Expanding air in the panels?

[amiklic1]

I've built four panels, each with 36 WB-29 cells from Plastecs. I got all cells alive, and also soldering was very easy thing to do.

My construction is like this:

3mm ordinary window glass as the back of the panel.

Each cell is glued to that glas with two small dots of glue (silicon).

Front glass is 4mm tempered glass.

Between two glasses I use 6mm aluminium square tube with some kind of "sand" inside, that is often used in double-glass window production to prevent moisture.

It is all sealed with some back automotive-type silicon around the edge, and put into 20x20 U-shape aluminum profile. It stays pretty tight in that profile.

I am now a bit afraid about air expanding. In hot weather, when air temperature raises, it would expand. Can it break the back glass of the panel? I put two 3x20 mm aluminium bars in the alu frame behind the back glass to support it a bit.

[Clifford]

I've seen car windows that have exploded...  They are quite spectacular, at least in the aftermath.

Not quite sure what the cause...  perhaps a mix of factors.

Heat, Thermal Expansion of the glass, Pressure (unlikely too much because cars typically aren't well pressure sealed), Imperfections in the glass.

You might consider trying to put as much of a vacuum as you can on the glass.  A Vacuum can't expand.  The less air, the less expansion.

----------------

The other thing to keep in mind...  double paned windows are inherently problematic about blowing their seals, and trapping moisture inside which causes them to cloud up.

---- Clifford ----

[dinges]

Hi Amiklic,

Don't know the correct equations from memory, but you should be able to find them quickly on the internet; you need to calculate 'linear thermal expansion'.

Hmm, from memory, but I might be mistaken...

dL=alfa*dT*L

(in words: your change of length will be bigger when alfa is big, temp.difference is big or when the length is big)

Where

dL=change of length [m]

alfa=thermal rate of change (term?); unit unknown to me (probably 1/(deg.C*m)

L=length of your 'thing' that expands [m]

You should calculate dL for both your glaspane and your aluminium frame.

Let's say your frame is .5m * 1m in size; and it would heat up from 10deg.C to 60deg.C in the sun (not very unrealistic, I think, depending on the color; glass might have less dT)

So for glass: dL=alfa-glass*(60-10)1=.... m (depending on alfa)

and for alu frame: dL=alfa_alu(60-10)*1=....m (dep. on alfa_aluminium)

The dL's should be roughly the same, but they won't be. Therefore, stresses will occur within the glass and aluminium.

You can calculate the stresses with Hooke's Law:

sigma=epsilon*E

where sigma=stress (N/mm^2)

epsilon=strain (dL/L) (no unit)

and E=Elasticity-modulus of the material (for alu, from memory, about 80.000 N/m; steel is 210.000 N/m)

You know epsilon from the previous equation; E-modulus you can look up in a material's table; then you can calculate the stress (for either the glass or the aluminium, assuming 'the other' material has infinite stiffness (not very realistic); You could calculate it without this assumption, but for that I need to put my thinking cap on :-) )

This should get you on the way.

Peter,

The Netherlands.

[dinges]

I forgot:

the stress should be less than the max.stress for the material; for steel it's about 250 N/mm^2; aluminium probably about 100N/mm^2; don't know about glass, look it up; we didn't do much mechanical engineering with glass in engineering school :-)

Peter.

[MountainMan]

Peter,

Interesting observations, but I think he was worried about PV=nRT.  He's worried about the air inside the sandwich expanding enough to break the glass.

I wouldn't sweat it.  I've never heard of a double pane window exploding, and that's effectively what you've  made there.

BTW, the "sand" in the aluminum strip is called silica gel.  Used all over the place to soak up water vapor.  The little packet of stuff that comes in your bottle of vitamins or your bottle of expensive blue pills that says "don't eat this" on it, is the same stuff.

jp

[amiklic1]

Thanks for reply, but I had something to add to your story. Neithr I heard some double paned-window exploded, but windows are usually transparent, so most of the sun rays pass thru. In the case of the solar PV panel, it has black surface in it (solr cells) that would improve temperature to much higher levels than ordinary window did.

I was affraid about temperature expanding of the inside air.

[DanG]

Just once I'd like to see someone post their finalized PV Array plans before actually commiting to them. I take all sorts of gruff for sitting on these but the heartache of failing arrays would be worse... here's 40 pounds of proof I'm not a troll bent on hammering your play:

Including spacing you have about 350 square inches of collector area - did you use a square shape or rectangle? We Fieldliners would likely praise and admire any photos you could upload ; )

As outlined your mechanics seem sound, remember all dimensions will expand with heating which might self-regulate interior pressure. Use a pliable mounting system to keep pressure points down as heave & shrink happens, and I wouldn't allow the back-glass to EVER see cold water when it's been sun heated, some sort of drip-edge will help keep back glass clean and dry & no noon time hose downs just for giggles please!

