Should ice take longer to melt when used to run a heat engine?

[Tom Booth]

If you are not tapping that energy away, out of the system as useful work, then surely it will wind up in the cold reservoir?

That is the prevailing wisdom, is it not:



So logically, without a load (work = 0), all the heat entering the engine ends up at the sink. In this case the ice. The ice melts quickly, I would guess, about the same as if exposed directly to the heat source.

Add a load, (so work > 0) less than 100% of the heat entering the engine ends up at the sink. In this case the ice. (which is otherwise well insulated against heat infiltration.) So with a load the ice should not melt as quickly as with no load

Of course even at no external load, there is friction dissipating heat from the bearings to the air, heat transferred to the air by the flywheel, noise, vibration, etc. Which in this case are beneficial rather than just being inefficiencies, act as a partial load and divert heat away from the ice. So, any load, even the engines own friction results in a slowing down of the melting of the ice.

This all seems like basic thermo to me, and rather obvious but more often than not, I'll end up in a debate about it with someone

 

[jbriggs444]

Of course even at no external load, there is friction dissipating heat from the bearings to the air, heat transferred to the air by the flywheel, noise, vibration, etc.

And where does this energy go? Is there a third reservoir?

 

[Tom Booth]

And where does this energy go? Is there a third reservoir?

It basically remains in or is transferred back to the source, but in a different form. Noise, vibration, motion of the air, are no longer counted as heat but "work" in one shape or form or another.

When a heat engine performs work otherwise, like lifting a weight or driving an electric dynamo, where does the heat go? Do we need another reservoir to account for the "lost" heat?

I don't think so.

 

[jbriggs444]

It basically remains in or is transferred back to the source, but in a different form. Noise, vibration, motion of the air, are no longer counted as heat but "work" in one shape or form or another.

Why to the source? Why not to the sink? And what makes you think that it counts as "work"? Neither source nor sink as heat reservoirs have any capacity to individually sink "work".

 

[Tom Booth]

Why to the source? Why not to the sink? And what makes you think that it counts as "work"? Neither source nor sink as heat reservoirs have any capacity to sink "work".

If I hear squeaking from the flywheel at least that portion of the energy is going to my ears, true, some small fraction of sound may reach the sink, under all that insulation.

The air being pushed around the room by the spokes of the flywheel, the piston displacing air, transferring energy to the atmosphere, these are all heat transformed into other forms of energy that don't reach the sink as heat.

They may not be "useful work" but I don't think that the expanding gas in the engine pushing the piston to make all that happen makes any distinction between "useful" or otherwise.

 

[Tom Booth]

"Neither source nor sink as heat reservoirs have any capacity to sink "work".

True enough, but since when does "work" require a heat sink? Work represents heat energy transformed. Work does not come from nor sink to that unfortunate term "reservoir".

The term "reservoir" tends to force the mind to think in terms of heat as a fluid, which it is not.

 

[pbuk]

Nevertheless, it is the path through which the heat must travel to get to the sink. Like a gate. If the gate is locked, there is no access to the garden.

That is not a useful analogy. Think instead about a room with a window facing the sun; does the room get hot even though the window is closed? The heat energy of the sun is being transferred by radiation which is the same way it it transferred from the plate on the bottom of the toy Stirling engine.

That is the Crux of the issue: the misconception that for some reason heat energy must, by some law of the universe, travel from source to sink in order to accomplish work.

The law in question is the second law of thermodynamics, and it doesn't quite say that (because it is a general law rather than one specifically applying to heat engines).

If, in a heat engine, energy is converted into work, then that same heat cannot also radiate, conduct or otherwise transfer to the sink it is already accounted for as "work".

That is correct, but it is not the heat energy that is doing work that is transferred to the sink, it is the 'lost' energy due to the inefficency of the Carnot cycle.

Personally I'm not satisfied with the current quagmire of assuming it is some combination of these two competing philosophical notions regarding the nature of heat and how a heat engine actually operates.

We don't deal with philosophical notions in these forums, we deal with physical laws which have been derived from consistent theories and confirmed by experiment or observation.

The truth is just the opposite IMO. Because the engine is not doing much work, the amount of energy that needs to be disippated is greater.

If the engine were doing some actual external work, there would be less waste heat requiring disippation to the sink.

