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What exactly does "100% efficient" mean, though?

I'm always hearing this "no engine is 100% efficient" but when I looked into what that actually means, in terms of thermodynamics and heat engines, I was quite surprised to find out that it means something entirely different from what it sounds like.

If I heat a Stirling engine with an alcohol burner, or whatever. I assume by "efficiency" is meant, how well does the engine utilize the heat actually supplied to it, above the ambient baseline. But, correct me if I'm wrong, but in thermodynamics, in calculating the efficiency of a heat engine, Absolute zero is used as a baseline.

So, if I supply heat at a temperature of 200 F degrees above ambient, (which is, say, 60 F), my heat source is 260 F.

If my engine utilizes every bit of the heat I have supplied to it, so that the cold side never goes above ambient and no heat is transfered to the sink, the "efficiency" will only be what?

19.7%

Suppose I figure a way to actually use even more heat so the cold side of the engine is refrigerated down to 40 F?

At best, the efficiency is still only about 24.3%

Even if I run my engine on ambient heat using a wet sponge for cooling with a temperature difference of 20F and my engine gets so cold the sponge freezes, the "efficiency" is still just 7.6%

How is that possible? Because thermodynamicist calculate heat engine efficiency on a measuring stick that starts at absolute zero, so if your engine has not cooled itself down to absolute zero then it has not utilized "all the heat available" and therefore is less than 100% efficient.

In other words this measure of "efficiency" is complete nonsense, and for the purposes of these experiments it is completely meaningless and irrelevant.

These rules about how to calculate heat engine efficiency were made up something like 200 years ago, and are nonsensically simplistic to the point of being laughable.

That, of course, is just my opinion based on nothing but my own research on the subject and common sense.

All of that engine efficiency nonsense should have been thrown out the window a long time ago IMO. It doesn't make any sense and does not actually represent engine efficiency in terms of how well it actually utilizes the heat supplied to it.

In other words, to prevent the ice from melting, assuming ambient temperature is 70F a Stirling engine would only need to have an efficiency of about 7.2%

Sockmonkey wrote: ↑Fri Jul 17, 2020 2:00 pm The ice will always eventually melt because there is no 100% perfect insulator, and no machine is 100% efficient. There are always mechanical losses.

As far as mechanical loses and no perfect insulation, my freezer does not have perfect insulation either, and likely has numerous mechanical loses, but I've been keeping ice in there for years and years. It only melts if I take it out.

So, a machine does not have to be 100% efficient or perfectly insulated to make ice, or to keep it cold.

Tom Booth wrote: ↑Fri Jul 17, 2020 7:39 pm

Sockmonkey wrote: ↑Fri Jul 17, 2020 2:00 pm The ice will always eventually melt because there is no 100% perfect insulator, and no machine is 100% efficient. There are always mechanical losses.

As far as mechanical loses and no perfect insulation, my freezer does not have perfect insulation either, and likely has numerous mechanical loses, but I've been keeping ice in there for years and years. It only melts if I take it out.

So, a machine does not have to be 100% efficient or perfectly insulated to make ice, or to keep it cold.

Your freezer has to expend energy to keep things cool. If your engine has to expend energy for cooling, you aren't getting more out than you're putting in.

An engine running on ambient heat, if such a thing is possible, is still expending energy.

The question is; not are we getting more out than we are putting in, but is it possible to run an engine on ambient heat at all?

A Vuilleumier heat pump expends energy and can produce cold at cryogenic temperatures.

A Stirling engine running on ice is expending energy.

The drinking bird toy is expending energy.

Heat is energy. Or so one theory of heat goes. Cold is not energy, it is absence of energy.

A Stirling engine "running on ice" is not running on energy from the ice. The energy is derived from everywhere surrounding the engine except the ice.

Carnot, and other early fathers of thermodynamics, literally believed that a heat engine that could utilize atmospheric heat, and required no other source of energy would cause the annihilation of the universe by "destroying" heat. They believed that heat was a literal thing, a literal fluid, like water, and that a heat engine operated, if not by heat flowing through altogether than by DESTROYING heat. I suppose like burning wood appears to "destroy" a log. Their heads were full of fanciful nonsense that turns out to be entirely untrue.

A drinking bird toy does not run on the cold produced from evaporative cooling, it runs on the heat energy surrounding it, everything except the cold water on its head.

A giant drinking bird could power a refrigerator to make ice and it still would not be using any energy from the evaporative cooling, nore would it bring an end to the universe by spiraling out of control and absorbing all the heat from the sun and stars.

Kinetic energy, which heat is, theoretically, does not have to "flow" like water, it can be transfered, like one billiard ball striking another. The first ball stops dead in its tracks and the other takes off.

We can sidestep the whole issue with a compromise and say that a Stirling engine does not run on heat OR cold, it runs on "a temperature difference".

