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12-31-2011, 12:45 PM
(This post was last modified: 12-31-2011, 12:47 PM by PonderThis. Edited 2 times in total.)
The internet makes these points, and I'm only parroting what I've read. I don't really know personally.
I had a parabolic oven, with the parabola formed such that the focal point was down inside the parabola, not outside. I thought that was better, but I see some solar cooker designs being made commercially now that focus outside the parabola as well. I could be wrong on that one too.
If I liked that parabolic shape, I've seen a design made out of mylar that folded out like an umbrella. That seems like an exceedingly simple and effective idea, I'm still trying to wrap my mind around how to do something similar for a parabolic trough collector instead. Ideas are welcomed.
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(12-30-2011, 06:09 PM)chuck white Wrote: I'm thinking a mold 10' long to match 10' pipe length.
paper mache' would work.
Shape a large heavy gage aluminum foil (I've seen 4' wide rolls) over the mold.
Place box around mold and fill with expanding foam.
Should end up with a box half filled with foam with the parabolic shape lined in shiny foil.
Just think'en,
I think if you have just the right thickness of sheet metal that yould could just bend it by hand and it would naturally form the radius you need.
Forget the aluminum foil because it wrinkles no matter how careful. If you go that route you need mylar.
But I think I have a better idea. You can get highly polished aluminum sheeting that is virtually as reflective as a mirror.
I got a bunch of it at a scrap yard from the inside of a big florescent sign.
Unfortunately I used it to make a bell in a parade float
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(12-31-2011, 12:45 PM)chuck white Wrote: The focus point should be where the feed horn sits, If you dish still has the mount for the feed horn this is where you would heat your water at.
Exactly. I used the same braces that used to hold the feed horn. I mounted a coffee can and spray painted it black.
What's kind of amazing is the light is so intense it actually makes the can look white.
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12-31-2011, 12:55 PM
(This post was last modified: 12-31-2011, 12:57 PM by chuck white. Edited 1 time in total.)
So what is the cost for the mylar film, I'm seeing 11 cents per square foot.
Al foil is 4 cents a square foot.
I have had some success fusing Al foil to fiber board with poly plastic sheets. I place the plastic down, then the foil and use a clothes Iron to fuse the Al foil, plastic and wood together. My test pieces has seen two winters with no sign of de-lamination.
Not sure how it would work on metal dish? Heating to the melting point may be a problem with a thick metal plate behind the plastic film.
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12-31-2011, 01:03 PM
(This post was last modified: 12-31-2011, 01:07 PM by PonderThis. Edited 2 times in total.)
Here's one website I looked at for mylar films, it comes in numerous thicknesses:
http://www.mirrorsheeting.com/
Oh, I'm pretty sure your local grow shop carries at least some of this stuff, too.
I suspect (but don't know) that this is similar to what's on vinyl sign material. The only thing that makes that special is, it comes with a peel-off self adhesive backing. I think that sells for about $1.50 a square foot, not cheap, but cheaper than polished aluminum which would be the premium product.
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(12-31-2011, 12:47 PM)tvguy Wrote: I think if you have just the right thickness of sheet metal that yould could just bend it by hand and it would naturally form the radius you need.
You would need to make plywood ribs to hold the proper shape. If you had a form you could press the metal in the right curve and then you would only need to support the edges. The pre-pressed metal may be available, I don't know.
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I guess you could glue 6" X 6" mirrors on the dish this would still focus the light to a 8" X 8" spot.
If you could come up with a way to point and glue mirrors, you wouldn't even need a dish.
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12-31-2011, 03:06 PM
(This post was last modified: 12-31-2011, 03:07 PM by PonderThis. Edited 1 time in total.)
Not anywhere I've seen yet. I'm frankly surprised by that, too.
Before you go making a pattern, someone I thought knowledgeable emailed me that while this idea is valid, the drawings I showed have some flaw in them, and he had told the website creator that 2 years ago who then ignored the suggestion. He sent me about 30 pages of math and told me the answer for figuring it out was spelled out therein. Math makes me roll my eyes unfortunately.
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(12-30-2011, 02:37 PM)chuck white Wrote: The greater the temperature difference the more efficiency you get. Some issue with roof top temp to ground temp is thermal conductivity. you pump the heat under ground and now it heats the ground up around your pipes, until that heat dissipates, you've lost your temperature difference, same thing on the roof.
Water is a could conductor and vessel to dump or extract heat. Heat exchange water pipes on the bottom of a holding pond could be real useful in a home energy system.
Also your thought of using a refrigerant can be costly. Why not just run a steam turbine with solar heated water. you don't recycle the water like you would have to with a refrigerant. I'm liking this thread a lot.. The stuff you guys are talking about is fascinating.
