Solar Panel Prototypes

Since the last page showed the solar cells attached to the backing, it's probably best that this page show that step.

I had decided early on that I wanted aluminum as a backing, but it wasn't until much later that I had an astonishing, and potentially crippling (to the project, that is) revelation: aluminum is conductive. Just one of those minor things you overlook sometimes. This meant that, as the cells were wired in series, they'd short out against the backing to which they were to be mounted. Sure, I could have just wired them in parallel and left the backing positively charged, but at 100A @ 0.5V per panel, I'd probably need 1" thick copper rods to connect the panels - not exactly a very reasonable solution. So I needed an insulator that would hopefully serve to transfer at least some heat to the backing.




TO-3 Thermal Pads
I knew that there were insulator pads out there that were thermally conductive. Hosfelt Electronics had these TO-3 insulators really cheap.



4 Corner Thermal Pads
Cut up, they fit on the corners nicely. I later cut out the middle sections into small strips that went over the ribbons soldered to the bottom. 		1 Pound Silicone Compound
I planned on using epoxy to attach the cells to the backing, but it wasn't thermally conductive, so I bought a full pound of thermal epoxy, which is probably way more than I'd need for a set of panels, but it's much cheaper than buying it in lots of little tubes. So, whenever I mixed epoxy for the cells, I'd put in a good bit of this silicone stuff. Still probably not the best in terms of thermal conductivity, but the mix was a good balance of thermal transfer, adhesion, and cost.



Time to seal the thing. For the longest time, I wasn't quite sure what to use for a sealant. Silicone caulking and anything of that sort would be applied to only the outside edges, exposed to the elements, thus increasing the risk of allowing moisture into the panel. Then I came upon polyurethane foam. Quite interesting stuff it is. Once it comes out of the can, it expands quite considerably, almost immediately, but then continues to do so for almost an hour afterwards at a very slow pace.
I wasn't looking forward to this step much, because once the epoxy was mixed and applied to one of the C-channels, there was a chemical timer running, dictating how fast everything had to be attached. Spraying the foam in there just added to it, making it tough to find anything to grip without getting the sticky stuff everywhere.

Prototype, Preliminary Sealing
Epoxy went on the inner edges of the C-channels, and then the polyurethane foam was sprayed down the inside of each. They were then forced in place over the aluminum backing and the polycarbonate front. The extra thickness given by the epoxy made the fit extra tight, which was good, as it helped insure a good seal. There's a lot of the stuff everywhere on the cardboard, as some of it was testing of the foam's expansion, since I'd never used it before. And some of it was just from setting the can down, after which the foam continued to expand and squeeze its way out of the dispensing straw. 		Prototype, Connection Closeup, Outside
There's another blurry picture, showing the negative therminal, and next to it, is a small hole previously drilled out for the helium inlet. I decided that there was no good way to keep the hole from being filled, so I just let it get filled over - I used the same drill bit later on to just drill the foam out of the way, allowing for insertion of the inflating needle. 		Prototype, Connection Closeup, Inside
And an inside view of the negative terminal. The foam expanded very nicely, forcing its way into every single space it could find, which was a really good thing, as it should give an excellent seal. The corners of the panel didn't have any caps over them, and the joints weren't cut perfectly to begin with, so the foam formed weird looking "cysts" over the night. These were just cut off with a razor blade.



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