From Stirling engines to thermoacoustic generators

[greybeard]

Halfway through the construction of the prototype mirror(applying the aluminium foil) I'm underwhelmed by the surface I'm producing.
I've made the 'petal' type ply base, with 32 sections, and cutting into the central area was a pain. The template leaves a fairly large flat area(16 cms diameter) in the centre if one is to avoid the likely possibility of a petal snapping off during the sawing.
This was then stapled up before the joins were glued together.
First lesson was that a ply sheet bends easier one way than the other, so the whole thing has a tendency to become a trough.
Second lesson was that cutting out foil is most easily done with scissors. The sharpest scalpel rips the foil, instead of slicing through it.
Spraying a mount adhesive was straight forward, as was laying it down and burnishing the surface with a small pad of cloth.
As this was a prototype I'd left in the staples and made no attempt to improve the ply surface. Burnishing the aluminium showed up all the imperfections, even the grain, though it is a very fine surface ply I'm using.

So, what next? At the moment we have thick fog so I'm not in a position to play around, setting fire to bits of paper, but it does give me the opportunity to pose the following questions.

How 'good' a curved surface is actually needed ?
What I have in mind is that if the solar energy hitting an 8' diameter parabolic surface is reflected back to hit a target 6" in diameter(the hot plate of my stirling engine), does it matter if the image is greatly distorted ?
Suppose a second 8' mirror was made up of 2" square flat tiles. Each one would produce a 2" (roughly) square patch of light on the target, so the total light hitting the mirror would still be all hitting the 6" target after reflection.

Is there any difference in the amount of energy received that I have overlooked ?

The aluminium foil, as noted above, shows up all the blemishes, so now I'm considering using a plastic sheet instead of ply. If I managed to keep it dust free(no small task) it should produce a major reduction in light loss through 'scattering' from the surface. I'll keep to the foil as a reflector, as plastic sheeting and kitchen foil are readily available to most people, whereas aluminized plastic isn't, never mind the cost.

[delt0r]

Been in NZ for 3 weeks and had generally limited internet.

Anyway, How good a curved surface do you need.

It depends. Its not high quality optics level, but if you need a high concentration ratio they need to be pretty good. Also there is usually a non-imaging optics 2nd stage focuser.

non imaging optics are very cool. again if after a Google your interested i have some papers on them.

But for low level focusing the only problem with a poor reflector is uneven heating of the surface. this is not really a problem for low temperature absorbers (ie copper <1000 C).

I have used a hand made Al foil and got to 250 C will a little insulation on the absorber. It was pretty bad so doing better should be easy.

[greybeard]

non imaging optics are very cool. again if after a Google your interested i have some papers on them.

delt0r - thanks for the heads up on non imaging optics.
Did as you suggested, an googled the subject, and read a couple of papers.
Interesting to discover there's a name for what I had in mind

I'd be glad to read anything you have on the subject. It may be a bit of a side issue, but I'm a great believer in acquiring as much knowledge as I can - you never know where it may lead, and doesn't take up much room.
Regards
John

[delt0r]

There was a good article back in 1991 in Scientific America (march). There digital archive does not go back that far however.

I can't find any good links now, but generally the curves are pretty complicated and are not easy to make without CNC. I have just found a good way to full text index my PDFs (swish-e) but I can't find my really old PDF archive. I will have a look at work tomorrow.

[cozmicray]

Lamina Flow Engine

http://www.stirlingengines.org.uk/thermo/lamina.html



The basic parameters of the engine are

1. Bore - 20mm
2. Stroke - 18mm
3. Hot cylinder - 14mm ID X 140mm test tube
4. Flywheel - 75mm diameter 3 spoke
5. Con Rod - 75mm long with ball bearing big end.

[OneAndy]

I realize this is grave digging... but any progress?

I'd love to start building my knowledge and skill-base in this area so I can eventually try to build a usable Stirling engine genset.

[greybeard]

I realize this is grave digging... but any progress?
.......

Bit too close for comfort Andy, but thanks for posting, as it's got another part of the brain working again.

.....I'd love to start building my knowledge and skill-base in this area so I can eventually try to build a usable Stirling engine genset.

I've got as far as re-building my first cnc, which is meant to be the workhorse and first step to the ultimate goal of generating our own power.
My plan has been(is) cnc/solar mirror/hot water/hotter working fluid(oil?)/generator.
So you can see I've a long way to go, never mind the domestic life that slows us all down.

But now the sun has started to shine again in this neck of the woods, I feel encouraged to change up a gear.
Any information you can bring in, Andy, would be most welcome, and help to keep my brain going
Regards
John

[epineh]

While that brain is going again John ... what sort of temp diff would be needed to make any sort of usable power using a stirling generator ?

We have so much sunlight here it seems a crime to be using coal fired power stations (during daytime hours anyway) Our plumbing is run through the roof space and on a hot day when the cold tap is run the water that has been in the pipes in the ceiling is hot enough to scald you if you put your hand under it, until the cold stuff kicks in. For me a dish that could go above 100deg c would be enough to generate steam which is easily converted into electricity using good old fasioned technology, the used steam/water could be kept in the roof space at a nice warm 70ish degrees. I know that last 50 or so degrees takes a lot of effort but it seems worth a shot.

Surely a simple mirror array like those solar furnaces but on a small scale is not that hard, and I am sure a few steppers/servo's could be used (this is CNCZone after all ) to keep alignment of the mirrors while the sun moves about.