This is not correct.

 

[pbuk]

My reason for posting to this forum is primarily to find out if it is already well known, or considered "normal" or accepted that by converting heat into work, OF COURSE ice being used to run a heat engine will take longer to melt, EVERYBODY knows THAT!

No this not considered normal.

The science of what makes a heat engine tick does not seem to be well established. There is no real consensus. It has not been reduced to formula to the extent that something like electricity has for electric motors.

This is not correct. The science is well established and can be found by searching the internet.

Is there a formula for heat flow resistance through a conductor like Ohm's law?

Yes, $\frac{Q}{t} = \frac{k A \Delta T}{d}$ where $Q \over t$ is the rate of heat transfer (similar to current $I$ in Ohms law $I = \frac{V}{R}$), $\Delta T$ is the difference in temperature (similar to the potential difference $V$ in Ohm's law) and $d \over k A$ is the 'heat flow resistance' (similar to $R$). But this is only an analogy and like all analogies it only works up to a point. Current is simply the flow of electons in a conductor, heat is much more complicated (for one thing it is clearly not constrained to a conductor - think of the sun shining through the window).

Edit: sorting out $\LaTeX$. Edit: added window/conductor contrast

 

[pbuk]

When a heat engine performs work otherwise, like lifting a weight or driving an electric dynamo, where does the heat go? Do we need another reservoir to account for the "lost" heat?

Yes. A good treatment of this subject can be found by downloading these notes. There is quite a lot of mathematics in there; this is necessary for a thorough understanding of any topic in physics but I think it still works without the maths.

 

[sophiecentaur]

The model stirling engine works between, say 273K and 302K, which gives it a thermodynamic efficiency of less than 11%. The mechanical energy available is only just enough to overcome internal losses in the engines described. Applying the logic of energy flow, the only difference in the rate of melting the ice an 'ideal' engine, running or not, would be no more than 11%.
The evidence is that the difference is much more than that (from the rates of melting the ice = 16%). Doesn't that imply that another mechanism must be responsible for the evidence?
If we acknowledge that the Laws of Thermodynamics must apply then the apparent error is around 50% high. If we were trying to prove the thermodynamic laws, wouldn't we need quite a bit more evidence than that, in the form of many experiments and tighter controlled conditions? It just so happens that the answer comes out one way but that's all.

 

[russ_watters]

You quoted without the context.
It was with regard to the time taken to freeze water in the insulated containers.

I'm still not seeing it. What does the Mpemba effect have to do with that?

Though to cut down on time, one could freeze the water in the container, then insulate it, then put the whole thing back into the freezer. When freezing it, though, it would be very difficult to tell when it is actually frozen.

Obviously it is not relevant to the running of the engine, or to the melting of the ice.

Agreed.

 

[russ_watters]

Running the engine on ice, the engine is powered by the incoming ambient heat. Correct?

Yes.

So if energy is being extracted from that incoming heat faster, less incoming heat is left to enter the system for melting ice.

Maybe, but it would depend on the existing mechanism of heat flow and how you are harnessing it. I would think that when you are circulating a working fluid it makes heat flow faster. That's why I'm suggesting adding a load should increase heat flow.

This isn't like a river with a fixed flow rate and elevation drop, it's more like a water tank with a leak in it. To me, the functioning of the engine is independent of the source of the leak.

It makes no sense really, and I'm probably wrong, but the idea has kept nagging at me, which is why I started this line of experimentation. To settle the question one way or the other.

It's an interesting problem/experiment.

 

[sophiecentaur]

That's why I'm suggesting adding a load should increase heat flow.

I was wondering about that too. The heat flow out of the hot sink would depend on the temperature difference across the input heat exchanger and cannot be assumed a 'constant power source'. (or can it?) My mind goes naturally to the Electrical Analogue for this sort of problem but that may not be applicable. @jbriggs444 could perhaps give an opinion.??

 

[pbuk]

It's an interesting problem/experiment.

Yes it is.

Here is what I think may be happening. Start by assuming an engine that is very close to the Carnot efficiency limit (i.e. has very small internal losses). If this engine is operating without load then the work done is very close to zero, and therefore the heat transfer to the cold sink through the operation of the engine is also very close to zero, let it be $Q_w$.

