Stirlingengineforum.com

According to the Carnot efficiency theory, or so-called "Law", supposedly a "Law of Nature", a Stirling engine running on hot water should have a heat conversion efficiency of only about 15% to at best 20%

Reputable scientific sources say this means that at least 80% of the heat entering into the engine must be "rejected" by the cold side of the engine.

Years ago, here on this forum, it was generally agreed that insulating the cold side of a Stirling engine would cause it to overheat and stall.

See, for example this post (edited for relevance):

jesterthought
Re: Stirling Engine Thermodynamics
Post Fri Feb 26, 2010 9:48 pm

Hi Tom,
1. Insulating the cold end will not help (or we would all have been doing so).

As you say, model hot air engines sometimes slow down because the cold end gradually warms up, thereby reducing the temperature differential. If the cooling is by ambient air, the warmth of the “cold” end can be felt. Since it is above the temperature of the cooling medium, that medium cannot be heating it!

To insulate it would cause the cold end to heat more rapidly and slow the engine even sooner.

...The reason the cold end is warming is that we are putting heat into the engine. Engines are not yet 100% efficient, therefore the power out is less than the heat input, and most of the excess heat is rejected at the cold end – which must be cooled.

Heat is being fed to the cold end by the working fluid, internally, not from the atmosphere. Ambient air is still cooling it (although, sometimes, not quite enough).
...
More heat also short-circuits the thermodynamic process by conduction through the structure, increasing the cooling required by the cold end.

...
Heat flows down only to ambient. It takes power to cool the working fluid below that temperature (despite the heat being released from it).

Jester.

viewtopic.php?p=1228#p1228

So it is, or was, common knowledge that the majority of the heat goes through the engine to warm the cold plate. Heat cannot be "rejected" to ambient unless the cold plate gets warmer than ambient.

So insulating the cold plate would disrupt this necessary process of "heat rejection" causing the engine to overheat, run more slowly and soon stall.

Well, if all that is true, than how is this possible?:

https://peoplesresearchcenter.com/video ... ments.html

.

Here is some opposing data:


viewtopic.php?t=5410&hilit=Alphax&sid=9 ... b&start=30

Alphax wrote:FIRSTLY: I've measured the heat flow through the static engine and through the running engine. What I have found (subject to repeats and confirmation) is that both heat flows (static and running) are almost the same.

And next, one of your antisocial, derogatory, and hurtful remarks:

Tom Booth wrote:Perhaps the cheapest most inefficient piece of junk ..

Probably the one that drove Alphax off. Too bad that we lost his help. Would have liked to correspond with him some.

Please obtain real data in the form of an indicator diagram, power output measurement, or temperature readings inside, and outside the hot and cold plates. Your wild camera waving, blurry images, lack of instrumentation, and lack of any laboratory plan or write up.leads to nothing more than an inconclusive, magic show.

Only with that data will we begin learning the answer to, "How this is possible?". My guess is laboratory/garage experimental error.

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Probably the one that drove Alphax off. Too bad that we lost his help. Would have liked to correspond with him some.

Although I think Tom's experiments are flawed, he is at least attempting to use an engine that has some degree of thermal separation. I can confirm that the engine used in Alphax's testing was horribly inefficient from a thermal short perspective, and for him to not recognize that seems strange considering the level of precision he was after.

I've said it before and I'll say it again, the heat passing though the gas itself must be significantly greater than the heat passing through the body of the engine/displacer chamber itself. And the heat must only pass through the gas during expansion. So far I have not seen any testing with any such engine. My epoxy chambers may come close but still they have too much thermal mass in the hot and cold plates relative to gas mass.

Until someone builds such an engine it seems Tom is beating a dead horse, and we are left denying what could be an anomaly.

In this video, the former Space X engineer said he and his team had to contact NIST to verify the freezing points of various gases under certain conditions. They were informed by NIST that some of the charts and graphs contained data that was never physically verified and only extrapolated from other data.
https://www.youtube.com/watch?v=r4OovdoG80w

I've gained some respect for that type engine since then. VincentG has engineered some amazing modifications, but all the same here was his engine posted:
1, no regenerator
2. Steel bolts conducting heat
3. Convective air currents conducting heat from bottom to top plate around the engine.