Just adding another project to the list

Russell.

[greybeard]

Hi Russell.
I suppose there's no simple answer - the stirling's great attribute is that it can work with very small differential temperatures. However, like all energy conversion processes, the greater the temperature difference, the greater the efficiency of the conversion.
This in turn raises problems with avoiding heat losses/choice of materials/capital costs of the system.
Many years ago, a friend had the idea that with a low temperature difference, the way to go was to use huge lightweight pistons, 6 foot diameter polystyrene foam. We never got to the fine details of his ideas, which might have been just as well, and instead, concentrated on finishing the bottle.

With your situation in Oz, perhaps using the solar energy to purify waste water might be more useful.
Another fundamental problem, of course, is how to match the timing of the solar energy input to your energy needs. Storage of the energy will kill most schemes from an economic point of view, I think, so if you can identify an electrical energy need that matches daylight hours, then a sterling/generator should be a good way to go.

My most recent thought on this topic in general was to modify my mirror design to make use of a local sheetmetal company with a seriously big laser cutting facility.
I'm thinking of laser cut stainless sheet, with suitable notches/tabs, that will assemble without fastenings to form a reasonable parabolic surface.
The reason I'm not considering a simple "trough" shape, is that I want a high temperature area to become the hot end of my stirling. The trough is perfect for producing hot water in a very cheap fashion, but there's a limit to how many hot baths I can take

Regards
John

[skippy]

Hi John, I was in contact with Lance (Evodyne) for quite a while some years back but we lost contact and I don't think he checks in here any longer. You wouldn't happen to have an email address for him would you?
regards
Phil

[CJL5585]

John,
A couple of years ago I downloaded from a thread on this site (I think), a program for parabolic dish and trough reflector design. I have since that time lost a hard disk or two, along with the program.

Do you have, or know of a link, where I can get such a program? I have done a couple of searches without results.

Also, any luck on the CNC?

Jerry

[OneAndy]

Bit too close for comfort Andy, but thanks for posting, as it's got another part of the brain working again.



I've got as far as re-building my first cnc, which is meant to be the workhorse and first step to the ultimate goal of generating our own power.
My plan has been(is) cnc/solar mirror/hot water/hotter working fluid(oil?)/generator.
So you can see I've a long way to go, never mind the domestic life that slows us all down.

But now the sun has started to shine again in this neck of the woods, I feel encouraged to change up a gear.
Any information you can bring in, Andy, would be most welcome, and help to keep my brain going
Regards
John

Thanks. My plan is similar, tho I'm skipping some steps and going a slightly different route.

CNC (nearly done). It's a moving gantry style for hardwood cutting to ramp up my main money making business with gun grips, stocks, signs, engraving, etc...

Even though it's primarily for wood, I've taken great pains to make it very stable against load in alignment of leadscrews to bearings and nuts, spindle to table, and flex in parts. This baby ain't going nowhere.

And I beefed up my travel. 50x36. Yep, I'm going to get into METAL CASTING using the lost foam process! I figure any adventure in making engines from scratch will be greatly aided by the ability to cast somewhat precision parts. Precision cut-foam for investment casting seemed alot easier and precise for metal work than the traditional green sand casting method.

So while waiting for my flanged bearing upgrades for the CNC to come in from MSC, I went ahead and poured a heavy(ish) duty foundry in an old air bottle I had a welding buddy of mine cut the top off of. Inswool and 2.5 inches of satanite refractory morter! I'll be able to stick a 5x7 crucible made of 3/8 well pipe in there. Satanite is rated for 3,000 degrees F, so I should be able to cast something like Brass, once I've cut my teeth on Aluminum.

The overall objective here is complete energy self sufficiency. While I am totally intrigued by Stirling technology, it just doesn't seem to be developed to the point of real feasibility, yet. I've not given up and will be happy to brainstorm and experiment, but I need/want to become self-sufficient as quickly as possible.

So I hit upon extremely low rpm Lister Diesels. They're ridiculously long-lived, easy to maintain, extreme fuel sippers, and will probably run on at least some used motor oil (not to mention veggie oil), plus road diesel has been coming down in price pretty fast recently($1.95 at the closet station). The only problem is, yeah you guessed it... new EPA regs came out. I don't think they can be imported here any longer. I just love our Federal overlords.

I'm stuck either trying to find an old one or building a new one from scratch. Which, I'll have the means, if not the ability right off the bat, to do with a CNC, Foundry, Metal lathe w/ milling attachment setup.

Tomorrow I'll know whether or not I've won a decent Atlas lathe from the wonderful world of government auction. If so, I have the dubious honor of making a 1500 mile round trip to go get it. Whee!

So I suppose my setup process so far is:

CNC, Foundry/casting, metal lathe, Lister Diesel restoration OR Lister Diesel fabrication OR maybe an old Bamford or similar.

Simultaneous effort on Stirling research and design. One big roadblock I've come up against in THAT tho, has been finding books by the big names in the field that are actually still in print!

I tried to find a copy of James Rizzo's two volume set "Stirling Engine Manual"... and could only find Volume I. Volume II is where most of the meat is at, I think.

Nor was I able to locate Andy Ross's book "Making Stirling Engines." It appears to be in revision.

I may eventually buy "The Regenerator and the Stirling Engine" but I haven't bought a $200+ book since college and I'm sure I don't know enough about the Stirling Engine field for this book to be worth the outlay, yet.