Constant volume compression/expansion-displacer chamber analysis-heat powered mechanical amplifier

[Tom Booth]

Tom I do fundamentally disagree with that, but putting that aside...I think its strange to view dumping heat into the cold sink as a waste when the whole reason these engines run is through temperature differences.

The fact that we have unlimited ambient cooling should be taken advantage of to the fullest extent IMO. We are essentially given a free cold power stroke with no net work loss.

To me not taking advantage of this would be like not using hydro electric power from a dam. Or driving your car up a hill and then not coasting down.

The elusive "cold hole" is just an exaggerated version of what we already have with ground temperature or ambient cooling. Its far easier to make heat than cold, and as you have stated(correctly I believe) there is more power to be had from 300k to 600k than from 0k to 300k, for instance.

Home on a break from the shop, so thought I'd look in.

The problem with that ("I think its strange to view dumping heat into the cold sink as a waste when the whole reason these engines run is through temperature differences")

The "whole reason" as it turns out (IMO) these engines run is not a temperature difference. It's a pressure difference between the internal pressure and external pressure.

Adding heat increases the internal pressure to do work, the work output uses up the energy and the pressure drops back down. Your playing against atmospheric pressure.

To.my way of thinking a "cold sink" doesn't utilize or "use up" the input energy through work output, it just throws it away.

Cooling might be useful in moderation but I imagine it would be better to use a heat pump cooling the cold side if needed by transferring the heat over to the hot side, but you can't go below zero pressure, so even that has its limits. Too much cooling and you'll just be robbing heat from the hot side before it can do any work.

If the engine is utilizing all the heat input the pressure drop comes naturally.

On top of that, a Stirling engine is already it's own heat pump.

I just think that it may be possible to design a more powerful heat engine if the whole apparatus for transferring heat to the cold side were eliminated and instead, more attention was given to fully utilizing tbe heat, then the cooling would take care of itself.

I can't say at this point in time that this is a proven theory, I may be all wrong, but it's the direction all my experimenting has led me to and why I designed my "Ringbom" engine conversion without a cold sink or a displacer. Granted, it may not work but I feel I have to at least give it a try.

[VincentG]

I can't say at this point in time that this is a proven theory, I may be all wrong, but it's the direction all my experimenting has led me to and why I designed my "Ringbom" engine conversion without a cold sink or a displacer. Granted, it may not work but I feel I have to at least give it a try.

Absolutely give it a try because the solution usually lies somewhere in the middle of the extremes.

[MikeB]

Tom/Vincent,
Interesting thoughts. As ever, the real world proves more complex than any 'model/theory' however one thing to bear in mind here is that there are generally considered to be two entirely separate methods of heat transfer: Conduction (which seems to be what you are talking about above) but also Convection. It is surely the latter mode that leads to the misnomer that "heat rises" when in reality this 'fact' is based on something that we know well, which is that (unconstrained) hot air expands, and becomes lighter.

In a VERY slow running Stirling, this probably would lead to differences in heat transfer one way up vs the other, but surely what we need to be looking at, for several reasons is 'turbulence' ?

[VincentG]

The primary means of heat transfer to the gas would be conduction after the displacer has moved. Convection should be avoided at all costs, and occurs while the displacer is in transit. The faster the displacer moves, the less convection should take place.

My favorite thing about the gamma displacer in the real world is the turbulence that is introduce by it's movement through the gas. With some thought, the movement of the displacer can be used to not only introduce turbulence but also to direct the flow of gas through the heat exchangers more effectively.

[Bumpkin]

Joining in late here, but in my way of thinking, turbulence is convection. But no matter the wording I’m sure we agree that the air and the heat have to somehow get cozy with each other in a timed process. I know I’ve harped on it for years now, but I think we should give radiant exchange more consideration. Solar heat doesn’t get here by conduction or convection and it has quite an affect, just not on clear air. One of my favorite free-thinkers from some years back in the forum; “Vamoose,” mentioned it would be neat to have “black gas.” That got me to thinking that a thin black regenerative displacer moving through the air, instead of the air moving through it; might do the same thing only better, because it would alternately blanket the hot and cold ends. The blanket would do the work of conduction and convection without any further trickery needed. To my thinking the only question would be whether radiation to/from the blanket would be sufficient. I think it might, and additionally the blanket could contact and dwell at each end for an instant of conduction too.

Bumpkin

[VincentG]

Right Bumpkin maybe just semantics. But like you say, whatever it takes to heat the air faster.

