"Thermoacoustic" Stirling - theory of operation

[VincentG]

In a flash there I thought maybe that forced expansion could be used in stead of water cooling. Whether you're powering a pump or using that work to "overstretch" the fluid to cool it further. I wonder which would be more effective and efficient.

Thats the ticket Jack. We are lucky to live in a world that is very close to zero pressure. There is no realistic limit to high pressure, but there is a realistic floor that can be used to our advantage(cooling). Once positive pressure(power) is high enough over atmospheric, it becomes beneficial to use the over expansion(low pressure) for cooling. Pressure and temperature can continue to rise to incredible numbers during compression(think fire piston, lightning, nuclear, etc.), while during expansion we are already close to 0, and so trading some power for cooling should become not only practical but greatly beneficial to net power.

[Tom Booth]

In a flash there I thought maybe that forced expansion could be used in stead of water cooling. Whether you're powering a pump or using that work to "overstretch" the fluid to cool it further. I wonder which would be more effective and efficient.

Thats the ticket Jack. We are lucky to live in a world that is very close to zero pressure. There is no realistic limit to high pressure, but there is a realistic floor that can be used to our advantage(cooling). Once positive pressure(power) is high enough over atmospheric, it becomes beneficial to use the over expansion(low pressure) for cooling. Pressure and temperature can continue to rise to incredible numbers during compression(think fire piston, lightning, nuclear, etc.), while during expansion we are already close to 0, and so trading some power for cooling should become not only practical but greatly beneficial to net power.

Just musing but...

With cooling to about 80°K (-315°F) you could have a "wet thermoacoustic" engine leveraging the power of phase change of liquid air.

http://hyperphysics.phy-astr.gsu.edu/hb ... iqair.html

Interesting that:"

The liquification point of a mixture of gases with different boiling points will typically be between the boiling points of the pure gases. The boiling point of pure oxygen, O2, is 90.6 K and the boiling point of nitrogen, N2, is 77 K. If air is cooled at atmospheric pressure, it remains a gas until the temperature reaches 81.6 K, 9 K below the liquification temperature of pure oxygen. It will be completely liquified at about 79 K, 2K above the liquification point of pure nitrogen

That seems to be what Tesla was aiming for. Not "thermoacoustic" necessarily but some "cold hole" engine (or turbine or something) operating down to the liquid air temperature range.

https://youtu.be/woR5sSKSkKQ?si=LDaxoE72RibKqYHM

WooWoo video, but no question he was working on something of the sort as he plainly stated in his 1900 "Increasing Human Energy" article.

[VincentG]

This guy seems to have really developed something. Its so simple in layout you know it's a good idea. The one tank of water is heated while there is a vacuum pulled in the other cold tank. That causes the hot water to boil easily, power the turbine and condense in the cold tank, preserving the vacuum.

https://youtu.be/P3VplkAD_-M?si=QhEa4bOzaFGjgX4b

[Tom Booth]

This guy seems to have really developed something. Its so simple in layout you know it's a good idea. The one tank of water is heated while there is a vacuum pulled in the other cold tank. That causes the hot water to boil easily, power the turbine and condense in the cold tank, preserving the vacuum.

https://youtu.be/P3VplkAD_-M?si=QhEa4bOzaFGjgX4b

IMO a version of the "wet thermoacoustic engine", at least in principle. Or the previous video with the crushed 55 gallon drums.

Imagine the steam in the collapsing drum, cooled by ice, condensing to create a vacuum, but instead of just collapsing drums, use the vacuum to power a steam turbine.

You get a partial vacuum in the steam boiler as well, which lowers the boiling point.

They have gotten the boiler down, well below 100°C while spitting snow out the turbine at times, all while powering a considerable load.

It started as an "open source" project, but some financial backing seems to be taking the project in the direction of possible commercialization, and, so, less and less "open" as time goes on it seems.

There is, or was a forum, but there has been no activity there in several months:

http://www.energeticforum.com/forum/ene ... ce-project

[VincentG]

Yup, he's probably gonna get a big pay day out of that one. It would be fun to try and recreate, doesn't seem too hard with the benefit of hindsight. I've heard that you can't patent natural phenomena, so I wonder if something like that and any other cold hole style system is considered as such.

