aluminum lithographic offset printing plate

My local college prints a weekly campus newspaper, and I asked around to see if they had any aluminum lithographic offset printing plates - they did, and that's where I got mine to make the reflectors for the box collector.

So, if you have a community college or local university - they might be a source for the aluminium lithographic offset printing plate.

dbk

How specular are these sheets? Diffuse reflection is lost in this design, and only specular reflection matters. So, the "aluminum is 85% reflective" statistic does not matter, only the specular component. As a test, stand up a book or some text on top of the material. In the reflection, how high up the page can you read? Ideally you should be able to read the line furthest away.

They don't seem shiny at all, certainly not enough to read by. Would covering the sheet with something like this make sense: http://www.ebay.com/itm/ADHESIVE-MIRROR-solar-water-heater-OVEN-PARABOLIC-AFT-/180857971128?pt=LH_DefaultDomain_0&hash=item2a1bf9a1b8

They're completely specular, and being almost pure aluminium, should be well into the nineties, visible plus near and far infrared.

The reflective surface does seem quite blurry, but in a one meter trough the focal line I was getting was about 15-20 mm wide, which is narrower than the collector tube, so all you need. With reflective mylar it was about half that, but that doesn't hold up against any kind of environment, and is just aluminium anyway.

You could stick a separate mirrored surface to the plate, but would kind of defeat the purpose of having it in the first place...

admin, I think you don't know what specular means. You say its "perfectly" specular and then you say the reflection is hazy. The apparent thickness of the bright area is not sufficient evidence of adequate specularity. Is the surface like a perfect mirror or is it like the shiny side of aluminum foil, or even less? I agree adding a film would be a shame, but it sounds like the intended reflector might not be sufficient.

Ah, right, I'm thinking in terms of 3d animation, where the definition of specular is slightly different.

The reflectivity of aluminium is in the high nineties, cross spectrum. The printing plates visually resemble the shiny side of tinfoil, and in practice gives a focal line of about 20mm at 50cm distance, which is less than the diameter of the collector tube and therefor all you need. (So does tinfoil, for that matter, if you stretch it over the backside of a mesh.)
The plates can be polished to give better results.

Not that the printing plates are in any way essential to the device, anything sufficiently flexible and reflective will do.

I tried agricultural mylar, which is about 98% reflective and gives much sharper reflection, the focal width was about 10mm at 50cm. But it doesn't survive more than a couple of weeks outside.

I haven't been able to find any specularity numbers for this online, so attempted to test it myself with a laser pointer, screen, and digital camera. Best I could figure is about 80% specular, tho I'm not entirely sure how much I trust that.

The printing plates are very highly anisotropic, in that they reflect tightly in on direction and widely in the other. So if you bounce a laser off it you'll get a long narrow line as a reflection. It actually looks quite cool. It is nice and tight on the tight axis, about a centimeter at 3 meters distance, which is better than I would've thought. On the other axis it's almost 180 degrees blown out.

Long story short they should be fine for parabolic troughs as long as you make sure to bend across the grain of the metal, so that it's the tight axis which forms the focal line. I learned this the hard way.
We made a Solarflower in Spain last December and though didn't get a chance to properly test it all, after ten minutes in low winter sun the top (so the coolest part) of the collector pipe got to 140 C. I'm going to take this to imply the plates are at least usefully reflective.


I've also been using them to make a vertical axis wind turbine, which I'll post to the Other Tech page here and on the main site once the tutorial is done, which should be soon. The plates are really nice to work with, you can easily form precise shapes by scoring with a craftknife and giving the metal a couple of flexes, the line can then be torn out. Also, bending the edge of the metal into a triangle makes it surprisingly strong and rigid. The turbine, which consists only plates, a bike wheel, nuts bolts and rivets, was successfully tested on the back of my father's car up to 105 km/h, and it should take more.

So with that in mind I've an idea now of how to make the SF collector entirely out of printing plates, so no wood or plywood required. It should be about a quarter the weight, easier and cheaper to make, and more precise. I'll get it prototyped and update the tutorial when I get the chance, but this might be a while as I'm madly busy preparing for the trip through Asia in a couple months.

If you're planning to make anything get in contact with me first, I'll take you through the new design.