Can you post exactly which automotive-type sealant you used?

A note on silicon sealants - Most release acetic acid acid during the cure process; gave the space program fits trying to find a formulation that wouldn't outgas corrosive compounds - RTV is another military technology giving a peace dividend now...

With your panels I hope there is enough base metal available to neutralize it w/o compromising front-plane traces fatally but know it keeps outgassing even when 10 days and more cure time is allowed before permanently sealing an enclosure. Hopefully the waxy fluxes or some other PV cell OEM handling aide will provide some level of protection unless they've been solvent washed & then the acetic will be growing salt crystals where the sun don't shine so to speak.

Expensive acid-free RTV silicone pastes are: white GE RTV 162 & gray high strength RTV 167; clear Loctite 5140; clear & gray Dow 3145. Supposedly GE Silicone II is a inexpensive acid-free formulation, you'd have to check the msds.

Since its done already I'm thinking remedial fix to flush out that 6mm airspace - hope you post back what compound you used!!

[dinges]

Yes,

After re-reading the post I think you're right, he meant overpressure. That problem has a relatively easy solution though: drilling a tiny hole somewhere. When our 'thermopane' (double glas) in our house went defect (manufacturing defect, but no warranty because we put it in ourselves), lots of problems with condensation; drilling a tiny hole solved the problem (why I don't understand, because there was already probably a leak somewhere). Insulation value got worse by this, but what use is a window when you can't see through it :-)

In my opinion, different thermal expansion rates would prove to be more of a problem and not so easily solved as the overpressure problem.

Peter,

The Netherlands.

[dinges]

Oops,

I should have read your response before posting :-)

As far as expansion of air is concerned, it too can be calculated; if you know the excess pressure, and the surface area of the window/glass, you should be able to calculate whether the glass survives or not.

I'm not sure on this, have never made calculations of this sort, but I think that when the resulting force is greater than the 'transverse rupture strength' (TRS; [N/mm^2], just like stress) of glass, you may have a problem. For brittle materials like carbides, they don't specify 'stress' because it's difficult to determine, but TRS.

My advice: ask a manufacturer of glass panes how much overpressure glass of various thickness can withstand. If you know the max. allowed pressure of a window, you can easily calculate whether or not excessive pressure may be a problem.

Good luck,

Peter,

The Netherlands.

[amiklic1]

I just did something like that you mentioned.

I drill a small hole thru the sealant and the aluminium tube that's holding silica inside it, so I can insert a tube (in fact wire insulation of the 15AWG/ 1.5mm2 wire) inside the panl. Since it's a very tight hole, I doubt it would cause much condensation problems inside.

I thought, can we put some kind of a baloon (or a similar thing, maybe some thin rubber) to the outher end of a tube, and so to not alow the air to get from outside to the inside of the panel.

I know it is difficult to find such a membrane, but if we could, when the panel is getting hot, the air expands, and instead of glass breakage, the "baloon" is getting bigger. When getting cold, the baloon is geting smaller.

The ony problem is how to find some kind of membrane that would last long time. (maybe even a few condoms, so the outer are protecting inner one???)

[dinges]

You might try an airlock?

I.e., a bend the hose like this:

to panel---

           |

           |

           |

           |      |(to outside environment)

           |      |

           |      |

           --------

Fill the bottom part of the 'U' with water (or perhaps better, some kind of oil or something that doesn't evaporate over time; this would require periodic refilling).

This way, the air can expand, without moisture getting inside the panel.

Or you could use a piece of rubber (e.g. a condom; not joking) to seal it of; however, the inflation of your balloon would still cause overpressure in the panel (but less of it than without it).

Before you try re-inventing the wheel; how do professional manufacturers of solar collectors/panels deal with this issue?

Good luck,

Peter

The Netherlands.

[amiklic1]

The principle in the comercial built PV panels is totaly diferent thatn in most homebrew.

They use just one glas, and (looking from the front to the back):

EVA film, PV cells, EVA film, TEDLAR film. It is all laminated at cca 170-200 degrees celsium in vacuum laminator, so it is compact unit, with no air inside. It means no condensation is possible, and no air expansion. Here is a poicture of SOLARIS panel cutout. Hope you'll understand.





I'm just beggining the production of a simple vacuum laminator, to avoid all mentioned problems.