[Tom Booth]

The law of diminishing returns has something to do with this. I think because catching and using all of the heat would just take too long. And forcing it to expand takes work.

In a flash there I thought maybe that forced expansion could be used in stead of water cooling. Whether you're powering a pump or using that work to "overstretch" the fluid to cool it further. I wonder which would be more effective and efficient.

There is an interesting phenomenon in connection with what you refer to as "forced expansion" or the gas cooling by being "overstretched".

When a flywheel or the momentum of a heavy piston continues to expand the gas causing it to cool...

This description or conceptualization of what is happening is apparently not correct - from the point of view of the gas or working fluid.

Now this is not exactly my idea, but the way it has been presented in thermodynamics demonstrations.

Say you have a piston in a cylinder and the piston is pulled out "stretching" or mechanically expanding the gas. Forced expansion.

Well, apparently the gas itself does not "know" it is being "forced" to do anything. The gas "thinks" it is doing work to push the piston and actually cools due to it's own work output as it pushes against and moves the piston.

You, (or the flywheel, or piston momentum), by pulling the plunger are HELPING to remove some of the outside atmospheric pressure that would prevent the gas from expanding, but as the piston moves out, increasing the volume, the gas is moving much faster than the piston so, is still colliding with the piston, "helping" move the piston, and doing "work" to move the piston, and that is why the gas cools, due to its work, pushing out the piston, although to you, it feels like you are doing all the work "forcing" the gas to expand.

Does that make a difference?

I think it does, because if the gas was not doing any work it would not actually be loosing, or using up energy. The heat or thermal energy would not be removed from the gas, it would still be there spread out more in the larger volume.

But from what I've described, if true, the "overstretched" working fluid does still actually continue to give up internal thermal energy to the piston and any load being driven by the engine. Heat is still being transformed into "work".

[Jack]

Once you're going below ambient temperature and pressure you're putting in the net work to get the gas that far. And the last little bits of heat want to transfer into work very slowly, if you're forcing that you're again putting in the net work. You're basically sucking the heat out in stead of letting it transform into work.

So what I'm trying to say is by over stretching you're creating a bigger Delta so the heat is taken out of the gas faster. You're creating it by putting in work.

That's my idea of it at least.

[matt brown]

Well, apparently the gas itself does not "know" it is being "forced" to do anything. The gas "thinks" it is doing work to push the piston and actually cools due to it's own work output as it pushes against and moves the piston.

Hmmm, and how about 2 identical engines, except piston A has 2x the friction of piston B...

[Tom Booth]

Well, apparently the gas itself does not "know" it is being "forced" to do anything. The gas "thinks" it is doing work to push the piston and actually cools due to it's own work output as it pushes against and moves the piston.

Hmmm, and how about 2 identical engines, except piston A has 2x the friction of piston B...

Whatever the case, from what the guy says in this video for example:

https://youtu.be/PMKPZuCj9a0?si=1BkYHpqmpW1PW58D

as long as the piston continues to move, the gas molecules are "hitting a moving target" and loosing energy.

If a gas expands into a vacuum, not doing any work, the temperature does not fall, so it is not a matter of occupying more space, it is the way the gas molecules "hit a moving target" that causes them to "come away with less energy".

Also the instructor is pulling the piston forcing the gas to expand but the gas molecules are still hitting a moving target and cooling.


If the friction in your second engine is not too great so that the engine continues to run, and there is a crank and flywheel so the distance the piston travels is the same then I would guess the piston would be more difficult to move and the engine would run slower but the work output for the gas would be greater. The gas in the engine would probably cool more, the same as it would with an external load.

Though some of the work "output" due to friction would no doubt heat up the cylinder, so the cooling would not be as effective as it might be due to an external work output, such as turning a generator.

It reminds me though of the time I ran an LTD on ice with lapping grease compound on the piston.

I had run the same identical engine on ice in the same way many times before and the ice had never re-froze. But that time with the additional friction and work involved, the engine apparently ran cooler than usual so that the ice kept refreezing and sticking to the bottom of the engine.

