# Thread: Low Temperature Differential Stirling

1. development.1318cermak.com

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i was working on a project that aimed to program micronctrollers using a macromedia flash-like timeline interface and possibly a scripting language.. this is obviousely a good idea. however, the software development proved exceedingly difficult, so i dont know... i never wanted to be a great programmer... making a full featured application is obviousely possible, given alot of hours i dont have..

anyways, seting up a sweet forum for people interested in developing an alternative energy project is easy. that i can accomplish, with passable graphics, and alot of features.. my inexpensive hosting would accomidate a small but active group of misguided visionaries.... plus, as soon as a usefull idea emerges, i look forward to producing some extremely high quality 3d models and related drawings...

parabolic dishes... surely it is simple to begin designing around an 'ideal' concentrator geometry, based on some actually usefull persons research?

2. Years ago, (1967)a fellow eccentric and I were discussing stirling engines(he was doing most of the talking and I was looking for flaws in his logic) and he made a point that I've remembered but not verified. He said that, all other things being equal, the power from the engine would be proportional to the diameter of the piston. Therefore if you double the diam, you get x4 the output. So he thought that the way to go would be enourmous pistons, probably made out of foam, with only a relatively small and slow movement. It might be possible to think in terms of the flexing of the surface of the "piston" as the movement, rather than a sliding piston in a cylnder.
We thought of a surface made up of a honeycomb of foam, each hole being filled with coarse steel wool as a "displacer" if that's the right term.

Re parabolic mirrors - start with a large hoop(plastic tube pulled into a circle) with each surface covered in aluminised melinex film and pump out the air between, each surface is a pretty good parabola. Use the pipe as an air duct as well as being the frame.
If you control the pressure, you can control the focal length.

John

3. I believe this one was done for experiments in making ultra-pure high temperature alloys.
Yes, they are fiddling with ceramics amongst others. The place is also a general test site for solar energy, because they have (quote from Trevanian) "no weather".

Re: Greybeard. One thing when sizing up is mentioned, reminds me always of the question: Why does a mouse eat, measured in proportion, more than an elephant, to maintain by burning food it's temperature?

If we dimension (dimensions chosen for ease of calculation, not for reality) a mouse 1 cubic centimeter, it's cooling area is 6 square centimeters, dimension a human 1000 cubic centimeters (cubic decimeter), cooling area 600 square centimeters, elephant 1000000 cubic centimeters (cubic meter), cooling area 60000 square centimeters. From these ratios you can derive that everything has an ideal size. Human: not eating all day and yet not massive. Massive pistons: slow through dead areas. Small pistons: high surface and friction losses.

Carel

4. power is force times distance, so a large piston over a short stroke would produce more force, but is not more power than a small piston over a longer stroke. What I am saying is it is a larger displacement that produces more power, not a larger diameter.

Eric

• Originally Posted by fkaCarel
Yes, they are fiddling with ceramics amongst others. The place is also a general test site for solar energy, because they have (quote from Trevanian) "no weather".

Re: Greybeard. One thing when sizing up is mentioned, reminds me always of the question: Why does a mouse eat, measured in proportion, more than an elephant, to maintain by burning food it's temperature?

If we dimension (dimensions chosen for ease of calculation, not for reality) a mouse 1 cubic centimeter, it's cooling area is 6 square centimeters, dimension a human 1000 cubic centimeters (cubic decimeter), cooling area 600 square centimeters, elephant 1000000 cubic centimeters (cubic meter), cooling area 60000 square centimeters. From these ratios you can derive that everything has an ideal size. Human: not eating all day and yet not massive. Massive pistons: slow through dead areas. Small pistons: high surface and friction losses.Carel
A lazy person like me just says it is the surface to volume ratio.

Originally Posted by Greybeard
Re parabolic mirrors - start with a large hoop(plastic tube pulled into a circle) with each surface covered in aluminised melinex film and pump out the air between, each surface is a pretty good parabola.
Are you sure? I thought this method gave you a spherical surface, the force acting on the film is normal to the surface at all points

• < Massive pistons: slow through dead areas. Small pistons: high surface and friction losses.

Carel - I'm not sure what your analogy is leading me towards. I go with there being an ideal ratios in all areas, but if one's "ideal" is lowest capital cost/power ratio, presumably this would lead to a different result than if your ideal was say maximum power/size. To be honest I've no idea, but to look at off the wall ideas may be useful in both learning and achieving, don't you think ?

Geof - I'm sure you're right. I'm probably getting mixed up with chains draping in catenary curves. Put it down to the drink - I need another one.

John

• Eric - I'm sorry, I made a cardinal error of condensing several thoughts into one. What I should have said was given the larger output of a large diameter piston(strokes the same) then the flexibility of the larger area would allow you to consider it as a flexing surface, rather than a sliding piston. This in turn would remove frictional forces and replace them with the stiffness of the "membrane" to be considered.
The drift of the ideas at that time was to look for an approach that would allow the use of cheap materials/engineering even though it would be radically different.

John

• A lazy person like me just says it is the surface to volume ratio.
Yes, it is lazy. I did an effort to explain the volume/surface ratio. This explanation was an introduction of the general principle that things can be too small or too big.

Carel

• If you have Pyreneean weather, solar furnaces are a great starting point.
But if you only have a large dirt patch, consider a hole in the ground as the "cold" end of your design, with a moving solar panel above it as the "hot" end, and with a large diameter membrane over the hole as the "piston"(or should that be "displacer" ?)
Just a thought.
John

Edit

Geof - if the target of the mirror is a piston end, then the spread of focus that a spherical surface will give you may be an advantage ?

• Originally Posted by greybeard
Geof - if the target of the mirror is a piston end, then the spread of focus that a spherical surface will give you may be an advantage ?
I don't know that it would make much difference; for the amount of curvature you are talking about a spherical reflector and a parabolic reflector are pretty much indistinguishable. Your statement that "each surface is a pretty good parabola" is quite correct. I only asked if you were sure .

• Just found this idea/site when I googled "sterling engines diaphram".
Thought it was a very clever way to get the 90 degree phase lag into a simple construction based on the diaphram idea.
90 x 4 = 360 = swashplate ... brilliant.

I'm now trying to think of my own method.

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