4. Or if running on ice. The ambient heat flowing directly to the bottom plate into the ice is unchecked. No heat would need to pass through the engine to get to the ice, running or not running.

If someone is going to duplicate an experiment they really need to make modifications as I did to ensure the heat flow is through the working fluid of the engine, otherwise the flow of heat via other channels will be overwhelming.

He got exactly the results I would expect under the circumstances. Like trying to test an electrical circuit with uninsulated wire shorting out the circuit.

Fool, you have not explained the results I obtained when such measures are taken, you only referred to a very poorly conducted experiment that got no results.


In addition, no actual data, picture of the setup, video or anything, so, just his statement.

Regardless, too many unchecked pathways for heat transfer other than through the working fluid. I have almost exclusively worked with engines on which I had already made basic changes, such as replacing the steel bolts.

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In all do respect. Thanking him for trying would have been a better path.

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Fool wrote: ↑Thu Oct 31, 2024 11:31 am .

In all do respect. Thanking him for trying would have been a better path.

.

Maybe so, but I'm not here to win a personality contest or win new friends, I'm interested in accurate and reliable information about Stirling engines, as I think is the purpose of the forum.

Aside from that, I did not find his account credible. At any rate, his experiment is unrelated to the question here.

I've smothered the cold side of the engine(s) with all manner of insulation, glass surroundings to reduce draft, even put insulation inside the displacer chamber, used non-heat conductive cold plates plus space age aerogel etc.

Rather than causing the engine to overheat and stall, it ran faster. Aside from the temperature measuring near, at or even below ambient even after hours and hours of continuous operation.

You, fool, have no sensible answer so your making me out to be a reprehensible meany.

Here is another example: (2nd video)

https://peoplesresearchcenter.com/video ... ments.html

The engine was designed to be solar, so the top is acrylic and the base aluminium.

Attempting to run it "upside down", letting heat IN through the metal bottom should not work.

The engine is essentially a greenhouse. Sunlight is supposed to go in through the top to heat the black displacer. The metal bottom is intended to let the "waste heat" OUT.

So if run in reverse, putting heat in the bottom, the engine should not run if most of the heat needs to pass through and out the top as stated, according to the Carnot efficiency limit theory.

Acrylic is non-heat conducting. A kind of insulation. Good for retaining heat but not good for getting rid of "waste heat".

Regardless, the engine, while letting heat in through the bottom still runs like the wind.

This should be 'impossible" according to the Carnot limit theory as the overabundance of "waste heat" would build up inside the engine with nowhere to go causing the engine to stall.

Proof the Carnot theory is wrong. It does not adequately explain these readily observable facts.

VincentG wrote: ↑Thu Oct 31, 2024 6:16 pm https://youtu.be/0ZzPWfLDAOI?si=61ji52cfwpwkJYg6

Perhaps you would like to comment on the video and why you consider it relevant?

.

VincentG, thanks for finding that video. Sort of puts the kibosh on any claim of insulating a LTD engine. As long as the heat flow through the insulation is faster than the engines requirement.

It also proves the point I was making about the Stirling Engine blocking heat flow through. Bottle necks.

.

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Tom Booth wrote:You, fool, have no sensible answer so your [Sic] making me out to be a reprehensible meany.

I've done nothing of the kind. You prove that all on your own. It is associated with the personal defect called narcissism.

The only way out of it is, to realize it and grow up. We can't help, only point out your offenses.

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Fool wrote: ↑Thu Oct 31, 2024 9:59 pm .

VincentG, thanks for finding that video. Sort of puts the kibosh on any claim of insulating a LTD engine. As long as the heat flow through the insulation is faster than the engines requirement.

It also proves the point I was making about the Stirling Engine blocking heat flow through. Bottle necks.

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The video does not make clear what he means by "bottlenecks".