Now during operation the temperature of the hot heat exchanger (the top plate) is very close to ambient (or if there is an additional radiative heat source, very close to the equilibrium temperature) and so the rate of heat gain into the system other than that transferred via the hot heat exchanger will be small - let it be $Q_h$.

When the system is not operating, the mechanism will achieve an equilibrium temperature governed by the rate of heat transfer through the hot and cold plates. As seen in the experiment when the pistons froze, this can be significantly lower than ambient.

In this state, the rate of heat gain into the top plate from the atmosphere (and/or radiative heat source) will be significantly higher than $Q_h$ - we could easily imagine that the temperature difference could be 20° in this case vs. 2° in operating, implying 10x the rate of heat gain other than through the mechanism when not operating, but let us simply define $k$ so that the rate when static is $k Q_h$.

Now we can see that if $Q_w + Q_h < k Q_h$ then the ice will melt more slowly with a running engine. In other words no matter how small the 'non-productive' heat transfer coefficient (i.e. heat that is not transferred through the hot heat exchanger), we can always make the ice last longer if we have a sufficiently efficient engine that is doing no useful work.

 

[Baluncore]

I'm still not seeing it. What does the Mpemba effect have to do with that?

The Mpemba effect is a process in which hot water can freeze faster than cold water. Maybe you are fixated on heat engines and not considering the process of quickly producing blocks of ice.

Though to cut down on time, one could freeze the water in the container, then insulate it, then put the whole thing back into the freezer. When freezing it, though, it would be very difficult to tell when it is actually frozen.

I would eliminate the insulation from slowing the freezing process by using calibrated silicone mixing cups to hold the water during freezing, then transfer the ice blocks to the vacuum insulated cups below the heat engine. Silicone conducts heat quickly, and you can feel when the ice is frozen in the flexible silicone cups. There is no requirement that the ice blocks fit perfectly in the insulated cups at the start of the process.

I would test the Mpemba effect in an experiment by freezing boiled and cold water. If boiling did speed the freezing process, then I would boil the water before freezing.

 

[russ_watters]

The Mpemba effect is a process in which hot water can freeze faster than cold water.

I'm aware of what the Mpenba effect is. I'm still not seeing the relevance, and you aren't actually saying what you think the relevance is. Please be explicit: why, exactly, do you think the Mpenba effect is relevant here?

Maybe you are fixated on heat engines and not considering the process of quickly producing blocks of ice.

1. Yes, I get that what you are focusing on is the process of freezing water, not the experiment the OP is doing. I think that, in and of itself, is a mistake/derail.
2. Are you aware that the Mpenba effect is largely an apocryphal phenomena? It sounds like you are trying to suggest a better process for freezing water, but you aren't actually saying what you think the OP could be doing differently or how the Mpenba effect could be affecting the OP's work.

I would test the Mpemba effect in an experiment by freezing boiled and cold water. If boiling did speed the freezing process, then I would boil the water before freezing.

Why? What does that have to do with what the OP is testing? And I'll save you/the OP the trouble: it doesn't.

The OP tried to freeze water with a setup that used a large amount of insulation. It's pretty obvious that to freeze water faster he should use less insulation. The Mpenba effect isn't real, but even if it were, it wouldn't change the fact that insulation slows heat transfer.

 

[Tom Booth]

That is not a useful analogy. Think instead about a room with a window facing the sun; does the room get hot even though the window is closed? The heat energy of the sun is being transferred by radiation which is the same way it it transferred from the plate on the bottom of the toy Stirling engine.

(...)

We don't deal with philosophical notions in these forums, we deal with physical laws which have been derived from consistent theories and confirmed by experiment or observation.

I was making my earlier comments in the context of the video I had just posted, in the post just prior to my statements to which you are responding to.

That video was of a different earlier experiment, where I first constructed the styrofoam disk, for a completely different purpose.

I made the styrofoam insulating disk for the purpose of insulating the engines heat sink, in an effort to eliminate the sink, to see if the engine could run without a sink. Or perhaps even run better.

My comments about that video/experiment became confused with my additional comments regarding the insulating properties of the plexiglass in the clear solar engine.

I'll take the blame for the confusion.

