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Possibly the Perfect Reflective Surface for Solar?

05/18/2011 2:09 AM

For those aware of my open source solar energy project, I've just finished giving the first free construction workshop at the Riverside community in Motueka, New Zealand, which went really well. Details to be posted to the blog in the next couple days.

But the point of this post is; for the previous two prototypes of the device I used kitchen foil for the reflective surface, stretched over the back of a welded wire mesh. This worked actually quite well, but takes a bit of prep and you can put your finger straight through the thing.

But during the course of the workshop I was donated some 60 cm x 90 cm aluminium lithographic offset printing plates, which a guy had picked up for $1.50 a piece from a local newspaper. I rang their printers and was told that they were not only aluminium, but of a particularly high grade. His company alone, and it wasn't large, apparently scraps about 900 of these every week. He said it was a very common printing process, tho I don't know about countries other than NZ. I didn't really need to polish them, despite being over three years old, and they were a nice thickness to curve into a parabolic trough.

So: very cheap, very highly reflective, can get everywhere and nice to work with. Should take environmental wear and tear well.

Perfect?

We just make home concentrated solar a lot more achievable?

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#1

Re: Possibly the perfect reflective surface for solar?

05/18/2011 2:12 AM

Tho here's a tip; make sure the grain of the aluminium is running across, ie horizontally to the curvature. Vertically for some reason badly affects the optics.

Had to learn that the hard way.

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#6
In reply to #1

Re: Possibly the perfect reflective surface for solar?

05/19/2011 12:37 AM

I see a "grain" in bought aluminium sheet too. If you shine a laser pointer on the material, you will a strange scattering of the light. I think the grain is bad news. Maybe a little polishing will greatly improve your results?

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#7
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Re: Possibly the perfect reflective surface for solar?

05/19/2011 12:40 AM

Just buy the correct reflective material and be done with it - no big deal.

Seems like flogging a dead horse.

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#8
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Re: Possibly the perfect reflective surface for solar?

05/19/2011 2:31 AM

There is a price performance scale, so there is no correct material. It goes all the way from kitchen foil price to space aged price. And everything degrades over time. He is trying to do this as cheaply and robustly as possible.

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#9
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Re: Possibly the perfect reflective surface for solar?

05/19/2011 2:42 AM

I know that he is trying to do it cheaply - why not use someone else's experience?

Good material as George used in the link is not at all expensive.

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#10
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Re: Possibly the perfect reflective surface for solar?

05/19/2011 2:54 AM

Looks like the stuff he's ended up using is about 8-10 times the price of the aluminium, if you can find it. Seems like good stuff, but these sheets should be about as reflective and easier to source. Might require a polish...

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#19
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Re: Possibly the perfect reflective surface for solar?

06/03/2012 11:57 AM

Just a note that I suggested showing the effects of polishing with a laser pointer to Ed (who owns the solar projects group at yahoo groups). I cannot remember which aluminium surface he polished but the effect was remarkable! It might just have been a buffing. The "grain" in aluminium has a terrible effect on reflected light. I just came across this thread again by accident. He has a before and after picture of the pointer light bouncing off the material. http://groups.yahoo.com/group/solarprojects/photos/album/1550335105/pic/list You might have to join to see the pics. Anyway, it is the reflected "point" before and after polishing the aluminium.

Brian

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#20
In reply to #19

Re: Possibly the perfect reflective surface for solar?

06/06/2012 6:54 AM

Very interesting.

It looks like the reflection is only being blown out in the direction of the grain, so in the case of a parabolic trough as long as you have the curve running across the grain it shouldn't make too much of a difference.

Buffing/polishing is an option, but if it can be avoided then it's one less thing to do.

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#2

Re: Possibly the perfect reflective surface for solar?

05/18/2011 2:30 AM

I don't remember for sure, but I think that if a strip of material sags a bit, and you simply push the edges closer together, you get a parabolic trough. If so, this would work for a hardware cloth backing with a layer of foil in front, or for a somewhat thicker sheet of reflective material without backing. On that basis, mass manufacture would be easy. Then use a suitable ratio "clock drive" for tracking. This seems to be what you have done already, which looks quite good. Best of luck in advancing this concept.

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#4
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Re: Possibly the perfect reflective surface for solar?

05/18/2011 6:20 AM

Re: I don't remember for sure, but I think that if a strip of material sags a bit, and you simply push the edges closer together, you get a parabolic trough.