A standard heat pump loses efficiency the lower the outside temp gets. And somewhere around freezing it is running on inefficient strip heat.. So a friend used a pipe of maybe 150 feet, buried 3 ft, underground, to deliver a constant 50 degrees to his condenser.. Given the underground pipe as a starting point, a heat pump could be optimized..
Also about 30 years ago they were experimenting with salt vaults.. Very well insulated underground containers with pipes laid in at various levels, covered in a saline solution, which would store a lot of energy in the form of heat. Some applications pumped solar heated water(or other medium) in over the summer and back out in the winter.. One system used this in a heat pump configuration, and another pumped the medium to radiators summer and winter. I don't remember the final outcome but it shows some people were at least thinking.. I know that is quite old tech..
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(12-31-2011, 02:58 PM)chuck white Wrote: (12-31-2011, 12:47 PM)tvguy Wrote: I think if you have just the right thickness of sheet metal that yould could just bend it by hand and it would naturally form the radius you need.
You would need to make plywood ribs to hold the proper shape. If you had a form you could press the metal in the right curve and then you would only need to support the edges. The pre-pressed metal may be available, I don't know.
I was thinking that just bending a piece of just the right gage sheet metal that it would naturally bend in to a half circle radius.
But I guess what you mean is that wouldn't be the proper radius? for a trough style parabolic reflector.
because it should be shaped like this...
But it still seems to me like if I just welded some angle iron to the top and bottom and then bent it until you get the desired radius( or close) then you could weld braces to hold it in that shape.
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(12-31-2011, 12:45 PM)PonderThis Wrote: The internet makes these points, and I'm only parroting what I've read. I don't really know personally.
I had a parabolic oven, with the parabola formed such that the focal point was down inside the parabola, not outside. I thought that was better, but I see some solar cooker designs being made commercially now that focus outside the parabola as well. I could be wrong on that one too.
If I liked that parabolic shape, I've seen a design made out of mylar that folded out like an umbrella. That seems like an exceedingly simple and effective idea, I'm still trying to wrap my mind around how to do something similar for a parabolic trough collector instead. Ideas are welcomed. I would think that any parabolic collector must follow the sun, or face misalignment and the accompanying efficiency loss... I saw a system that auto follows, but was on a simple solar collector...
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IMO this would be the best way to make a parabolic reflector out of a dish. It looks like this guy used the same highly polished aluminum sheet metal I was talking about.
That would real make a nice durable and highly reflective dish. But IMO no where near as practicable as the horizontal troff design with a pipe running right along the focus point
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Quote:HC... I'm liking this thread a lot.. The stuff you guys are talking about is fascinating.
A standard heat pump loses efficiency the lower the outside temp gets. And somewhere around freezing it is running on inefficient strip heat.. So a friend used a pipe of maybe 150 feet, buried 3 ft, underground, to deliver a constant 50 degrees to his condenser.. Given the underground pipe as a starting point, a heat pump could be optimized..
YEP underground OR in a lake either way works well because in the summer the water is cooler than the ambient outside temp and in the winter it's warmer.
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(12-31-2011, 03:35 PM)hillclimber Wrote: (12-31-2011, 12:45 PM)PonderThis Wrote: The internet makes these points, and I'm only parroting what I've read. I don't really know personally.
I had a parabolic oven, with the parabola formed such that the focal point was down inside the parabola, not outside. I thought that was better, but I see some solar cooker designs being made commercially now that focus outside the parabola as well. I could be wrong on that one too.
If I liked that parabolic shape, I've seen a design made out of mylar that folded out like an umbrella. That seems like an exceedingly simple and effective idea, I'm still trying to wrap my mind around how to do something similar for a parabolic trough collector instead. Ideas are welcomed. I would think that any parabolic collector must follow the sun, or face misalignment and the accompanying efficiency loss... I saw a system that auto follows, but was on a simple solar collector...
Yes the dish type that I made didn't work at all unless it was pointed directly at the sun.
But I'm pretty sure the horizontal trough design still works fairly well even when it's not perfectly aligned.
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There are a couple of things I REALLY like about this video
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She is sort of top heavy.
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(12-31-2011, 06:26 PM)PonderThis Wrote: She is sort of top heavy.
Grilled cheese will do that to you.
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The parabolic trench running east/west doesn't need to track the sun across the sky, but would to be adjusted up and down to follow the angle the sun plane is on.
A dish would need two axis to track the sun, A trench only one.
I believe the double parabola was to eliminate or reduce the need for vertical tracking.
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(12-31-2011, 08:15 PM)chuck white Wrote:
Yep I saw that earlier when I was trying to see how they were bending the metal to make these.
People have been using fixed solar collectors for years to heat their water that are mounted on a roof top that don't track anything.
They work fine although I'm sure they would work better if you went to all the trouble to make them track the sun.
I have some and although I haven't used them yet I was shocked at how hot the copper plates and pipes were on a cloudy day.
Basically I don't see that's it's worth all the effort to make something that needs to rotate two different ways.
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