This earlier experiment which my comments primarily were referring to, (the video of which I posted, mostly just to show the original origin of the styrofoam disk) did not have anything to do with running an engine on ice, or the clear solar engine.

You certainly have a point about the clear engine allowing radiation through, like a window. No real argument there. Perhaps some radiation goes out the same way.

My comments you are responding to were, however, related more directly to the video.

Probably nobody went to YouTube to read my comments relating to this earlier experiment, so I apologize. In the future I will try to be careful to be more specific and clear what it is I'm talking about.

This video was the experiment I was mainly referring to, and to which my comments primarily were related.



To provide some context. The engine in this video is not running on ice. It is running on a cup of near boiling water from the tea kettle.

My theory that prompted that experiment was based on numerous observations of Stirling engines which, apparently, were running without any functional or effective heat sink.

I thought that since a Stirling engine basically acts as a refrigerating device -taking in heat and transforming it into work, and producing cold in the process, the so-called "sink" might actually be a source of heat which could lessen the efficiency of the engine.

By insulating the sink to prevent heat from getting into the engine through that path, (if my theory was correct), not only would the engine continue running but might actually run cooler and more efficiently as it's refrigerating functionality would not be hampered by heat infiltration entering "backwards" into the engine through the intended "sink".

Watching the video, I think it can be seen, that by the end of the video, the engine is running better, at a slightly higher RPM with the heat outlet or "sink" insulated with the 1/4 inch of aluminized styrofoam insulation.

I could be wrong, but I didn't think much heat could conduct or radiate through that aluminized styrofoam. The aluminum foil is specifically to block such radiation.

Certainly, that is not perfect insulation. I'd like to repeat the experiment with the sink covered with Aerogel or something better, but think about it. If heat is flowing through the engine and out through the "sink", why would insulating the sink cause the engine to run faster? Or, if a cold sink is an actual requirement, how could the engine run at all without it?

The engine continued running for about three hours with the cold, ambient side or "sink" insulated, and apparently, by my observation ran better than without the insulation. It certainly, by experiment, ran longer with the insulation on the sink than without it.

When I first suggested such an experiment on the Stirling engine forum years ago, they pretty much dismissed the idea as insane. Someone said, "Tom, Insulating the cold end will not help (or we would all have been doing so)...".

At the time though I did not have an engine to try the experiment, so that never went anywhere, but now I do. Contrary to what "everybody knows" about Stirling engines and their theory of operation, it seemed to actually work.

Of course, I do not consider one such experiment conclusive.

That a Stirling engine would actually run better without ambient heat infiltrating it's refrigerating space, makes perfect sense to me., if we can dispense with the idea that heat is a fluid that powers a heat engine by flowing through it.

 

[russ_watters]

When the system is not operating, the mechanism will achieve an equilibrium temperature governed by the rate of heat transfer through the hot and cold plates. As seen in the experiment when the pistons froze, this can be significantly lower than ambient.

In this state, the rate of heat gain into the top plate from the atmosphere (and/or radiative heat source) will be significantly higher than $Q_h$

Why do you think this is true? Or, worse; if there's a working fluid moving, the delta-T can be lower while the heat transfer is larger.

Now we can see that if $Q_w + Q_h < k Q_h$ then the ice will melt more slowly with a running engine. In other words no matter how small the 'non-productive' heat transfer coefficient (i.e. heat that is not transferred through the hot heat exchanger), we can always make the ice last longer if we have a sufficiently efficient engine that is doing no useful work.

It seems to me that you are assuming the fastest heat transfer happens when the engine is shut off and then working the logic from there. What if the entire apparatus were wrapped with perfect insulation? Then the ice would never melt.

It should be possible to melt the ice rapidly by turning on a heat engine. Again: think of an elevated water tank and how fast you can draw energy from it.

 

[russ_watters]

FYI, guys, a pint of ice has a negative heat capacity of about 189 kJ. Over a day that's about 2 watts. That's a really small amount of heat transfer and in my opinion one should try to extract the heat a lot faster for this experiment.

 

[Tom Booth]

BTW, in the video in my previous post, yes the engine does slow down initially when I put the insulation over the "sink", but that is only because the flywheel rubbed against the insulation as I was putting it on, which caused the slow down.

