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    Member greybeard's Avatar
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    With helium readily available through part balloon shops, I would certainly seriously consider that as the best working fluid.
    As it's inert and not eco-unfriendly, those are also on the plus side, and as the engine can be designed to be a sealed system, there would be the minimum top-up requirements. There will be some, because helium will diffuse through anything eventually
    (Hands up who remembers Graham's Law of diffusion - or was it Dalton's )

    Geof - just to put one idea to rest, is it deffinitely best to use a gas as the working fluid, rather than a liquid ? I feel there may be pro's and cons, but I'm out of my depth to be honest.

    John

    It's like doing jigsaw puzzles in the dark.
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    Anybody got a chart on helium vs temperature expansion/contraction or pressure vs temperature?

    Phil, Still too many interests, too many projects, and not enough time!!!!!!!!
    Vist my websites - http://pminmo.com & http://millpcbs.com


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    Quote Originally Posted by greybeard
    ...Geof - just to put one idea to rest, is it deffinitely best to use a gas as the working fluid, rather than a liquid ? I feel there may be pro's and cons, but I'm out of my depth to be honest. John
    Stirling has to use a gas, the Stirling cycle is based on the change in the product of volume multiplied by pressure with temperature. Furthermore, I am fairly sure it has to indeed be a gas not a vapour. And you ask (maybe) what is the difference between a vapour and a gas? At temperatures below the critical temperature for a particular substance the substance exists either as a liquid or as a vapour depending on the pressure. Below the critical temperature it is possible to convert a vapor back to a liquid simply by increasing the pressure. Above the critical temperature no matter what pressure is applied the substance exists as a gas; it cannot be converted back to a liquid. The critical temperature for water is somewhere in the 700's degrees F (I think). With regards to steam engines and turbines, particularly turbines, you might have heard about them operating 'super critical'; this simply means the exit temperature of the steam is above the critical temperature for water.

    Stirlings are very different, thermodynamically, to steam engines in that they do not utilize a phase change from liquid to vapour as part of their cycle.

    Maybe all I have done is push you into deeper water, sorry, maybe you could find some books to read, all this stuff is quite interesting. Or take a trip up the Bolton Museum near Birmingham I think or the Museum of Industry (or Science and Technology it may have changed one way or the other) in London. As you might have guessed at one time in my history I taught senior high school and junior college level physics and chemistry; I can still drop into lecture mode on occasion.



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    Quote Originally Posted by pminmo
    Anybody got a chart on helium vs temperature expansion/contraction or pressure vs temperature?
    I think we're moving towards finding a formulaic method of sizing a stirling engine.

    Essentially as Geof said earlier the gas itself is irrelevant other than it's heat conductivity. That's true for pure working gases and gets a bit more complicated for mixes.

    You might be able to find a helium temp / pressure / volume chart from a Scuba resource, they ddo this to calculate trimix and have to deal with each gas seperately and together.

    Normally to plot pressure to temp of a gas you have to add a few factors and follow one of the gas laws. It gets complicated with mixed gases like air but as a noble gas, helium is covered by the Ideal Gas Law. You could use PV=NRT and plot away. P=pressure, V=Volume, T= Temperature in absoluteK, N= Mols of Gas and R= Molar gas constant based upon units used - for bar/atmospheres, degrees K, litres, grams etc R=0.082. Helium weights about 4grams per mol.

    To simplify it, if only Volume and Temperature are changing in the system you can get rid of the constants and end up with V1/T1=V2/T2. In other words the temp and volume are independant of the gas weight, the gas doesn't really matter other than its ability to transfer heat. A gas with a higher heat conductivity will work better so helium would be a gas of choice.

    If you want to be really precise you'd need to use Van der Waalls equations of state; (P+(n^2*a/V^2)(V-nb)=NRT. Where a & b are the Van der Waalls constants for Helium and I've no idea what they are.. you'd have to look them up. Once you've got them you could apply them to hot and cold end and work out the expansion/contraction at given temps.

    Andrew



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    Quote Originally Posted by greybeard
    Geof - just to put one idea to rest, is it deffinitely best to use a gas as the working fluid, rather than a liquid ? I feel there may be pro's and cons, but I'm out of my depth to be honest.

    John
    Jumping on geof's thread here, As he's said gas is needed for Stirlings but have a look at rankin cycle or phase change engines. More complex that Stirlings but use a liquid changing from liquid to gas and back again in the same cycle.



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    Hi Fyffe.
    Thanks for the idea, but I'm having trouble keeping focused on Sterlings as it is !!

    Geof - thanks for pointing out what should have been obvious to me( No problems with lecture mode - I was a lab tech of one sort or another for most of my working life).
    I've no idea what I was thinking of. How could I imagine the compression/expansion parts of the cycle would work.
    I was too focussed on the heat transfer I guess.

    Need another little drink.

    John

    It's like doing jigsaw puzzles in the dark.
    Enjoy today's problems, for tomorrow's may be worse.


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    Quote Originally Posted by greybeard
    .....Need another little drink.....John
    The next one should be a nightcap, yes?

    By the way I was lab tech for a while; one of my employers back in 1969/70 was DAFS - Department of Agriculture and Fisheries for Scotland. That got me interested in stuff so I took a few years out of the trade to get some degrees, did the boffin stuff for a while and then started my own business in 1981.