Since the cylinder was aluminum, I guess whatever heat was generated by friction dissipated quickly.

This was not an experiment and the re-freezing of the ice was completely unexpected, so I was not video recording, but when the engine kept sticking to the ice, because the ice kept re-freezing, I finally got my phone and recorded this video, just in case the ice re-froze again for a third time. It did.

https://youtu.be/2b2dIR8Eql8?si=Ly2ahBfegYzkKo0r

While uploading the video, and writing up a description, the refreezing of the ice happened for a fourth time.

The engine was not entirely identical though. The new piston I was lapping in with the grinding paste was DIY epoxy (JB Weld), rather than graphite, so was less heat conducting than the graphite piston.

That was 3 years ago. I really should do some actual controlled experiments to figure out exactly what was going on, now that you've reminded me of that.

[Jack]

So what you're saying there is that the engine with higher friction would run slower but have more work output by the gas?

I interpret that as saying that given two engine with identical displacement, the one running slower would use more of the heat available? You're proposing the cooling of the gas is linked to time spent decompressing the gas and not linked to the change in volume? That would be conduction, not all work.

[Tom Booth]

So what you're saying there is that the engine with higher friction would run slower but have more work output by the gas?

I interpret that as saying that given two engine with identical displacement, the one running slower would use more of the heat available? You're proposing the cooling of the gas is linked to time spent decompressing the gas and not linked to the change in volume? That would be conduction, not all work.

Conduction, during the heating and expansion of the gas would be thermal energy conducted into the gas which would raise the temperature of the gas.

If the piston required more energy to move a given distance due to friction, the piston would move less each time a gas molecules collided with the piston. Less motion of the piston per impact. More impacts over a given time, more impacts for a given distance traveled. More energy expended, more cooling as a result of work output.

Slower movement of the piston might allow extra time for more heat to be conducted into the gas to heat the gas, during the first 1/2 of the power stroke especially if more heat is available.

So there is an accounting of thermal energy being conducted in, to heat the gas, and thermal energy going out as "work"; heat being converted to work cooling the gas.

The change in volume per power stroke is fixed for an engine with a crankshaft and connecting rod. It could vary somewhat with a "free piston" engine.

So, during the power stroke thermal energy is conducted into the working fluid, increasing the temperature but CONVERTED into work, not conducted which results in lowering the temperature (or AVERAGE kinetic energy of the gas).

You would not have cooling by conduction.

For most Stirling engines the heat input is limited or constrained in some way during the power stroke, especially during the last half of the power stroke. If there is a displacer, the heat is physically cut off by being covered by the displacer. In a thermal lag engine the heat input is limited to what has accumulated in the steel wool and much of the gas has expanded out of the heating zone so heat can no longer be conducted into the gas. Heat is no longer available, or is less and less available as the gas expands.

In any event, any cooling is the result of work output, which CONVERTS thermal energy to "work". That is not cooling by conduction. Heating is by conduction. Cooling is by work output.

During the last half of the expansion heating by conduction is terminated and it is all, or nearly all conversion of heat into work and cooling by work output.

You could get some cooling by conduction depending on how effectively or how thoroughly the heat was converted into work, but apparently actual readings and measurements taken of working engines during operation indicates that the temperature of the gas falls below the temperature of the cold heat exchanger.

Such sub-ambient cooling, or cooling below the temperature of the cold "sink" could not possibly be due to cooling by conduction.

In a reciprocating engine the great majority of the heat input into the gas takes place at, or around TDC and heat input is often physically cut off well before full expansion or before the power stroke is complete.

IMO, any cooling by conduction during the power stroke is unlikely or very minimal in a functional engine. That is, an engine that actually works and runs.

Actual measurements of working engines indicate that the cooling is in excess of what would be possible as a result of conduction.

Heat cannot be conducted out from a gas that is already colder than the "sink" temperature due to the internal thermal energy having already been lost to work output.

If you have heat simply being conducted into the engine and back out of the engine, that would not be a functional engine since no heat is being converted to work.

The Carnot theory though, is exactly that.

