Author Topic: Making a parabolic mirror  (Read 9967 times)

billduck

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Making a parabolic mirror
« on: December 29, 2011, 07:38:38 PM »
I learned long ago that the net gain, for any solar heating device, is the heat in minus the losses.  To limit the losses, the collector needs to be small, so the cost of insulating same is small. Hence the use of parabolic mirrors. I heard of an Israeli who came up with the idea of using a flat mirror, but mechanically pulling on the center of the mirror, and restraining the edges to make the flat plate sort of parabolic. The idea might be of interest, if useful energy is going to be collected. Comments?


Gabriel

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Re: Making a parabolic mirror
« Reply #1 on: December 30, 2011, 08:42:34 AM »
Yeah, I've heard of people doing that before too.

I recently just finished a project from this last semester which "evolved" a parabolic shape using a genetic algorithm. (There is a good example program of a genetic algorithm which finds the best car for the given terrain at this link http://boxcar2d.com.) Basically, it was about the most convoluted way of finding a parabola that would give the desired focal point as you could imagine, but I plan on using much of the code for more practical projects in the future. For example, there is one neat example of a solar green house someone built where the back of it is parabolic. This way much of the light that hits the back of the greenhouse is focused into a tank of water to store excess heat. Maybe a genetic algorithm could be used to optimize either the same or similar ideas.


samd

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Re: Making a parabolic mirror
« Reply #2 on: March 04, 2012, 04:59:25 PM »
I remember a small but important fact about parabolas - from uni - that any material supported at both ends and subjected to a uniform force will form a parabola. Sounds boring but it's really useful.

So in 2 dimensions, if you look out your window at a powerline, it's a parabola. Held at two ends, and subject to uniform gravity.

In 3 dimensions, you can take an old empty window frame or some other four sided frame structure. Buy a mylar blanket off ebay for a few bucks, or buy some reflective film from a hydroponics store. Glue it to all four edges of one side of the frame. Then flip it over and glue a rigid panel to the other side of the frame. Waterproof plywood will do. Now apply a uniform force to it with a valve and vacuum. The mylar sheet will form a 3d parabolic bowl shape.

Adjust the vacuum level to adjust the focal length.

Youtube 'greenpowerscience' - they recently did just this. A very cheap way to make a large reflector. And if your material is only say 80% reflective, just make the unit about 20 percent bigger. A cheap way to trade of reflective materials versus size.

Hope this makes sense. Can't post links from this PC.

Added: Here is the link: 
« Last Edit: March 05, 2012, 02:13:36 PM by samd »

zininzelfdoen

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Re: Making a parabolic mirror
« Reply #3 on: March 10, 2012, 12:43:43 PM »
Interesting, saw the video.

I wonder if the vacuuum is really necessary when I think of your theory. If you just strengthen the Mylar (or whatever) in the middle, fix a piece of rope and you pull it and tie it on the back.

It would work in 2D. Maybe you need to have a round frame though. Well round is also a parabola no? So how do we make them easily?

samd

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Re: Making a parabolic mirror
« Reply #4 on: March 12, 2012, 02:22:20 PM »
A rope at the center would work to a fashion, but not ideal. Because it would be a point force.

If you can subject it to a uniform force you'll get a more accurate focus. Gravity, or vacuum. But perhaps several ropes would be an ok approximation.


beez

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Re: Making a parabolic mirror
« Reply #5 on: August 27, 2012, 10:59:40 AM »
I'm thinking about pulling on the center of a restrained pane of plate glass, then heating to the slumping point, then cooling to form a permanent paraboloid.  The other option would be to machine a parabolic plate from metal, heat and slump the glass over that.  Comments?

I believe the above comment about power lines may be incorrect.  Powerlines form a "catenary" shape.  This is more hyperbolic than parabolic.  For the purposes of focusing radiant energy I'm not sure if it makes a difference.

For our puposes a sphereoid or any similar shape may be good enough.

Incidentally this is a GREAT topic, thanks!
Chris

Brendan

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Re: Making a parabolic mirror
« Reply #6 on: September 17, 2012, 08:06:25 AM »
Just a heads up on this, it might be tricky in practice.  I was trying to get a round parabolic shape with mirrored acrylic on a frame where I could push the corners up a bit with screws.  Here are my unsuccessful results: http://www.heliostats.org/2010/04/gonna-have-to-switch-to-glass-mirrors.html

This guys have also done interesting work in this area: http://www.coolearthsolar.com/
But I haven't seen a lot from them lately.