That may make us all feel better but it doesn't really resolve anything.

If energy, let's call it "coldness", (or should we stick with "frigoric"?) Is somehow being sucked out of ice, causing it to loose it's coldness fluid, then we need to worry about accounting for the flow of this energy in and out of the ice.

If on the other hand a Stirling engine running on-top-of-ice, is actually running on the surrounding heat everywhere else, than the ice can just sit there.

If heat energy from the surroundings is what is really driving the piston in a Stirling engine running on ice then the kinetic energy (heat of the gas) is being transferred to the piston, not the ice. Then there is no reason why the ice has to melt due to heat being transfered through the engine as a necessary mechanism of its operation.

If our energy source is free atmospheric heat in the air then "efficiency" is largely irrelevant. Low efficiency is not going to increase our electric bill. Unless of course, we have to keep making ice to continue running the engine.

Going round and round with logical (or illogical) debate and argument doesn't really get anyone anywhere.

So, I've got some engines that I still have to put together, I have some vacuum insulated cups of Ice in the freezer. So I'm going to run some actual experiments and see what happens.
We have some ice in all but 100% thermally insulated containers, now I just have to put these Stirling engine kits together and we shall see what happens. I've had these cups in the freezer for like 2 months waiting for these engines to come in the mail.

If anyone would like to replicate any experiment using the same engines, I can say, I've ordered half a dozen different engines from various places on the internet. Most charged shipping.

The engines from stirlinghobbyshop.com were sent express, free shipping, were the last ones I ordered but we're the first to arrive and overall cost less.

Apparently, though I paid for shipping, the other engines I ordered were sent ground or something and still have not arrived, though ordered a month earlier.

I'm not in anyway associated with the supplier, I am just very happy some engines finally arrived and want to give credit where credit is due.

BTW, the engines I ordered on eBay were sent "economy", as I recall, I was not about to pay the hundreds of dollars supposedly required for express.

Stirling hobby shop had free shipping and the shipping label indicated the engines were sent express and at no extra charge.

I would suggest avoiding eBay merchants who advertise "Just One Left" and then want hundreds of dollars for overseas shipping within some reasonable time frame.

There is no shortage of these Stirling engines.

Tom Booth wrote: ↑Sat Jul 18, 2020 7:33 am BTW, the engines I ordered on eBay were sent "economy", as I recall, I was not about to pay the hundreds of dollars supposedly required for express.

Stirling hobby shop had free shipping and the shipping label indicated the engines were sent express and at no extra charge.

I would suggest avoiding eBay merchants who advertise "Just One Left" and then want hundreds of dollars for overseas shipping within some reasonable time frame.

There is no shortage of these Stirling engines.

Yeah, it's the same thing where some stuff has been "on sale" for years.

Well, I got one engine together.

https://youtu.be/415iIJtfdLA

I replaced the steel bolts that came with the kit with Teflon. I had to drill the holes slightly bigger.

It started up immediately, as soon as I put it on the coffee mug, 1/2 filled with hot water, and gave it a nudge.

I had to figure out how to upload videos to YouTube from my phone, download the app. Read the instructions and watch a video tutorial. And now as I write this the engine is still going. Not sure how long it's been but it seems like a long while.

I'm going to put the other one together with the steel bolts that came with it and run a comparison test.

These particular engines are infinitely adjustable though. The timing that is. Just loosen the large nut on the flywheel and set the advance at any degree you like. It comes with timing marks to set it properly, but they are just literal marking someone made with a magic marker, so getting the timing exactly the same on both engines could be challenging.

Eventually I may do some modifications on the displacer by adding a regenerator. I don't really think a Stirling engine is a Stirling engine without some kind of actual regenerator.

I'm currently doing a test to see how long the engine runs an a vacuum flask filled 1/2 way with hot water.

https://youtu.be/HQT5JviF-qk

So far, my impression is, (as the engine continues running); the engine is slowing down more quickly on the vacuum insulated tumbler than it did on the ceramic mug.

I think the reason is that, while the top plate remains cool to the touch, the bottom hot plate is really scalding hot. Too hot to keep my finger on.

The vacuum tumbler I used for this has a narrow mouth. It is about a full inch narrower in diameter than the ceramic mug so is exposing much more of the hot bottom plate to cooling by convection, so more of the heat is going around rather than into the engine.

I'm timing the run this time, though I forgot, (n the process of making the video), to measure out the exact amount of hot water, as I had intended to do.

Lesson learned is losses by convection due to exposure of the hot plate may be as, or even more important than the material the heat source container is made from, or it's insulating properties. Ceramic is also quite a good insulator. I may do a comparative test but I don't think a ceramic mug is actually a better insulator than a vacuum flask.

Anyway, in a few minutes it will be a full hour the engine has been running, but it is already slowing to a crawl.

