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  1. #81
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    Quote Originally Posted by Geof
    Make an open ring out of wood or metal. Support it horizontally and attach your film to the ring. Push the film down and fill it with water; the weight of the water will cause the film to form a catenary curve which is probably close enough to what you need. Now the more awkward part; fiberglas the bottom side of the film so it retains the shape when you empty out the water.
    Good idea Geof. Thanks.



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    Member greybeard's Avatar
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    How about filling Geoff's film with cement or plaster, then using that as a mould for the fibreglass ?

    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
    ...... but I would guess that aluminised acrylic might be a silly price ?
    Mirrored plexiglas? Yes, I haven't bought any recently but when I did it was two or three times the cost of regular. And I am not sure if you could vacuum form it without affecting the reflective surface.



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    11pm here -time for the nightcap and up the wooden hill.
    Night all.
    John

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


  5. #85
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    Quote Originally Posted by greybeard
    11pm here -time for the nightcap and up the wooden hill.
    Night all.
    John
    Nightcap should come after wooden hill.
    Night



  6. #86
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    Quote Originally Posted by greybeard
    How about filling Geoff's film with cement or plaster, then using that as a mould for the fibreglass ?
    Theoretically this would work to make a (almost) parabolic form but it might be tricky getting a film strong enough to take the weight of the concrete without sagging too far. I have heard of lightweight concrete which uses EPS beads as a filler so you might have to experiment to get the correct combination of film strength and concrete density. Of course for making a mold you don't need reflective film just smooth film.



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    I'm still researching along with my other activities. I wonder it a parabolic trough isn't more practicle. You only have to worry about 180 degrees vs 360, your spreadding the focused IR along the length of the trough, not one point. The heated pipe/chamber at one end would feed the Stirling, or be the hot portion of the engine. It would give more volumetric heat, although not as hot as a dish. The other possiblity in this arrangement is to use half/section for ammonia based refridgeration for the cold side. But I'm a long way from understanding it all and putting together.

    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
    ... I wonder it a parabolic trough isn't more practicle.....
    Recall I made a comment about this back in Post #13. This would be my preference for a few reasons: You can get quite reasonable concentration efficiency even if it is non-tracking with the long axis of the trough either horizontal or vertical and facing south with the trough inclined to the angle of noonday sun in the winter. You do lose efficiency early and late in the day but for a stationary trough you use a non-focusing modified parabolic form which means that the trough will still collect quite well from diffuse sun such as you get on slightly cloudy days. Depending on your location and whether you have a preponderance of clear sunny days or a mix of clear sun, haze and cloud you can get better accumulative efficiency from a trough compared to a focusing dish which will not work very well with diffuse sun. The trough is also easier to make.



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    Hi guys. There is a "simpler" version of a parabolic dish (which would also work for a trough). It's called a Cunutt concentrator. Basically it uses a grid of flat mirrors independantly aimed at the focal point. Performance? Here is a link. Google "Charles Curnutt" for more info.

    Evo



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    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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    Morning all.
    Just an early morning thoughts
    1. For a simple tracker - a clock.
    Put a 2:1 reduction gear on the hour hand drive shaft , and you have a sun tracker.
    2. Troughs v dishes - if space is limited, the rectangular shape of the trough "packs" better than a set of dishes ... 4/pi better.
    3. cds on the Curnutt type frame that Evo posted.( square mirror tiles would of course be better -see 2- but I've got to get rid of all these b****y free cds).

    Must wash up breakfast dishes.
    John

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


  12. #92
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    Downer for reduced pressure membrane idea - The mylar film I've got pulls down with only a small decrease in pressure, but stretches slightly as it does so. This might be problem, but I'm not yet sure.
    This could mean that a solid support is needed. Still, the aluminised film would still be a good non-corroding reflector when stuck over the chosen surface.
    John

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


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    Some place in Phoenix under the football field there is a rotating furnace that
    spins at a programed speed, they melt the glass, pour it in and let the furnace
    rotate till it sets up. I was supposed to go in and look at it but could not make
    the necessary connections. I would have liked to see that one.
    Regards Walt..
    This rotation causes the glass to form a very nice mirror for telescopes.
    Did I say big telescopes...



  14. #94
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    Nice one, Walt.

