Developing a 1000w generator based on approtechies' design

[danalinscott]

Trevor,

I'm not sure I understand what you mean by the upper and mid range areas.
Are you referring to the movement range of the diaphragm?

[Trevor]

Yes. When the diaphragm is mid way is when the engine requires more force to move up or down.
I guess it all depends how the diaphragm is manufactured.

[danalinscott]

As much as I hate to make changes to any working design early on I am considering replacing the aluminum bottle which forms the central core of approtechie's displacer with an aluminum cylinder to eliminate the dead space the neck area creates. It seems like a "no-brainer" but of course I only have one point of view and a single set of experiences. Is there any downside to eliminating dead space in a Stirling engine?

I am also considering increasing the diameter of the hollow displacer piston core slightly to lower the dead space created by the steel wool regenerator material. I have not been able to find any ideal ratio of hot end to regenerator space. Is anyone reading this aware of one?

[Blaf]

Dead space imho might be useful in cases you need lower compression ratio. I'm not able to tell if your build is the case though :)

[danalinscott]

I have begun receiving the "off the shelf parts" for the single cylinder test/development engine. As expected they are revealing flaws in my first design. I had originally planned on using stainless steel "Bain Marie" pots. Unfortunately the supplier who assured me that they were strait sided cylinders was incorrect. The sides are strait but not cylindrical being wider at the bottom than the top. Clearly they will not work as a displacer cylinder. Luckily they appear to be useful as the outer casing for the cooling jacket.

This has prompted me to use 6" exhaust pipe as the displacer cylinder instead. Unfortunately this material is not available in stainless steel but appear to be about 4 times as thick as either the coffeemaker tank approtechie used or bain marie pots I had hoped to use. .063" to be exact. Hopefully it will stand up to constant red hot temp it will be subject to.

Thoughts?

[7spoons]

I am also considering increasing the diameter of the hollow displacer piston core slightly to lower the dead space created by the steel wool regenerator material. I have not been able to find any ideal ratio of hot end to regenerator space. Is anyone reading this aware of one?

My understanding is that in ideal engine regenerator material would absorb and release heat from all of the air being transferred from hot side to cold and back. If we take for example an engine that has 100cc displacement and 50C dT, air as working fluid at atmospheric pressure - regenerator needs capacity for 100*50*0.001297 ~ 6.5 Joules of energy. (specific heat capacities @ wikipedia)

Then, if we're using steel wool for regenerator material, we'll need 6.5/(50*0.466) ~ 0.28g of steel to hold the heat, assuming 100% heat transfer efficiency. For real world I'd at least double that since there will always be temperature gradient within regenerator material... Also depends on geometry of the regenerator space (re: SP...) Not sure how tight you can pack steel wool without restricting airflow too much...

[danalinscott]

0.28 grams seems pretty light.
A penny weighs 2.5 grams.
Are you sure your calculations are correct?

[7spoons]

I agree, it does sound a bit low however I can't find an error in my math. According to the table 1g of steel holds nearly 1/2 joule per degree and 100cc of air holds about 1/8 joule per degree so yes, 100cc of air has about the same heat capacity as 1/4 gram of steel. Bizarre...

I guess the good news is that you can't possibly run out of regenerator capacity if you use amounts that look reasonable :)

For reference, a wad of steel wool that I buy from hardware store weighs about 11g...

[danalinscott]

If your calculations and the tables used for them are accurate the layer of steel wool surrounding the displacer piston core will be approximately 1/8 to 1/4 inch thick. That will be significantly different than the amount of steel wool used in Approtechies engine.

Luckily I am going to be able to test this.
The single cylinder test engine is designed to be easily disassembled, modified, and reassembled.

I have in the past found that nothing beats making a slight modification to a device after bench marking it and determining if performance improves or degrades. It is a simple but repetitive process.

My hunch is that a 7/8" layer of lightly packed steel wool will produce the best performance.
But to be honest I have no idea if this is what will test out as true.

[Bumpkin]

Hi danalinscott. A thought about layer thickness:
I believe the reason that engines that reciprocate a regenerator intead of a displacer perform well is that the blanket is doing more than just regenerating. In my opinion the effects of radiant heat transfer are not given enough credit in these engines. If you divide your regenerator in thirds, imagine the hot end accepting radiant heat from the chamber and then conducting it to the air in the flow. The same for the cold end, only in reverse. The middle remains the regenerator. If the radiant effects are profound, they probably taper off further into the depth of the bIanket.

It seems to me that in this design the different regenerator zones are compromised by their stroke into differing zones of the displacer chamber; and likewise, the chamber zones are compromised in an unavoidable reciprocating thermal short. Of course that's true of a conventional air-gap displacer too, and all in all it's a pretty good compromise. It seems a big-bore short stroke engine with a thin full bore blanket could use the same radiation-to-conduction principle and still regenerate without the thermal shorting. Likely more difficult to build though.

Looking forward to updates on your project.

Bumpkin

[danalinscott]

I agree.
Components, materials, and special tools are arriving daily.
However being part of a local virus response team has diverted my attention temporarily.
Luckily I received Principals and Applications of Stirling Engines a few days ago and am reading through it as time and my comprehensive abilities allow.
It is a very welcome break from my grim daily routine.

Rest assured the project is moving forward albeit at a slower pace than before the pandemic reached our shores.
Stay safe..help others.
Dana

[Trevor]

Hi Danalinscott
Again looking at the regenerator for you and what you are using - Using steel wool maybe not a good idea and this is due to the weakness of the fibres. You will find the steel wool will shred to pieces over some use. I found that out the hard way.
You may find that the use of stainless steel wool will be a lot better to use, though may be a little corser. The stainless wool takes awhile to heat and takes a few cycles to get there, however it retains it once it stabilized.
Trev

[danalinscott]

Thanks Trev,

Initially I plan to use the coarsest steel wool available to minimize shredding and experimenting with high temp binders as this is the least expensive option. If this fails I plan to try stainless steel "scrubby" material.

Is this the material you are describing?

[Trevor]

Sure is Danalin
You will find that the stainless is a lot more resilient than steel is and even that can shred.
I hope all goes well with you. I also noticed that you were going to use a alloy displacer. Ian gave me good warning that this may melt under high heat conditions, and! Yes, that is exactly what happened. The other failure of using alloy I found as it gets real hot not quite near melting point it will collapse under the vacuum of even swell if the pressure gets too high in the displacer piston.
Regards trev

[danalinscott]

The aluminum displacer core will not be sealed. So temp induced pressure changes will not develop.
In addition the top will be insulated with ceramic fiber mat.
Theoretically the air flow through the regenerator material should be sufficient to prevent the underlying displacer core sides from overheating. I believe that I can place a few small temp sensors in the displacer and regenerator to help determine if the heat stored in the regenerator is efficiently transferring or not.

I hope all are weathering this pandemic well.
Stay safe.