Brendan

samd

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Re: Making a parabolic mirror
« Reply #7 on: January 15, 2013, 07:11:56 PM »
Anybody seen GPS's new video? Dan does exactly what I proposed above with the mylar blanket subjected to a vacuum. I have had two sitting in my shed for about a year, I love it when someone does the hard work! Armchair construction :P

$25 dollar parabolic mirror.

http://www.greenpowerscience.com/videos.php?v=FyCLOXF1188&h=Daniel%27s%20Solar%20Heat%20Ray%20DIY%20$25%20giant%20parabolic%20mirror


Jim McMillan

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Re: Making a parabolic mirror
« Reply #8 on: January 18, 2013, 04:02:35 PM »
Here's a thought... make a parabola with the vacuum method and then lay up some fiberglass over it. Maintain the vacuum intil the resin cures and you're done. Or you could use pressure and do the layup in the convex side, starting with reflective mylar as the first layer.
Jim

Michael

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Re: Making a parabolic mirror
« Reply #9 on: April 02, 2013, 04:36:50 AM »
I find this topic utterly fascinating! Thank you.

I have a small industrial vacuum pump. If I used this pump to pull a partial vacuum on a Mylar (or other plastic reflector) how could I control a precise level of vacuum to control a focal point?

Michael

samd

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Re: Making a parabolic mirror
« Reply #10 on: April 03, 2013, 05:41:23 PM »
Put a small adjustable tap inline with the pump. Use it to finely adjust and hold the internal pressure while it sets, by pumping past the focal point and then let tiny amounts of air back into the system until you get it perfect. Then seal it and wait.

Select your focal distance at the beginning of the exercise by positioning your frame a set distance from the wall and use it as your target.

If your collector is going to be say, basketball sized, you don't need to be as accurate, you can focus it to the size of a dinner plate, not a tiny point....

Paul L

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Re: Making a parabolic mirror
« Reply #11 on: April 30, 2013, 03:16:37 PM »
Just made my first mylar mirror today - not planning on making it parabolic, just wanted to reduce the weight and cost of mirrors.  I'm planning on making the final one much larger, 4 feet by 6 feet.  Note the difference in reflectiveness - I accidentally put it on backwards, so the back ended up being more reflective than the front.  I used a cheap $2.00 emergency blanket for the mylar, which isn't that great quality wise; I might look around for mylar gift wrapping paper for the final mirror as it might be better quality.  I also used heat gun to get out smaller wrinkles.  Not bad for a first try!

05/01 - Update

Made a larger version using emergency mylar tent.  5'x4' frame using old 3/4" tomato steaks from last year and ripped into 3/4" strips.  Super lightweight.  Must remember next time to keep supporting internal members lower than exterior frame, and to sand exterior frame to reduce imperfections.  Also note, this particular mylar is semi-transparent, which sucks to find out after building the whole thing. Still, not bad.   
« Last Edit: May 01, 2013, 04:01:08 PM by Paul L »

Gabriel

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Re: Making a parabolic mirror
« Reply #12 on: May 01, 2013, 04:51:34 PM »
That emergency mylar stuff actually looks really good. I had bought some thin mylar years back which was mainly used for gardening in greenhouses. I was never able to get it to look as flat as what you have though.

I'll have to try and get my hands on some of that stuff.

Paul L

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Re: Making a parabolic mirror
« Reply #13 on: May 01, 2013, 05:56:18 PM »
Hey Gabriel!

   Just finished watching your new vids for linear actuator heliostat setup.  Great stuff - videos seem to be the most effective way to convey a bunch of info quickly and clearly.  I wish I had them when I was setting up my stats! :)  As for the mirror - it's not bad, but I must confess that it takes good pictures.  Definitely make sure that your cross-braces etc are lower than the outer edge of the frame - I thought I had mine low enough (or at least flush), but apparently not.  I'll most likely take it apart and redo it since I don't like the partial funhouse effect it creates.  Everything has to be extremely smooth - one of the biggest imperfections in the whole thing comes from a staple that I forgot to pull out. :(  In case anyone cares, I used mono filament tape for setting it since that's what I had on hand and it can take a whole lot of pulling without breaking.  It seems to work really well, though I'm not sure how it will stand up in the long run.  Eventually, I plan on bracing the mirror back to my heliostat since it is so lightweight, and will most likely warp in the wind.  From what I've read, mylar you mentioned for "gardening" (read " marijuana grow-ops") is pretty skookum, with good reflectivity.             

samd

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Re: Making a parabolic mirror
« Reply #14 on: May 01, 2013, 11:25:46 PM »
Hey this is good info. I am hoping to start mine next week.
I am thinking maybe 1inch square aluminium sides, with the plastic push in corners. Or still a bike wheel rim with spokes removed. I was thinking duct tape for the edges.

Have considered usign a blow dryer for the wrinkles - that's what the fake carbon fibre film guys do. How did you find it for getting out the wrinkles???