I guess a purist would tell you it's a catenary, but there is so little difference I don't think it matters--see the graph on this page: Catenary and Parabola Comparison - Math Forum - Ask Dr. Math

It's also interesting (to me ;-) to note that while an overhead transmission line is a catenary, the cables on a suspension bridge with a suspended flat roadway beneath it is more nearly (but apparently not quite) a parabola. A suspension bridge like a footbridge where the walkway follows the curve of the cables is a catenary. I found these in the Wikipedia articles on catenary and parabola.

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#5
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Re: Possibly the perfect reflective surface for solar?

05/18/2011 11:07 PM

I'm not sure, but I thought it was actually a parabola rather than a catenary. This is deflection/bending rather than tension/hanging by an "infinitely" flexible member. Your link sounds interesting; I haven't checked it out it but will go there next. Whichever way it turns out, it should still work as a solar reflector.

The weight of a catenary is spread evenly along its arc length; the weight of a suspension bridge is distributed nearly horizontally.

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#11
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Re: Possibly the perfect reflective surface for solar?

05/19/2011 7:57 AM

Re: deflection/bending rather than tension/hanging by an "infinitely" flexible member

I didn't think of it as deflection/bending rather than tension/hanging, so you may be right--I'm probably way out of my league. I just know that overhead transmission lines are catenaries, and assumed (I know) that this was a similar situation.

Just for curiosity sake, someday I'd like to know for sure, but I won't lose any sleep over it. ;-)

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#12
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Re: Possibly the perfect reflective surface for solar?

05/19/2011 12:21 PM

Not exactly as following picture shows:

The deformed surface is a sinus curve and as the second picture shows although there are not important differences the results for a solar mirror are very bad since as you will see the angles are variable and this leads to a broad region where the light is reflected. The concentration effect is a lot reduced.

It can be used but the collector has to broader and thus will present a higher loss via the bigger area available for radiation and convection to environmental air.

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#13
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Re: Possibly the perfect reflective surface for solar?

05/19/2011 8:09 PM

I managed to get a catenary into my cad package and compare it to a parabola. Basically what I took away is that if your curve is cut level with it's own focus, ie where it's 45 degrees, the difference between the two is pretty much within optical tolerances.

However, after that the caenary starts to deviate and after two or three times the focus depth is pretty far off.

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#14
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Re: Possibly the perfect reflective surface for solar?

05/19/2011 8:14 PM

Also, and this is getting a bit off topic, but I just used a catenary to approximate a focus level parabaloid, for use in a reflector dish I just made to get mobile internet into the valley here.

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#15
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Re: Possibly the perfect reflective surface for solar?

05/19/2011 11:03 PM

I made something years ago that will do the paraboloid. I called it the mechanical mathematician. It is basically an extension of the Make a parabolic curve with a t-square and string idea. (Which by the way is used to make the curves for SK14 solar cookers in Africa). http://home.germany.net/100-441770/amsi-model.html

Here is a picture of the idea http://images2.wikia.nocookie.net/__cb20080111003548/solarcooking/images/thumb/8/83/Mechanical_Mathematician1.jpg/702px-Mechanical_Mathematician1.jpg

Here is one of my links to it. If you use it, you can set up your focal point and mark all your wood or curves in place. I think you could also use it to make a fiberglass parabolic dish. Just make the mold from clay, etc and then lay the sheets straight on the clay mold. I don't think anybody else has made one of these things, might be something for your European tour.

http://www.altenergymag.com/emagazine.php?issue_number=07.10.01&article=cooker

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#18
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Re: Possibly the perfect reflective surface for solar?

07/12/2011 10:35 AM

Might be a catenary, which is superficially similar , but not the same as a parabola.

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#3

Re: Possibly the perfect reflective surface for solar?

05/18/2011 2:57 AM

Look at what this guy has done - neat and well laid out.

Materials sourced etc.

http://georgesworkshop.blogspot.com/

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#16

Re: Possibly the Perfect Reflective Surface for Solar?

06/04/2011 4:07 AM

What about Fresnel lenses made of broken bottles ;-)

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#17

Re: Possibly the Perfect Reflective Surface for Solar?

07/12/2011 10:10 AM

I have read your comments concerning these aliminum plates but how would you use them, you know set them up? How many do you need overall, to say, run a household? I'm rather quite curious? Maybe to curious for my own good some people say?

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