I really like the idea of radiant heating. I've thought that maybe a glass viewport that exposes an open flame or sunlight with a shutter type system could aid in rapidly heating the gas. My research has found that the air is directly heated to some extent by radiant. Gas seeding(black gas, etc) would help with this, but need to be careful to not raise the specific heat much.

At the end of the day I don't think it's a big concern. For this amplifier scheme to be most effective it would used a very small volume of gas, charged to high pressure. So the heat exchanger to volume ratio would be much higher than normal.

[VincentG]

The following is an excerpt taken from
Stirling Engine Design Manual by William R. Martini
Second Edition January 1983
Prepared by NASA for U.S. DEPARTMENT OF ENERGY
Conservation and Renewable Energy
Office of Vehicle and Engine R&D

As the gas is transferred at zero total volume change from the cold
space to the hot space the pressure rises. This pressure rise results in a
temperature increase in the gas due to adiabatic compression.
Therefore, at the end of the transfer process the mixed mean gas temperature
in the hot space will be higher than 900 K. Point 3 is calculated for all the
gas to be exactly 900 K. Adiabatic expansion then takes place. Then by the
same process as just described, the transfer of the expanded gas back into the
cold space results in a lower gas temperature than 300 K at the end of this
stroke. The computational process must be carried through for a few cycles
until this process repeats accurately enough. This effect will be discussed
further in Section 5.1.6.

Unfortunately, it was not explained further in any certain terms(at least in my eyes). It makes rational sense to me. The challenge then is to put this effect to good use.

[Tom Booth]

The following is an excerpt taken from
Stirling Engine Design Manual by William R. Martini
Second Edition January 1983
Prepared by NASA for U.S. DEPARTMENT OF ENERGY
Conservation and Renewable Energy
Office of Vehicle and Engine R&D

As the gas is transferred at zero total volume change from the cold
space to the hot space the pressure rises. This pressure rise results in a
temperature increase in the gas due to adiabatic compression.
Therefore, at the end of the transfer process the mixed mean gas temperature
in the hot space will be higher than 900 K. Point 3 is calculated for all the
gas to be exactly 900 K. Adiabatic expansion then takes place. Then by the
same process as just described, the transfer of the expanded gas back into the
cold space results in a lower gas temperature than 300 K at the end of this
stroke. The computational process must be carried through for a few cycles
until this process repeats accurately enough. This effect will be discussed
further in Section 5.1.6.

Unfortunately, it was not explained further in any certain terms(at least in my eyes). It makes rational sense to me. The challenge then is to put this effect to good use.

This "hysteresis" is usually considered a "bad" thing. An probably inevitable heat loss due to adiabatic compression.

In a "normal" Stirling engine with both a hot chamber or side and a cold chamber or side, the "extra" heat generated from "heat of compression" causes an elevation in temperature throughout the working fluid including in the cold side, which is viewed as detrimental. Heating up the cold side is not good. That extra heat will have to be removed or will degrade the temperature differential.

However, IMO, a Stirling engine already mostly resolves this situation by shifting all the gas to the hot side 90° prior to this high compression temperature rise. The "extra" heat is just added, or in effect "pumped" over to the hot side.

Likewise, the displacer again shifts during expansion cooling so the "extra" cold pulls heat away from the "sink" just cooling it further.

A thermoacoustic or thermal lag type engine, as I think has been demonstrated by recent experiments, has no "sink" to lose the "extra" heat to. During compression ALL the gas is pushed back into the hot chamber.

Another dilemma, as far as trying to utilize this hysteresis effect is, IMO, Stirling engines, contrary to popular opinion, do not run on a temperature difference. A Stirling engine runs on heat.

You have a gas at some initial ambient temperature, expand it with heat to do "work". Essentially that is all. The so called "compression" is really a "contraction" which is a natural consequence of the conversion of the heat into work.

So in actuality, the "hysteresis" is a myth. How can you have this "heat of compression" resulting from contraction? The "analysis" based on so-called "ideal" gas behavior is mostly a mathematical fiction in all likelihood.

So how to utilize an effect that doesn't actually exist?

Not to say that heat of compression and cooling from expansion is not real generally, it's how a heat pump operates.

I think that generally, Stirling engines already take advantage of this effect to a greater or lesser extent as described above.

It helps, I think, to have an idea of what is actually going on so as to more clearly determine ways to maximize the advantage.

The usual approach is to escort "waste" heat through or over to the "sink" for disposal as quickly as possible. To me that seems counterproductive. Thermodynamics "LAW" insists upon it as mandatory.

Pushing heat back towards the heat source does not really seem like the best possible solution to me either. To one degree or another you have a heat engine/heat pump at least partly working against itself.

