Best way to build a PV?

[orochi8]

I just picked up a 2 pound shipment of cells from cusdn on ebay, and Im fixin to build me a couple panels, so I thought Id run my plan by you guys and ask for some suggestions before getting started.

Im thinking the easiest way to do it is to make a backing of aluminum sheet metal with the biggest and baddest aluminum heat sync I can get my grubby hands on attached to it. Then I would get my self a nice big 3/16ths sheet of that more solid neoprene gasket material and cut sections out for each cell as a stand off between the aluminum backing and the glass to give the whole pannel some stability and shock absorbtion potential without making it rigid. Then like many others, attach the cells to the aluminum with a glob of silicon (im actually thinking a small glob in the center, and one at each courner of the cell), and do the whole wd40 coating thing. To seal it up Im counting on the neoprene gasket and some gentle force from the frame (which I think a standard aluminum picture frame might prove effective for) to keep things tight, rather than silicone arround the edges, simply because Im guessing I might want to open it up at some point for repairs.

I'll probably go with a standard 36 cell configuration, but Im thinking it would be best to do the whole lot of them in series rather than a series parallel config, just so the panel will actually do something when it has a little cloud cover. Otherwise Im may do a smaller number of cells on several easy to carry pannels that I can just string a bunch of together in series for what ever voltage floats my boat at the moment. These are going on the top of a little van conversion RV, so easy handling does have certain advantages for me that it wouldnt for people setting up an array that is just going to sit in the same place on a tracker all year long.

So what do you guys think? Any suggestions?

-Thanks

-Orochi

[orochi8]

PS:

Sorry for any conviniance this post, and the "mystery of the disapearing post" threads may have caused the mods when I posted them in the solar and rant sections rather than in a diary entry. It took me a while to figure out that you have to go to look under posts made by a specific user, then check the post, before finding editor's notes.

-Thanks

-Orochi

[Volvo farmer]

I'm assuming you're going to use tempered glass for the front? That's the first thing I'd do is start scrounging for tempered glass. If you have to buy it new, the economy of the homebuilt panel goes in the toilet. At $75/lb for cells, it's already getting there. You can cut your backing plate to size, you cannot cut tempered glass.

Definitely go at least 36 cells. Some of my Vocs with 36 were around 19V and I notice commercial panels are often around 21V so I'd bet that 38 or even possibly 40 cells per panel would work well.

I'd only attach the cells to the backing in one place. The expansion and contraction of the cells vs the material you mount them to would likely stress and crack the cells if you attach them at the corners, at least that's my theory.

Aluminum scares me because it's conductive and there's a bunch of electricity running around on the backs of these cells. Screw something up with the spacers and you make a dead short, not necessairly dangerous but possibly hard to repair.

I studied everyone else's method and still prefer my own. I've made up 600 watts of 100W panels now and a couple of 50W ones. They have not been outside in the weather at all yet so my designs are untested. If you search my postings, you'll find my methods, probably in diaries.

[ghurd]

I agree.

Al. is going to have a short... somewhere.

Go 36 cells minimum.

Don't bother with a heatsink,

just mount them with clearance around and under them.

FYI- for anyone interested.

Got some 7 cell, "4V, 0.100A" PVs.

Probably surplus from yard lights, but nice.

With 4 in series (28 cells), NO diode.

Open is almost 17V.

Short is about 80ma, but its just a tad hazy out.

Full sun into 12.8V is about 50ma.

Heavy overcast is 10ma.

A cloud takes it to around 1ma!

Have to get another PV to get the mini array

up to 35 cells for lead-acid charging.

The main use is for 3/6/(sometimes 12V) for Ni-Mh's.

Works fine for those because that is the finish voltages.

The 3.2V per PV is about past the limit of decent operation.

These are in low lead glass, both sides.

'Tedlar' type material on the back.

The 36 cell PVs do not drop like that!

G-

[Jeff7]

Neoprene - excellent thought. Quick search on it shows that it resists sunlight and heat well. It sounds nice if you can get a cheap source of it.

Volvo Farmer - how are your panels doing against weathering so far?

[orochi8]

You know, I thought about the same thing, but two panes of glass would make it heavy, and fraigile, both things that could prove very problematic when mounting something to a vehicle. Im sure there are coatings I could put on the aluminum surface that would keep anything from contacting. All the same though, I may just go with your approach anyway, as it would appear that I have located an abundant source of free glass localy.

-Thanks

-Orochi

[scottsAI]

Something to think about.

The cells are dark, they absorb heat, get hot.

With the cells raised off the backing the air becomes an insulator.

Where will the heat go?

Have fun.

[orochi8]

Well, my hope is that the wd40 will help to transfer some of the heat to the aluminum backing. I am contemplating using thermal grease, but Im worried that it would just wind up making too much of a mess and get all over the top of the cells blocking light.

-Orochi

[jimjjnn]

Before my retirement, we used thermal grease (silicone) on power supply transistors that got to 60 degrees Centigrade without any migration problems to other components. I don't know what temp that is Fahrenheit tho. I think it is near 150 degrees F.

I don't know at what temp it would might start getting liquified and  migrate to other parts.

[Tom in NH]

I like your idea of using aluminum picture frame. You will be able to make a nice rigid panel with it, and that will be good for your RV application where they will be handled a lot. I agree with the others who advised against the aluminum backing.

I would also advise against the wd40. Someone on this board, (was it Fred?) was relating how it causes more problems than it solves.

You are wise to use a gasket material between the front and back, rather than trying to seal it with silicone.

You're gonna have fun making your own panels.

--Tom

[orochi8]

Thanks for the input. Well, it seems like most everyone here agrees that I shouldnt use aluminum, so Im just going to trust that you all know whats up in that respect. I have to wonder though, why all the comercial panels are built with aluminum backings. Im sure there are alot of things you can do in a comercial production facility that just arent practical for us though. Im still rather weary of using another pane of glass as the backing, so I guess I'll keep my eye out for some other material with good heat disapation.

-Orochi

[Volvo farmer]

It's not aluminum. On my BP panels, it looks to be some sort of plastic or fiberglass sheet. The cells are laminated in a sandwich between the non-conductive backing material and the tempered, low-iron glass. Then they wrap an aluminum frame around the whole thing to stiffen it up. I could not think of an easy to obtain, non-conductive material to use for my backing and I had this free glass laying around so that's what I used.

I made up a 50W panel for one of my glass guys and he found an aluminum frame to wrap around it. Since glass is his business, he was able to scrounge aluminum and has the tools to join the corners correctly. It looks really professional that way. Maybe I'll get a picture of it posted this evening.

[ghurd]

Most commercial panels only have an Al frame.

I did have some kind of Gov't Surplus PVs mounted in an aluminum or stainless steel pan.

The cells were suspended in the middle of what appears to be 1/2" deep silicone.

Looks poured somehow.

The silicone yellowed, and did not adhere well to the pan.  It lifted at the corners.  I suspect water freezing in cycles lifted it more and more.

Siliconed them back down.  Everything still works, even after some flexing.

G-

[ghurd]

VW PVs have 42 cells.

Maybe because of the windshield being extra layer of glass.

G-