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    Question:
    Anyone have specs for a 1 meter or 4 foot dish?

    What would be the temperature at the focus point of a highly reflective (Chromed or polished aluminum finish) 1 meter dish?

    Am thinking about constructing an actual model stirling power generation system to see what I can accomplish.

    Jerry



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    Member greybeard's Avatar
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    Jerry - I guess you can only think in terms of how much energy you can bring to a "focus". If you use a small lens to focus the sun onto a piece of paper on a bright day you can set fire to it, but you wont boil a kettle of water.
    Like you, I should like to build a working model first, so my starting point will be to get hold of a "space blanket" - one of those survival sheets of aluminised/gold coated plastic sheets - and work out how to make a focusing mirror. I'll be working on results, rather than design !

    John
    John

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    Greybeard,
    Thanks for the reply. The reason I was asking for the approximate temp at the "focus" point was due to the fact that what I had read indicated that at the focus point the temperature could be as high as 1200 degrees F, and on the other hand "would set anything on fire immediately". Well, to me that seems like it could cover a large temperature range.

    I think that one might want to de-focus the unit at the focal point depending upon how large an area one wanted to heat. Smaller area = sharper focus.

    As for using a space blanket.
    To begin with, I thought about trying to find an OLD fiberglass or metal satellite dish from the 80's and apply a new type reflector for it. One could probably get one for free if he was willing to move it. A space blanket should work very well for this. It would also work on the newer smaller dishes that are approximately 30 inches in diameter (.76 meters).

    I am considering building a solar powered SINGLE CYLINDER sterling engine that I have been thinking about for some time that incorporates all the functions of a 2 piston engine with regenerator.

    Jerry



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    Hi Jerry.
    Just bought myself 20 metres of space blanket off ebay !
    My approach to the mirror construction is to use an evacuated double skinned drum.
    Hope that makes sense. First thought was to bend a piece of plastic pipe round in a circle, and add a pyramid of struts to support the stirling. Now I think that I'll laminate a thin plywood drum. This has the advantage of being able to see how deep a curve I can pull down, so that the focal length isn't too great. The pipe idea would be too shallow I think.
    John

    Last edited by greybeard; 07-12-2006 at 11:54 AM.
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    Duplicate posting. No idea why.
    John

    Last edited by greybeard; 07-12-2006 at 11:53 AM.
    It's like doing jigsaw puzzles in the dark.
    Enjoy today's problems, for tomorrow's may be worse.


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    Quote Originally Posted by greybeard
    Duplicate posting. No idea why.
    John
    Well, Not exactly.

    One had the info on using double sided tape to hold down the solar blanket. I think that might be the one gone.

    Jerry



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    Oh b***er ! Thanks Jerry, at least you had a chance to read it before I erased it ! So I assume you've got the picture.

    I've also been looking at a German enthusiast's site at http://members.aol.com/hstierhof/index.html who has some interesting small scale ideas, but unfortunately his assembly techniques and drawings are a bit difficult to fathom.
    I'm particularly interested in his "Free Cylinder Free Displacer Free Piston Stirling engine" which begins to look like the ideas I have rattling around in the back of my head.

    John

    It's like doing jigsaw puzzles in the dark.
    Enjoy today's problems, for tomorrow's may be worse.


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    If your interested in a Stirling project I've created a wiki on my website:
    http://www.pminmo.com/wiki/index.php...irling_Engines
    Feel free to create a user account and add/edit/contribute. In 10 minutes you'll know enough to edit and add links and information.

    Phil, Still too many interests, too many projects, and not enough time!!!!!!!!
    Vist my websites - http://pminmo.com & http://millpcbs.com


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    PMINMO:
    If your interested in a Stirling project I've created a wiki on my website:
    -------------------------------------

    Hope to have some info to post in near future. Thanks.

    Jerry



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    Likewise

    John

    It's like doing jigsaw puzzles in the dark.
    Enjoy today's problems, for tomorrow's may be worse.


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    CJL, what is your website?

    Phil, Still too many interests, too many projects, and not enough time!!!!!!!!
    Vist my websites - http://pminmo.com & http://millpcbs.com


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    Quote Originally Posted by pminmo
    CJL, what is your website?
    I do not have a website set up. I was just attempting to say that I intend to send some photos and designs to the wiki on YOUR website as soon as I get something going here and get some photos of same.

    I am presently engaged in an effort to purchase a parabolic mirrored reflector made of Glass ( similar to WWII aircraft searchlights which scanned the skies ) as a basis of a SINGLE cylinder stirling electrical powerplant. The single cylinder sterling is my own design.

    Sorry for the screw-up.

    Jerry



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    Default don't re-invent the wheel.

    I hope I don't step on any toes here. I've been reading the posts here, and doing my own stirling research, Re: stirling engines and power generation.
    I am looking at a project to use as "proof of concept", to re-use one, or some of the thousands of "C" Band satellite dishes littering our landscape as a framework for a stirling generator project. Thinking along the lines of designing a solar/stirling generator to mount directly in the dish framework and then make use of the power to either charge batteries or to sell the power back to the grid......
    Just like Mac Donalds, duplication as a business model. I'd like nothing more than to have to get up early in the am, and see to several generators, and then cash the check from the utility co. to supplement my retirement.
    John



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