According to the Carnot theory, the heat engine is like a water wheel and heat is like the water that goes in and then comes out the other side at a "lower level" or lower temperature.

Yes I do spend a lot of time, perhaps too much time, trying to point out how absurd that is, but I think it is an important point to drive home since this Carnot theory is still being widely taught and advocated.

A Carnot engine doesn't exist because the Carnot cycle is not at all functional. It is, IMO, in no way "ideal". It is completely non operational as ALL the heat is conducted into and then conducted back out of the engine. ZERO heat is converted into any work output. A Carnot cycle engine requires 100% outside work input to do anything at all. How is that "ideal"?

[Fool]

The caloric/heat coming out of the bottom of a Carnot Engine contains less energy than that going in the top, as depicted by a change in temperature, as Carnot plainly explained in his papers. This is a result of being converted to work, and corresponding temperature drop. Lower entropy.

By what experiment was Efficiency = 1 - Tc/Th ever verified? Who did such an experiment? When, where, how?

Nobody, ever.

Period.

Perhaps you are asking the wrong question. Your question seems to be similar to asking to empirically prove a "perfect" circle. I doubt that's been done either.

Instead try asking yourself, "why, in 200 years, no one has come even close to the efficiency of (Th -Tc)/Tc, let alone ne beat it?"

Matt, Jack, and VincentG, all seem to be trying to do so, I even try to so. I would guess that thousands of engineers before us have tried to do so. So I present to you again, "why"?

The answer is observation, empirical data that it hasn't happened, not even close.

Matt I support your comments here. Thanks.

[VincentG]

I for one am not trying to beat Carnot, so much as question its meaning, or perhaps just sidestep it all together. You can't beat a mathematical formula using the same formula. We live in a time where even mainstream science is questioning everything, even the nature of reality. So what makes this different?

Fool, can I propose a question to you out of genuine curiosity; If an internal combustion engine (assume it's frictionless and perfect thermal properties) has a Tmax of combustion (say 1300k), and a Tmin exhaust temp (say 300k), what is it's calculated Carnot efficiency? Now, to avoid zeros, let's say another engine has an exhaust temperature of 1 degree k. What is the calculated Carnot efficiency?

Finally, can you explain the means in which cooling ambient air from 300k to 1k is more "efficient" than just exhausting it at 300k (on Earth)?

[Tom Booth]

... You can't beat a mathematical formula using the same formula. ...

Exactly.

[Tom Booth]

The caloric/heat coming out of the bottom of a Carnot Engine contains less energy than that going in the top, as depicted by a change in temperature, as Carnot plainly explained in his papers. This is a result of being converted to work, and corresponding temperature drop. Lower entropy.
...

Could you (or for that matter, anyone who knows of the existence of such) kindly provide a reference to any such explanation to that effect by Carnot.

As far as I know, Carnot considered Caloric itself to be heat, and that it did not diminish in any way in passing through a heat engine.

He stated:

The production of motive power is therefore due in steam engines not to actual consumption of caloric but to its transportation from a warm body to a cold body

And also:

The motive power of a waterfall depends on its
height and on the quantity of the liquid; the
motive power of heat depends also on the quantity
of caloric used, and on what may be termed, on
what in fact we will call, the height of its fall,
that is to say, the difference of temperature of the
bodies between which the exchange of caloric is
made. In the waterfall the motive power is exactly proportional to the difference of level between
the higher and lower reservoirs. In the fall of
caloric the motive power undoubtedly increases
with the difference of temperature between the
warm and the cold bodie

Carnot believed, by all accounts, and by his own words, that "caloric" was conserved. Caloric did not "contain" energy. Caloric was not "converted" to work.

Carnot did give a very clear, simple and easy to understand explanation. Caloric "in fact" falls, just like a waterfall. A heat engine only transports the caloric exactly like a water wheel transports water from a high to a low level.

The above are quotations out of the text of Carnot's book. Translated from the French, but maybe you have a different rendering or some other passages, or maybe you could explain or point out in what way I'm missing the actual intent of Carnot's written words, or maybe I'm twisting the meaning and you can give a better, more accurate translation or interpretation or cite some other passage.