Tom is right, no heat sink req'd

[matt brown]

Yep, Tom is right, you don't need a heat sink for a closed cycle Stirling cycle when scheme is carefully designed. For alpha yes, for gamma no, but this gamma scheme requires an unusual event sequence and a good regenerator. Let's just say that the typical 90 deg out-of-phase crap won't cut it here. I gained this conclusion from a simple mid-sized gamma scheme (no tinkertoy) with a 300-600k cycle. Yeah, everything idealized distinct events, but there should be enough wiggle room for real vs ideal values when the event sequence is close enough. Although unusual, this event sequence is almost boring and I'm working on options now.

I found this while messing with some numbers for my open cycle scheme (1000cc gamma thread). I'm still stabbing in the dark there, running various values while trying to find a simple relationship between volume and thermal ratios.

This deep dive led me to grasp how Tom's LTD ran with cold plate insulated (which is interesting in itself) but I can't explain the lacking regenerator (which would nix my scheme) except that maybe the thermal ratio was so low that the heat moved around thruout cycle, especially considering the out-of-phase dynamics.

As a long time alpha guy, I'd like to think that I would have found this years ago if not stuck in alphaland, but unsure. Meanwhile, Tom, go ahead and stick a couple feathers in your cap !!! The main question I have now is how did all the academics miss this ??? When I get a grip on various ratios, I'll have a grip on various constraints. Until then, all I can say is that...you don't need a heat sink for a closed cycle Stirling cycle (sometimes).

[Tom Booth]

Yep, Tom is right, you don't need a heat sink for a closed cycle Stirling cycle when scheme is carefully designed. For alpha yes, for gamma no, but this gamma scheme requires an unusual event sequence and a good regenerator. Let's just say that the typical 90 deg out-of-phase crap won't cut it here. I gained this conclusion from a simple mid-sized gamma scheme (no tinkertoy) with a 300-600k cycle. Yeah, everything idealized distinct events, but there should be enough wiggle room for real vs ideal values when the event sequence is close enough. Although unusual, this event sequence is almost boring and I'm working on options now.

I found this while messing with some numbers for my open cycle scheme (1000cc gamma thread). I'm still stabbing in the dark there, running various values while trying to find a simple relationship between volume and thermal ratios.

This deep dive led me to grasp how Tom's LTD ran with cold plate insulated (which is interesting in itself) but I can't explain the lacking regenerator (which would nix my scheme) except that maybe the thermal ratio was so low that the heat moved around thruout cycle, especially considering the out-of-phase dynamics.

As a long time alpha guy, I'd like to think that I would have found this years ago if not stuck in alphaland, but unsure. Meanwhile, Tom, go ahead and stick a couple feathers in your cap !!! The main question I have now is how did all the academics miss this ??? When I get a grip on various ratios, I'll have a grip on various constraints. Until then, all I can say is that...you don't need a heat sink for a closed cycle Stirling cycle (sometimes).

To clarify my position. It seems a temperature difference is necessary, one portion of the engine hotter than the other, but the heat going into the engine on the hot side does not, of some necessity have to spill over and out of the engine through the cold side.

This does not have all that much to do with any design parameters. It has to do with the nature of heat.

Heat is not a "flow" of liquid, or even a flow of energy THROUGH the engine, in one side and out the other.

Rather than two "reservoirs" with heat flowing from a higher to a lower temperature to power a water wheel set between the two levels it is more like WAVE ENERGY "vibrating" a single stationary body of water.

I'm also talking about Stirling engines with a sealed quantity of air or other working fluid, not necessarily other "heat engines" that have steam or fuel/air passing through them

Adding heat to a Stirling engine through the action of a displacer (heat valve) introduces "waves" into the internal gas. The engine is designed to intercept and utilize.this "wave energy". Stop adding heat and the wave activity subsides.

Are there potential loses? Sure. But they are not like the "loss" of water from a reservoir to power a water wheel. That kind of power generation absolutely requires that the water pass from a high to a low level. "Wave energy" can and does exist in a single reservoir. Capturing and utilizing that energy has nothing to do with any flow through the engine from the hot side to the cold side.

As yet, though, Stirling engines are not all sophisticated enough to take energy from individual "waves" so there needs to be a wave, and a no-wave condition alternating.