Once the insulation was firmly affixed the engine quickly regained it's speed, and, in my estimation, seemed to start running noticeably faster. Unfortunately I did not have a tachometer for an objective reading.

 

[russ_watters]

I made the styrofoam insulating disk for the purpose of insulating the engines heat sink, in an effort to eliminate the sink, to see if the engine could run without a sink. Or perhaps even run better.

That would be a violation of the second law of thermodynamics.

I thought that since a Stirling engine basically acts as a refrigerating device -taking in heat and transforming it into work, and producing cold in the process, the so-called "sink" might actually be a source of heat which could lessen the efficiency of the engine.

A heat engine and refrigerator are opposite devices. You can't do both at the same time. No; a Stirling engine is not a refrigerator. The purpose of the cold sink for any heat engine is to pull heat away from the engine. The cold sink is colder than the output of the heat engine.

 

[Tom Booth]

A heat engine and refrigerator are opposite devices. You can't do both at the same time.

I disagree with that opinion. My own careful observations over many years studying how Stirling engines work, led me to the conclusion that a Stirling engine in its cycle goes through two phases.

In the first phase heat expands and drives the piston. In the second phase, stored momentum in the piston/crank and flywheel expand the gas, which has already effectively converted it's heat energy to work. This mechanical expansion has a refrigerating effect.

The ambient temperature or "cold reservoir" limits the degree of cooling the engine is able to produce because when ambient temperature is reached by refrigeration, or mechanical expansion of the gas, heat flow reverses, flowing from the "sink", back into the engine.

That is the real reason for the Carnot efficiency limitation. Backward flow of heat from the sink.

 

[russ_watters]

I disagree with that opinion. My own careful observations over many years studying how Stirling engines work, led me to the conclusion that a Stirling engine in its cycle goes through two phases.

Have you looked at a thermodynamics book or online equivalent to see how they describe it? You don't have to try and figure it out yourself -- they are designed and built based on well-known thermodynamics.

 

[Tom Booth]

Have you looked at a thermodynamics book or online equivalent to see how they describe it? You don't have to try and figure it out yourself -- they are designed and built based on well-known thermodynamics.

Sure, I became rather obsessive about studying thermodynamics about ten years ago:

https://stirlingengineforum.com/viewtopic.php?f=1&t=478

 

[Baluncore]

The Mpenba effect isn't real, but even if it were, it wouldn't change the fact that insulation slows heat transfer.

Ah, so that is your problem. You think it is not real, but cannot prove it is not real.
Note; the spelling is Mpemba, NOT Mpenba.

I did not say anything about insulation and the Mpemba effect. That was your straw man, and you have blown it up out of all proportion. I simply said the OP should be aware of the Mpemba effect.
Interesting, I will now consider under what conditions the Mpemba effect might be aggravated by insulation.

You could have just ignored the original reference to the Mpemba effect. Instead you have given it lots of publicity, and you have persisted in derailing this thread from the subject of heat engines.

 

[pbuk]

Why do you think this is true? Or, worse; if there's a working fluid moving, the delta-T can be lower while the heat transfer is larger.

I am attempting to separate the total heat transfer into two components, $Q_w$ which is the heat transferred into the working fluid and $Q_h$ which is the heat transferred into the structure of the engine. All I am saying is that with a greater temperature difference, $Q_h$ will be greater.

It seems to me that you are assuming the fastest heat transfer happens when the engine is shut off and then working the logic from there.

That is the (second hand to me) observation. I am asserting a possible explanation.

What if the entire apparatus were wrapped with perfect insulation? Then the ice would never melt.

And with no temperature differential the engine would never run; I am not sure what your point is here?

It should be possible to melt the ice rapidly by turning on a heat engine. Again: think of an elevated water tank and how fast you can draw energy from it.

I think I would modify this to "it should be possible to melt the ice rapidly by turning on a heat engine and extracting useful work from it". I am trying to get away from analogies as I think the OP may be misled by them - let's stick to the laws of thermodynamics.

However I would imagine that these toy atmospheric heat engines have such a low power output due to the small working fluid volume coupled with the low efficiency due to the small temperature differential that it is very difficult to extract useful work without grinding them to a halt.