    Thinking about the basics for sun-collector part of the stirling, I realised that so long as all the light that's hitting the mirror then hits the collector surface of the stirling, it doesn't matter how distorted the wavefront of the image is. Hence, a large number of flat mirrors, each the same shape/size as the hot end of the stirling will be just as efficient as a carefully made dish of the same total area focussed onto a tiny spot on it.
    Is this correct ?
    In fact you might then have problems coping with the extreme high temperature of a small spot focus. Designing the hot surface to run at say 300 degrees might be a lot easier than 1500.
    (I'm thinking big mirror here ! )

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


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    Greybeard:

    You are on the right track. If you look at the French super sun collector, you will see that they use flat mirrors in the field. The parabolic mirror, mounted on the building can also consist of flat mirrors. It's all a matter of concentrating the entry surface to the focal surface. For sunlight mirroring is easy, it comes in a straight line, so angle of entry is angle of exit. For mirrors: if you can find, in plenty, the variety that is used in laser printers, where the mirror is on the glass, instead of the household variety where the mirror is behind the glass, you reduce focal losses.



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    Hi fkaCarel.
    Yes, I was only underlining the simplification that the use of flat mirrors might bring to some of us, along the lines that Evo mentioned in his link to the Curnutt system.

    As I've already bought about 15 square metres of aluminised film, I will be using that, even if I cut it up into small pieces and stick it on MDF
    But is more likely that I will use it as a stretched film over a large frame/multiple frames with a low pressure back pulling it into a curved surface. I'm going to try this with a square frame to see what the shape of the sun's image is at the best 'focal' distance.

    My idea followed this line - if I make 7 circular frames in a group, or even 7 fibreglass parabaloids, I could produce a very hot single point. However, the packing fraction of the circles is only pi/4 (roughly 78%), so there is a lot of wasted space in the mirror 'footprint'.
    If I use rectangular mirrors, I can approach 100%, and although there are more units to make, the construction is easier.

    So the question is, is it worth the hassle of making accurate parabaloid surfaces when the stirling doesn't 'know' which direction the energy is coming from, just that it all arrives ?

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


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    If any one would visit the site for the Keck Observatory in Hawaii, they might notice they have a single mirror made up of 36 mirrors. If you were to arrange 37 hexigon mirrors in a hexigon then remove the center mirror, that is what they have.
    Of course, they have hydraulic pistons on the back side of these mirrors to keep them in adjustment. They are working for an optical image concentrated at a vocal point out in front of the "dish" and then reflected back through the center of the mirror where you removed the center hexigon nut. This has to be the best way because the Japanese copied it and built one just like it some short distance away on the same hill. Now they have Keck I and Keck II.
    Maybe a frame work made to support the 36 mirrors (could be flat mirrors) and the 36 mirrors form a mosaic to reflect light to some type of reciever.
    Only thing left to do is follow the sun.
    I ramble Walt



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    Greybeard:
    The construction Walt mentions is the same principle. If you take 10 square mirrors and you point them at a mirror of the same size, the energy is 10 times concentrated. If you put a parabolic mirror at that point, you can focus the energy with only a small (parabolic) mirror, reducing the focal problem in size. Another approach for bundling could be a lens. But for heating the Stirling engine, there is only an area to be heated, so it does'nt need to be "spotted". Area heating is even better. Thus it's flat mirror size versus area to be heated size.

    I suspect looking at angles of the various field mirrors that the French take a further advantage of flat mirrors: they adjust them one at a time to keep the second mirror in focus, making continuous tracking of the whole system unnessary.



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    Quote Originally Posted by greybeard
    ......So the question is, is it worth the hassle of making accurate parabaloid surfaces when the stirling doesn't 'know' which direction the energy is coming from, just that it all arrives ?
    The answer is no and you have hit on the reason a non-focusing, non-tracking parabolic trough collector can have better overall efficiency. All the light entering within its acceptance angle finishes up at the collector surface.



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    Thankyou Geoff for seeing the point I was making.

    For everyone else, I'm afraid you'll have to get used to my rambling style, aimed, I say in explanation, at giving other people alternative ideas they may wish to persue.

    I go my way, learning as I can, but I scatter my thoughts like falling leaves.

    John

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


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