Except possibly some thermoacoustic type Stirling engines which apparently manipulate the waves more directly.

Either way, the old Carnot water wheel model is misleading and can lead to design errors.

[Tom Booth]

That "wave energy" analogy, it should be recognized is an analogy, not intended to be taken too literally, but if we are going to liken HEAT to water in any way, then to imagine a Stirling engine as a kind of wave energy machine, that can function in a "single reservoir" is, I think, more apropose than the waterwheel analogy which absolutely requires two "reservoirs", a high and a low.

Another analogy of a kind of energy harvesting that only requires one "reservoir" could be tidal energy. But really, any reference to a water-like FLUID is not going to be accurate in relation to heat since heat is conceived to be only a form of energy (not a fluid or any other substance)

Beyond that, however, setting aside theory regarding the nature of heat, the conclusion that a "sink" is not necessary, for me, is based mostly on simple observation. As you mentioned: "Tom's LTD ran with cold plate insulated".

Also, by way of observation, generally, with "thermoacoustic" engines, there is no flywheel to overcome the pressure from the remaining "waste heat" to complete the cycle and there is no "displacer" to drive the gas over to the cold side for cooling. Nor is there always a gradual buildup of heat under these circumstances.

I do not particularly concur with these statements:

"...you don't need a heat sink for a closed cycle Stirling cycle when scheme is carefully designed. For alpha yes, for gamma no, but this gamma scheme requires an unusual event sequence and a good regenerator. Let's just say that the typical 90 deg out-of-phase crap won't cut it here..... Although unusual, this event sequence.... maybe the thermal ratio was so low that the heat moved around thruout cycle, especially considering the out-of-phase dynamics.
."

I've been waiting for you to elaborate beyond: "I found this while messing with some numbers" before commenting, but it has been a few days. But what I'm saying I disagree with, is I've made such observations mostly in regard to ordinary Stirling engines running in the standard way with standard 90° phasing. Sometimes with, and sometimes without a regenerator. Nothing particularly "unusual" about it, though what you are actually talking about I have no real idea.

[Bumpkin]

If true, it logically leads to an ambient powered engine, where the only exhaust is”cold.” I’ve never thought that was impossible; and have been down some of those rabbit-holes myself. I sure wouldn’t discourage others from sifting through those same weeds, or even looking in whole new fields. It would be the height of arrogance to think we already know it all.

Bumpkin

[VincentG]

Tom I think you are spot on with wave energy in a literal sense. Tesla would agree. The secret to the universe is vibration. Heat energy IS vibration(waves) whether it be molecular or light(IR), the hurdle is turning that into something useful to us.

Right now all we can do is capture that energy as an opposing
force between a solid object and a piston.

The solution will have no pistons.

[Tom Booth]

... Heat energy IS vibration(waves) whether it be molecular or light(IR), ...

Perhaps another "particle wave duality" conundrum is lurking, not too far around the next corner somewhere.

I've already encountered references to "particles of heat" as "phonons".

[Tom Booth]

There is a statement in an article I just came across, looking into "phonons" which reads:

This movement is generally random, and its energy then corresponds to the temperature of the material. But if carefully triggered, it may happen in unison, leading to billions of atoms moving together as a whole.

https://www.eurekalert.org/news-releases/498052

Though the article is not specifically related to heat engines, this statement closely parallels what I imagine takes place in a "thermoacoustic" Stirling, and possibly all Stirling engines to one degree or another.

The generally random motion or diffused heat is triggered into oscillating.

[Tom Booth]

Put another way, the traditional view of a heat engine is to add heat to cause expansion and then remove that same heat to cause contraction

The general consequence of this is engines that are designed to siphon off the "waste heat" which results in abysmal efficiencies as the fuel(heat) is being continually and intentionally thrown away to make room for more heat, to likewise be wasted.

If viewed as an oscillation, the design goal should be to add but also retain as much heat as possible in a way that reinforces the oscillation.

[Tom Booth]

https://youtu.be/5v5eBf2KwF8

[VincentG]

While I think its important to theorize these things, to get back on topic a bit....who cares if we use active cooling when it's available in unlimited amounts with ground source or air? While it may not be ideal, we can learn far more from just building and testing than years of theorizing.