 

[sophiecentaur]

Don't get me wrong, I have no criticism of the content of the OP. It was an interesting exercise which, in the light of 'useful' replies from PF, could be repeated and extended to yield some valid answers. I love the idea of home experiments and the toy Stirling is something I would like to get involved in.

But I have to wonder why we are spending so much time and effort on trying to 'explain' / 'justify' the result of an experiment that is, as yet, as full of holes as the one described in the OP. If we were discussing a report from CERN or Fermi Lab and trying to understand an apparently odd result, then the experimental details and the statistics would all be discussed in depth. This time, we have dozens of posts ignoring all that. The numbers are important in this - just as in the search for Higgs Boson.

Forgetting whether or not, the mechanism is a 'heat engine', there is a cylinder of warm air which is moved to a cooled cylinder - and on and on. That will transfer heat to melt the ice (method of mixtures - my school lessons in 1960) . If the machine is not turning then that will not be happening so that particular transfer path will not be there.

In the reported experiment, something is turning the machine and that will increase the rate of heat transfer - all things being equal. That something is explicable in terms of a heat engine. The rest of the experiment is just not explicable in the light of the scarcity of data. I can't find mention of the condition of the room used for the experiments. How about an hourly plot of air temperatures over the day? A ' control' would need to be run at the same time and, of course, the experiment should be run with the control roles being swapped, to reduce the uncertainty.

What are we actually trying to explain here? There is, actually nothing specific that demands an explanation. Heat transfer across one of several different paths with a temperature difference of less than 30C is hard to quantify. We are trying to do the equivalent of explaining the behaviour of a Wheatstone bridge that uses 10MΩ wires, mounted with rusty nails on a water soaked wooden board.

I think the point has already been made that any mechanical energy generated by the Stirling Engine is not going anywhere outside the engine; work against friction is all contributing to melting the ice. Some microwatts of sound may be escaping.

 

[pbuk]

But I have to wonder why we are spending so much time and effort on trying to 'explain' / 'justify' the result of an experiment that is, as yet, as full of holes as the one described in the OP.

From my point of view I have been spending time and effort on this because the OP appears to believe that a heat engine can produce a refrigerating effect without receiving mechanical work input, and that his experiment demonstrates that.

I think it would be a mistake to give the OP the impression that all he has to do is improve the accuracy of his experiment and he will be successful. Instead I hope to show him that this effort would be futile by providing both information about the (well understood and experimentally verified) science of heat engines and a suggestion as to an alternative explanation for the effect that he is seeing.

 

[pbuk]

We are trying to do the equivalent of explaining the behaviour of a Wheatstone bridge that uses 10MΩ wires, mounted with rusty nails on a water soaked wooden board.

I take back what I said about analogies not being useful here

 

[Tom Booth]

I can't find mention of the condition of the room used for the experiments. How about an hourly plot of air temperatures over the day? A ' control' would need to be run at the same time and, of course, the experiment should be run with the control roles being swapped, to reduce the uncertainty.

These are certainly all valid criticisms And as I've mentioned, I do not myself consider any of the observations conclusive of anything.

I ordered six Stirling engine kits, four of the type seen in the video (low temperature differential engines) and two high temperature. in order to run various experiments, with controls running concurrently.

So far I've managed to assemble one of the six kits.

To get some preliminary data and, save some time and get some ballpark, baseline numbers, I decided to do a few informal experiments with the one engine, which I could leave running while I work on assembling the other kits, as well as attending to other responsibilities. My "free time" for playing with Stirling engines is actually very limited.

It would be rather foolish to give any experiments posted on YouTube by amateure experimenters any real weight. For all anyone knows, I've installed a small motor and batteries under all that insulation to keep the engine going under any circumstances.

In other words, I would not believe MYSELF, if I stumbled across these experiments on the internet, conducted by someone else.

I am extremely skeptical by nature myself and am rather disbelieving of my own results, which is why I asked the questions. As an untrained amateure researcher, I don't even know for sure if the results would be considered unusual or expected.

I'm doing these experiments only because I could not find any account of anyone else ever doing them.

I proposed the experiments repeatedly for years and years on many forums, because I did not, and still do not have all the time or resources to conduct proper experiments, but nobody ever thought it would be worth the bother, so I feel compelled to do the experiments myself.

I'm not asking anyone to wast time on it if they consider it a waste of time.