I'm not looking for 100% efficiency or "perpetual motion". I'd be content with a practical, working, power producing heat engine of just about any kind.

Ultimately, I'm tending towards the conclusion that the reason these engines are not already available at every corner hardware store has more to do with politics and economics than any actual inadequacy in the engines. They are already very practical and very efficient, and have been for over a century, and have only been improving. NASA has already sent some off beyond the solar system somewhere, I think. They can run continuously for 30 years without maintenance. Such high efficiency Stirling engines COULD BE, or could have been mass produced at a low cost for decades already. The technology has been proven over and over and over again.

INFINIA SOLAR Stirling engines were ready for "automotive-scale manufacturing" ten years ago.

http://santamarta-florez.blogspot.com/2 ... ipcard&m=1

Where are they now?

Before INFINIA there was Stirling Engine Technologies.

https://youtu.be/_rl1H-53Mks?si=o4bwF6bHFwuJygtQ

Before that was Advanco

https://youtu.be/vGdT9w4ubLc?si=lrX5udiLaogDH7uQ

As far as I know, about the only one of these that hasn't been turned into scrap metal is down in my workshop. One of these, but without the dish.

https://youtu.be/VEpq-WCTOrM?si=hzkvXRWWXW8fNqL7


Still not sure what if anything I might be able to do with it, without any dish, control panel or software to operate it.


https://youtu.be/6-OlbCAVBdo?si=yFpVEb837UQB_R4P

How much more advanced could this technology get?

I have a feeling our efforts to improve our little tin can engines must be rather amusing.

[VincentG]

Just to make sure we are on the same page, they are specifically referring to a gain in temperature above and below the source temperatures during constant volume heat addition or removal. So there is no work being done (by a volume change), except internally on the gas that results in an extra pressure and temperature beyond what one would calculate based on PVT alone. So an admitted deviation from the ideal gas law.

The heat source would then become a heat sink for a brief moment just like the cold sink would become a heat source.

It seems that the effect would be a result of a defined constant volume adiabatic cycle. This doesn't exactly occur without dwelling the power piston as well as the displacer, or in a displacer chamber with no piston as shown above. So this brief moment of temperature extremes may or may not go unutilized. If nothing else, it could serve to reduce the input energy required for a given power output. Philips called this effect a semi-adiabatic cycle.

How much more advanced could this technology get?

I have a feeling our efforts to improve our little tin can engines must be rather amusing.

I am under no delusion that we are uncovering anything groundbreaking. My only aim is to simplify this advanced tech into a DIY friendly design. Any other design goal will just keep this stuff reserved for industry as it has been for decades.

[Tom Booth]

Just to make sure we are on the same page, they are specifically referring to a gain in temperature above and below the source temperatures during constant volume heat addition or removal. So there is no work being done (by a volume change), except internally on the gas that results in an extra pressure and temperature beyond what one would calculate based on PVT alone. So an admitted deviation from the ideal gas law.

The heat source would then become a heat sink for a brief moment just like the cold sink would become a heat source.

It seems that the effect would be a result of a defined constant volume adiabatic cycle. This doesn't exactly occur without dwelling the power piston as well as the displacer, or in a displacer chamber with no piston as shown above. So this brief moment of temperature extremes may or may not go unutilized. If nothing else, it could serve to reduce the input energy required for a given power output. Philips called this effect a semi-adiabatic cycle.

....

The cycle is described as "sinusoidal", so, the "analysis" "assuming" a "constant volume" and so forth is idealized. Schmidt analysis, Finkelstein analysis whatever. You can't, or IMO shouldn't, take these descriptions too literally.

I'm cutting through the (my opinion) BS "analysis" that "assumes" this, that, and the other thing.

Admittedly I have not read the entire paper, but there are real engines that actually run, then there is these kinds of academic analysis that fudge things this way and that and assume things that are not actually real.

That may or may not be the case here, but it appears to me at a glance that they are actually talking about "hysteresis", in a regular Alpha(?) Stirling with sinusoidal motion. If that is the case then any "constant volume" is decidedly brief at TDC and BDC, but in an Alpha that isn't actually clear, at any rate too brief for any significant "constant volume" heat transfer.

Maybe I'll back track a few more paragraphs to get the context, though, generally, I find this kind of a academic fantasy "analysis" rather fruitless.

[VincentG]

I see what you mean. I read the whole report and despite the comprehensive equations, they admit to them being incomplete as well as assumptive of many things. The author mentioned that most of the existing experimental knowledge is held by private companies and therefore not in the report.