Tom, if you scale up your HTD conversion 5x, place it on the wood stove and add some active cooling you'd have a real power producer.

[matt brown]

"...you don't need a heat sink for a closed cycle Stirling cycle when scheme is carefully designed. For alpha yes, for gamma no, but this gamma scheme requires an unusual event sequence and a good regenerator. Let's just say that the typical 90 deg out-of-phase crap won't cut it here..... Although unusual, this event sequence.... maybe the thermal ratio was so low that the heat moved around thruout cycle, especially considering the out-of-phase dynamics.
."

I've been waiting for you to elaborate beyond: "I found this while messing with some numbers" before commenting, but it has been a few days. But what I'm saying I disagree with, is I've made such observations mostly in regard to ordinary Stirling engines running in the standard way with standard 90° phasing. Sometimes with, and sometimes without a regenerator. Nothing particularly "unusual" about it, though what you are actually talking about I have no real idea.

Matt Brown's voodoo cycle.png (18.45 KiB) Viewed 4161 times

Here's my non compression gamma scheme and sorry I can't post a sketch (never used phone camera for such).

Anyways, look at this timeline for 720 deg where

Dis A = 1000cc swept vol
Dis B = 1000cc swept vol
power piston = 500cc swept vol
Dis A,B, and power piston all have the same stroke length (so Dis A & B have ~1.4 greater bore than piston)
300-600k cycle

Assume a rotary valve is located on power piston cylinder head with a first conduit connecting valve to cold space Dis A, and a second conduit connecting valve to cold space Dis B. Rotary valve runs 1/2 piston speed (like ICE) with 2x 180 deg duration events: 1 to Dis A, 1 to Dis B. Just so no one complains about lacking means to drive displacers...let's add a cam running at 1/2 piston speed with 2x 180 deg duration events: 1 for Dis A, 1 for Dis B (are you guys seeing a pattern here). However, note that the valve events are "interleaved" the displacer events.

Now, for the thermo part where this particular cycle is 300-600k. Let's follow this timeline from 0 deg and only focus on piston and Dis A (for now) where piston is TDC with 0cc vol, and Dis A is BDC with 1000cc hot vol and 0cc cold vol. Assume that there's no dead vol, just to make this easier. 0-180deg piston expands gas from 1000cc hot space thru regen to cold space and fills 500cc piston cylinder. At 180 deg the engine has a combined vol of 1500cc. Pressure thruout engine has decreased, but remains equal thruout engine (Schmidt assumption). However, since the hot space is 600k and the cold space is 300k, the gas density in cold space is 2x hot space.

Now, between 180-360 deg, if we have the piston AND Dis A move towards TDC at the same speed, we can have a 1:1 constant volume transfer from 300k piston cylinder to 500cc of 300k displacer cold space while simultaneously having a 2:1 constant pressure transfer from 600k Dis A 1000cc hot space thru regen to 300k Dis A 500cc cold space.

Next, between 360-540 deg, the 1000cc gas in 300k cold space with return thru regen to 1000cc 600k hot space, but this time regen is 1:1 constant volume transfer. Last, between 540-720 deg, gas continues heating (bogus if ideal, not so in reality).

Meanwhile, Dis B and piston follow the same sequence.

This scheme requires careful design, since swept vols and thermal ratio are inter-related. I'll cover some more tidbits in the future. But for now...there you are guys, Tom was right, isothermal compression is not mandatory !!!

[matt brown]

I'm gonna call this the voodoo cycle, since it defies common thinking. The 2:1 thermal cycle combined with the 2:1 swept volume ratio (displacer:power piston) is a coincidence where both are 2:1. This is a "crossover" point between these ratios and they don't scale up & down linearly. Don't get me wrong, there's various cycle possibilities, just not as simple as swept vol ratio = thermal ratio. I have various examples worked out and still chasing down the tiny equation between the vol ratio vs the thermal ratio.

[Tom Booth]

While I think its important to theorize these things, to get back on topic a bit....who cares if we use active cooling when it's available in unlimited amounts with ground source or air? While it may not be ideal, we can learn far more from just building and testing than years of theorizing.