1 hour and five minutes now and still going as I post this.

I thought I would mention something very strange I discovered while making this video.

I wanted to use a magnifying glass to make the lettering on the vacuum flask lable more readable, but through the pinhole lense on the phone camera, l found that a magnifying glass doesn't work. Does not magnify anything. Has no effect.

BTW the engine finally stopped, just now, after 1 hour and 48 minutes.

I did not expect it to run so long.

On the ceramic mug the engine ran fast for a long time but came to a halt more abruptly. On the vacuum flask it slowed down more quickly in the beginning but ran rather slowly from then on for quite a long time.

https://youtu.be/fFByKkGr5bE

Runtime with the cold "heat sink" insulated, about 20 minutes, so far. (still running).

The engine has continued to run with the cold side completely insulated with 1/4 inch foil faced styrofoam and Owens Corning fiberglass.

It is still also insulated from the hot end by insulation and by eliminating the steel bolts, replacing them with nylon bolts.

I'm actually running these experiments with the cup only half filled to save some time.

Later I'll do some comparisons once I have the other engine put together.

The engine ran for two hours and 35 minutes with the top insulated. That is, I think, the longest run so far.

When it finally stopped, I removed the styrofoam from the top. The top plate felt a bit warm. Just slightly. The water was still a little hot.

Without the insulation I was able to start the engine again. It seems willing to run a bit longer but is now turning quite slowly.

It ran for an additional 29 minutes.

About 3 hours altogether. That is a 5:1 ratio, for what that might be worth. That is it ran for five 1/2 hour periods "without a sink" and one additional 1/2 hour with insulation removed.

Of course, as noted. No insulation is perfect, so there was always some sinking of heat, no matter how slight. But adding insulation to eliminate the sink, to a degree, did not seem to hinder the engines performance.

The engine had slowed down when adding the insulation as the insulation had rubber against the flywheel a few times, but the engine seemed to recover speed once the insulation was taped down, and it ran longer than ever.

Taping something down with one hand while shooting a video with the other was the most difficult challenge.

I'm working my way towards the ice trials, but I wanted to see how the engines work under more ordinary circumstances. Not that insulating the sink is ordinary, but I just mean using heat rather than ice.

One thing to keep in mind: Unlike cups of hot water, average ambient heat never really cools down. I expect the ice experiments to last much longer.

I also have yet to incorporate a regenerator.

I also, eventually will be experimenting with timing and throw. I think these can possibly be adjusted to some advantage.

And, using some refrigerant other than plain air could have a dramatic effect. But at the moment I don't want to risk dissolving the styrofoam displacer, plastic body or rubber O rings. Or accidently blowing up the engine with butane gas.

While I work on getting the other engine put together, and the many other things that need doing every day, I thought I may as well give the little engine something to do, and so, broke out one of the cups of Ice from the freezer.

Ambient heat this evening in Fort Plane is about 85 degrees F this evening.

https://youtu.be/dQVRVL1OUBg

Let's find out, at least as a baseline for further experiments and engine modifications, how long a Stirling engine will run on ice when some effort is made to prevent the ice from melting, by first of all freezing the ice in a kind of Dewar flask or vacuum insulated thermos, set the thermos on styrofoam, wrap the whole thing in fiberglass, put the engine together with non conducting bolts, so that, hopefully, most of the heat that melts the ice will have to do so by going through the gas in the engine, while the engine continually tries to convert the heat into its own motion, as well as other things; noise, heat generated in the bearings, air pushed by the flywheel.

This engine isn't doing very much actual work at all, but it is doing some, just making itself go.

I think if it were under an actual load of some kind it would disipate more heat as work and less heat would reach the ice, allowing the engine to run even longer.

Later, I may add some fan blades or a little electric generator or something to try and test that theory.

At this point I'm wishing for a thermal infrared camera, and maybe some temperature probes.

This may be a little difficult to explain.

https://youtu.be/s4MzYMen2Ow

I noticed the top of the engine was getting quite cold, very cold to the touch. Almost looked a bit frosty, or condensing tiny droplets that made it look frosty. I got the crazy? Idea too much cold was escaping.

The engine as advertised, only needs a 20 degree temperature difference. At 85 ambient, that's more than twice what the engine needs to run, so I thought it might be ok to set the 1/4 inch insulating disk I made previously over the engine. I almost expected it to stop running immediately without any heat, but it kept going.

About twenty minutes later, I noticed it seemed to be running a little slow, so took the top insulation off again. The engine quickly got back up to speed. I was surprised though how long it kept running at the same speed, parrly isolated from the heat source by 1/4 inch of styrofoam insulation.

It has been running almost 3 hours so far.

You may notice I also added more insulation since the last video.

Going on five hours.

Condensation on the cylinder:

https://youtu.be/GLzTxVzMjIQ