He does make clear the vast difference in heat conductivity between acrylic and the various metals.

He also talks about acrylic exclusively as useful in solar applications for effectively letting light in. He says nothing in regard to acrylic conducting heat out.

I assume you are familiar with the greenhouse effect. Radiant heat after passing through greenhouse glass becomes trapped as it is no longer LIGHT once it strikes a solid object such as the displacer.

Nothing in the video states or implies acrylic serves as a viable heat sink for a Stirling engine. Just that it lets light in for solar applications.

At any rate, IMO he seems to be fumbling for an explanation why a metal heat sink or an acrylic heat sink make little difference, in fact he calls it very surprising.

He does not address the possibility of the heat being converted to work so that a "sink" is not required, nor does he show any actual temperature readings between a metal or acrylic "cold" plate.

So, undefined "bottlenecks".

Not very informative or conclusive IMO, but I was interested in hearing VincentG's take on it, as the video could be interpreted either way.

As a matter of fact, I have previously posted that video in support of my theories.

Because there is no heat to be "rejected" it literally makes no difference how heat conductive the "cold" plate is, as it isn't necessary that it conduct heat, since the heat was converted to work (mechanical motion).

viewtopic.php?p=16562#p16562

The point of the video is that the acrylic is not a bottleneck because other parameters of the engine are. I think Tom needs to do some testing on temperature gradients. These engines run from a temperature gradient. There is always a temperature gradient in real world materials. A piece of aerogel maintains a temperature gradient at all times when it is covering the cold plate, as does a pure copper cold plate, or even graphene. That means that there will always be a temperature gradient between the top and bottom of these engines Tom, regardless of how much insulation is placed on top.

If you want to conduct this test with any meaningful results, you will need to do your best to use a known heat source like 8oz of hot water in a vacuum cup for consistent results on the hot end. On the cold end you will need to build a very insulated box that fits over the mechanism and provides a known volume of air space.

Perform multiple tests and record air temperature of the box as well as water temperature with an accurate thermocouple over time, at least twice. Run the test from 190f to 150f(as an example) using an accurate thermocouple in the water placed at a consistent location. This will give you a rough but consistent quantity of heat to calculate.

Conduct a test with a stationary engine and record air temperature, at least twice.

Conduct a test with a running engine under no load and record air temperature, at least twice.

Conduct a test with a running engine under load and record air temperature, at least twice.

The room temperature will have to be consistent between tests.

The vacuum cup should be either cold soaked or pre-heated, consistently, before each test.

The engine should be cold soaked before each test.

These model engines produce so little power that I don't think any difference will be observed between running unloaded and loaded but I would like to be wrong.

I can't state enough that these infrared scanners are not nearly accurate enough to be used for these purposes. I have a nice one and it is always giving me irregular results depending on materials, finishes and temperature gradients across its viewing area.

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I think the point of the video was to explain why both the conductance/R-value of the hot and cold plates of an LTD of Stirling Engine, are of little importance. It didn't limit the insulation to the cold side. Applied to both hot and cold.

Because the bottle neck of a tiny amount of gas, it's mass, slows down the flow of heat more than the insulation.

Measuring that bottle neck in the heat flow is what an indicator diagram, or dynamometer measurement of power would tell.

I've tried to recommend he insulate the hot side, and also try inverting the engine. Tom, how is the two engines bolted together, cold plate to cold plate, experiment progressing? Its been about a year or so. Or did that give opposing results that you don't want to share?

Tom's conclusion, that 80 W of power supplied is converted to work, implies that an engine capable of 0.020 W, some how can be forced to produce/convert 80 W.

Verifying power output with a dynamometer is necessary. No conclusion can be drawn without it.

The scientific data is stacking up against Tom.

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Fool wrote: ↑Fri Nov 01, 2024 5:53 am ....
The scientific data is stacking up against Tom.

The opinion of an anonymous poster on an internet message board giving their interpretation of the, at best, ambiguous and inconclusive speculations about unidentified "bottlenecks" on a YouTube video hardly constitutes "scientific data".