Tom, if you scale up your HTD conversion 5x, place it on the wood stove and add some active cooling you'd have a real power producer.

The point I've been trying to get across, with considerable futility, for, I think, more than ten years is that active cooling is actually detrimental. This is not readily apparent, because, among other things, "everybody knows" putting an ice cube on a toy LTD for example makes it go faster. This is an observable fact. What is not so apparent though is why this works.

Another point that is difficult to get across is that power output, or the conversion of heat into work is a form of "active cooling" in itself which does not have an efficiency penalty.

Third, another difficult concept to get across is that a Stirling engine IS NOT a heat pump "running in reverse", it is ACTUALLY a heat pump operating in exactly the same way regardless if it is running as a heat pump or a heat engine. Only some outside conditions have changed, the actual working fluid goes through the same cycle, it does not know the difference.

How do you get a Stirling Cryocooler to work?

It is obvious that for a Cryocooler to be maximally effective, the "cold finger" needs insulation around it to prevent the surrounding ambient heat from warming it up.

The same is true for the cold side of a Stirling engine. It is being heated up every which way EXCEPT via the working fluid, which is actually acting as a kind of heat pump to keep the cold side cold.

Generally what is done is to neglect heat transfer to the cold side through conduction, through convection, through the heat in the surrounding ambient air. The cold side is heated up to 300K by the surrounding ambient before the engine even has a chance to start. The relatively small amount of heat added to the hot side is a drop in the bucket with much of that being conducted straight over to the cold side every which way EXCEPT through the working fluid, which is keeping the cold side cold by converting heat into work.

So "active cooling" works, only because it somewhat compensates for negligence in other areas. It allows more heat to be thrown at the engine, so it can run a little faster, but at the expense of ever diminishing efficiency. In some situations, maybe that's an acceptable solution, if you have a mountain of "free fuel" of one kind or another, but the engine will have to be bigger and it will use up the fuel much faster. In reality, it does not improve the power and efficiency of the engine in terms of weight and size of the engine to power output ratio and rate of fuel consumption.

[Tom Booth]

In an effort to show the heat pump action of a Standard Stirling engine with an ordinary 90° advance:


https://youtu.be/i_wcbfE364s


A few things about this. The "sink" for this engine is clear acrylic, a very poor heat conducting material, so, aside from the visual aspect of being able to see what's going on inside, the "cold side" of the engine is pretty much an insulator.

Contrary to what is frequently taught about these engines in academic circles, as well as generally, heat is not "rejected" during "isothermal compression".

Compression results in a rise in temperature, the "heat of compression".

In actuality, in a Stirling engine the heat INPUT, due to the lifting of the displacer exposing the air inside tbe engine to the hot plate, and compression, take place simultaneously, so that the compressed hot air (HOT due to the "heat of compression" like a fire piston) is pushed, by tbe displacer, down into contact with the engines hot plate.

This is how any heat pump works. A gas is expanded (cooled by expansion) when in contact with cold, so that it can take in heat from a cold source, then the gas is compressed (heated by heat of compression) so that it can "pump" the heat over to an already hot space.

If turned in the same direction with an electric motor, the heat pumping action would continue in the same way, except that more heat would be transfered because the heat is not being used to drive the engine and more "heat of compression" is being added due to the work input from the motor.

A Stirling engine does not operate by facilitating the "flow" or transfer of heat from the hot to the cold side, quite the opposite.

[Tom Booth]

To put it another way.

"Energy" follows "the path of least resistance".

So a Stirling engine RESISTS the natural dispersion of heat through the engine. It keeps pushing or "pumping" the heat back the way it came so that "the path of least resistance" then, becomes pushing the piston.

The engine has to have a greater resistance to the flow of heat than the path that results in power output to the load. The energy cannot dissipate as heat, so it goes out as "work".

So, I think, keeping the cold side insulated (protected and shielded) from the heat of the surrounding ambient, (or wood stove), would produce better results than providing the heat with an easy outlet through a water cooling jacket.

I'm not, or I am no longer just looking to have a functional engine that I can get some useable power out of, I'm endeavoring to redesign the engine based on first principles. The Carnot theory that you can improve heat engine performance by throwing water on it to increase the "flow" to the sink just doesn't cut it.