[Tom Booth]

Not to throw the baby out with the bath water, did you happen to take a look at that "monothermal" website I posted a link to earlier in the tread?

It may inspire some ideas as far as "The challenge then is to put this effect to good use". It appears to claim there is a way to do just that, sort of, I think, maybe.

The basic idea seems to be to shunt gas back and forth through a regenerator, which, theoretically takes next to no energy. Then, as you referenced, you get a greater ∆T than the given source and sink .

If you can then, so to speak, skim off this higher grade temperature difference to run a Stirling engine, the process could, according to the "monothermal" theory, be continued.

Kind of similar to my "Maxwell's Demon" contraption in a way.

viewtopic.php?f=1&t=5338&p=18503&hilit= ... s+d#p18503

Can you create a ∆T without any energy input?

[Tom Booth]

I shouldn't say without any energy input, as there is heat input.

The conclusion of the "monothermal" concept is, to summarize; (my understanding or interpretation):

Start with ambient as a heat source and some manufactured cold, such as what? Ice, dry ice,liquid nitrogen...

Between ambient and the "cold hole" you place a regenerator as illustrated:

Resize_20230926_120859_9621.jpg (50.21 KiB) Viewed 4816 times

As the displacer pushes the working fluid through the cold hole the gas, as it passes through reaches the temperature of the cold hole, then as additional gas follows, the contraction of the additional gas causes an expansion and cooling of the gas that has already passed through making it colder yet. Colder than the "cold hole".

If the ∆T of the cold hole and the COLDER temperature produced is then utilized to run the Stirling engine to keep the displacer in motion, we use the heat from the cold hole, converting that heat to work, which can only reduce the temperature of the cold side generally even further.

Heat, being replenished or "pumped" from the hot side is theoretically inexhaustible.

Having been translated from the French, I may be misinterpreting something, but that seems to be the gist of it.

On the hot end you have a similar ∆T between the ambient and the above ambient effect.

As heat passes from the regenerator through the ambient heat source it is heated to ambient, which heat is freely available from the environment, of course.

As additional working fluid passes through and is heated, the additional gas expands which causes the compression of the gas already heated to a temperature above ambient.

The suggestion is, I think, that this above ambient heat source can also be utilized without penalty, any "waste heat" passing through from the ambient-plus-heat-of-compression heat source simply returns to the environment.

The main thing to take away I think is that you are not utilizing the ∆T generated by the heat pump at the hot and cold ends dumping "waste heat" into the cold hole. Instead, you are just "skimming" the heat of compression off the top, or alternatively, the heat from the "cold hole", converting some of that to work, which can only increase the overall ∆T.

I've been examining this scheme, trying to discover the major flaw in reasoning that would prevent this ambient heat engine/heat pump from actually operating in real life.

I'm not even 100% sure I'm actually explaining it correctly, or as the inventor intended. Again, being my interpretation of a translation, though French to English presents no real difficulty.

[VincentG]

That was a fascinating read, missed that the first time you posted it.

"It is not uncommon for engineers to accept the reality of phenomena that are not yet understood, as it is very common for physicists to disbelieve the reality of phenomena that seem to
contradict contemporary
beliefs of physics" - H. Bauer

This is a great quote they added at the end.

Maybe the constant volume gamma displacer chamber can demonstrate this effect measurably. If I track the pressure swings with a good sensor and they start to become larger with time, that would be a good start.

It begs the question, will an idealized Stirling cycle exaggerate this effect even more? It would not be hard to add a dwell cycle to the power piston on the new model LTD. Combined with the displacer dwelling at either end, the mechanical end of this ideal cycle can be realized.

[Bumpkin]

“As the gas is transferred at zero total volume change from the cold
space to the hot space the pressure rises. This pressure rise results in a
temperature increase in the gas due to adiabatic compression.
Therefore, at the end of the transfer process the mixed mean gas temperature
in the hot space will be higher than 900 K. Point 3 is calculated for all the
gas to be exactly 900 K. Adiabatic expansion then takes place. Then by the
same process as just described, the transfer of the expanded gas back into the
cold space results in a lower gas temperature than 300 K at the end of this
stroke. The computational process must be carried through for a few cycles
until this process repeats accurately enough. This effect will be discussed
further in Section 5.1.6.”

OK Martini is a big shot so I’m open to persuasion, but —

I’ve always thought the power piston compression/expansion is mostly prior to the displacer shift, so you can only transfer half of your available temperature difference before you run out of difference. The rest is already taken by the pressure change. So for instance for a 300K to 600K source/sink (100% difference) you would only want at most a 50% volume change engine. — But I dunno.

Bumpkin