found experimentally, as well as logically based on actual "Real" gas behavior that reducing the cold air volume doesn't seem to matter all that much.

Take the experiments with the little acrylic/magnetic engine insulated with silica aerogel.

https://youtu.be/i9nz0vt7eQA

The displacer barely lifts away from the hot plate leaving a relatively huge cold space above the displacer, probably 20 times more cold air volume at minimum than the maximum hot air space volume.

This however did not deter the engine from running.

Temperature readings showed negligible heat transfer through the engine to the cold side even after several hours of continuous operation, and without even incorporating a regenerator. Adding a regenerator of course, improved hot/cold air separation further. (Which appeared to result in a below ambient temperature drop on the cold side).

I remember debates on this topic going back to when I first joined this forum a decade ago and there was no consensus.

Anyway, I'm pretty convinced at this point that the hot and cold gases are much less prone to mixing than generally supposed.

The "slug" of cold air seems to remain largely independent. The hot gas in contact with the hot plate apparently does not instantly travel and mix with the cold, infact it likely barely penetrates a nanometer, the "mean free path" on the macro-scale is virtually non-existent.

The pressure evolves throughout the working fluid, apparently without any necessity for the "hot molecules" to penetrate through the cold.

These are just my general impressions for what it's worth. Comparative experimenting with different configurations/engine designs could reveal what difference, if any, a total elimination of the cold volume makes.

Personally, a few years ago I agreed wholeheartedly, but I was also under the "kinetic theory" assumption that the gas molecules zip around freely at the speed of light so that the temperature equalizes instantaneously throughout the displacer chamber. This seems to be far from the truth.

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Tom Booth wrote:The displacer barely lifts away from the hot plate leaving a relatively huge cold space above the displacer, probably 20 times more cold air volume at minimum than the maximum hot air space volume.

So what you are saying is that very little heat gets into a LTD engine. I take that to mean very little heat is needed to run the engine that is producing zero work output. Duh! So by that nature the heat coming out of the cold side would be so low as to be almost unmeasurable. No wonder you measured nothing. Your crude measuring methods were far too primitive. Thus all your experiments are inconclusive.

Proved all your conclusions fraudulent and yourself the fool in one sentence. Clap clap clap...

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Fool wrote: ↑Sun Nov 10, 2024 6:25 am .

Tom Booth wrote:The displacer barely lifts away from the hot plate leaving a relatively huge cold space above the displacer, probably 20 times more cold air volume at minimum than the maximum hot air space volume.

So what you are saying is that very little heat gets into a LTD engine.

No, not at all.

The surface area for heat exchange is about equal top and bottom. The aluminium bottom is at least 2000 times more thermally conductive than the acrylic+aerogel top.

Obviously, given that the engine is running heat is entering at least at the metal/air interface. The air inside the engine is itself a very poor heat conductor so the air molecules not needing to conduct heat but only transmit pressure helps explain why any Stirling engine using air as a working fluid could ever run at all.

All that dead air space above the displacer is mostly just additional insulation that cannot transfer any heat, but CAN transfer PRESSURE.

The heat transfer is likely very localized at the aluminium/air interface - surface area so very little movement of the displacer is necessary to run the engine no-load.

Under load, a higher lift on the displacer could force additional air into direct contact with the hot metal plate but heat transfer to the piston would be non-existent, only indirect pressure would influence the piston.