Encapsulating with EVA

[Sandman57]

I encapsulated my first panel today with EVA and Tedlar. I soldered up 36 1.5v 3x6 cells to get a 60watt panel. I have access to a vacuum pump and autoclave so I wanted to see if I could do a profesional grade encapsulation. The laminate was stacked as follows:

1. Glass
2. EVA sheet
3. Solar cells
4. EVA sheet
5. Tedlar Backing

The laminate was placed in a vacuum bag and the pump drew a 27hg vacuum. This sucks all the air out of the bag and squeezes the laminate together.

The bag was placed in an autoclave and heated to 175 deg and pressurized. When the pressure got to 15psi the bag was vented and the preasure was increased to 20psi (i.e. 20psi outside the bag, one atmosphere inside the bag) and was left for 20 min. Pressure is preferable to vacuum becasue any air inside the bag is compressed though the vacuum pump should remove 99.9% of the air.

I wasn't sure what the correct pressure should have been but I was nervous about the glass cracking or the cells splitting so I didn't increase it any further. I let autoclave cool to 135 and then removed the laminate.

This process seemed to work well. The cells are flat against the glass with no air bubles. The EVA seems to cure clear and bonds the cells to the glass and the Tedlar to the back. Tedlar is an opaque smooth plastic sheet that should keep moisture from migrating through the EVA and into the panel.

The only thing I will do different next time is to let the pannel fully cool before removing the panel and possibly leaving the panel in longer at the max temp. I think taking the panel out at 135 degrees the EVA isn't fully cured and might be susceptible to delaminating.

Anyone out there know what temp/preasure/time the factories use with their laminating tables?

Regards,

Jack

[Mike90250]

Thanks for the update, and the excellent walk thru.

I think I would let cool till 95c, and then remove, my concern is thermal shock to the tempered glass from the "cool" air, and most plastic sets up/softens right at 100c.

Did you solder with lead free solder, or standard tin/lead? Rosin flux, water base flux, cleaned or uncleaned?

I've never heard of a 1.5V cell, are they 3 cells factory stacked ?

[Sandman57]

Oops

I meant 0.5v 1.7W 3"x6" cells

I used a rosin flux, type RMA pen to solder the tabing wire and a small amount of rosin - core solder on each bus wire.
Should I be using a different flux/solder?


I gently cleaned the cells and glass with alcohol prior to bonding with EVA.

do you really mean 95c as my temps were degrees F

I was origionally thinking that the EVA might need 250-350F but what little I find seemed to indicate lower temps.

[Sandman57]

EVA manufacture cure specs

This is the info that the vendor who sold me the EVA gave me for a cure cycle. They were not clear on the timing but did give me some temp guidence.

"The press cycle should start when the sandwich has deaerated
to 1 Torr or less, and the encapsulant has
softened (encapsulant temperature of 130-150

[Mike90250]

Sorry, I didn't see any units in your temps, so I thought they were C. I'd figured panels toasting on a roof would easily see 180F in the summer, so I figured you had to bond them together hotter than that.
Rosin core, tin/lead has 80 years of solid history.

[nayubis]

Would clarify, please, when you said "the bag was vented"
...when the pressure got to 15psi the bag was vented and the pressure was increased to 20psi ...
How do you vent the vacuum bag when it is inside the autoclave @ 15psi?

Thanks for the great thread!! You're a big help.

[Sandman57]

In the process I used a vacuum pump sucks all the air out of the bag and forces eva and cells against the glass. The difference between inside the bag and outside air preasure creates this force. When I put the laminate into an autoclave and increase the pressure outside the bag the force on the laminate increases because the differential between inside and out increases. At 15psi I shut off the vaccum pump and open the inside of the bag to outside the autoclave. The 15psi presure creates sufficient laminating forces and "venting" the bag ensures that if there are any small leaks in the bag the preasure inside and out won't equalize causing the bag to stop pressing down on the laminate. This is a common technique for building composite laminates. Preasure is preferable to vacuum because any air bubles in epoxy resin will enlarge under vacuum and shrink under pressure.

I used to work as a boat builder and have access to a large autoclave that is used to create carbon fiber hulls, masts, and components. When they fire up the autoclave they don't mind me putting a pannel in with a boat parts.

[Tazman]

New question

Can this be done without the autoclave? I have the Tedlar (TCP) but no info on how to apply it.

[Sandman57]

The Teddar is bonded to the backs of the solar cells with a sheet of EVA. The EVA bonds when it is heated. Commercially this is done with a vacuum press. If you don't have access to a press you might apply pressure with two aluminum sheets clamped together and heated in an oven. You would have to be careful to get even preasure and not to break the glass. The tough part will be to ensure you get no air bubbles without a vacuum.

[Tazman]

Thanks

Thanks for the reply. I guess I am out of luck. My panel is 52" x 96".
Do you think the liquid encapsulating might work with enough pressure evenly applied as it dries?
I have made several panels of increasing level of professional look. This one has an aluminum frame with the liquid encapsulation (took 4 jars). So far cost about $350 CAD so I figure it's a good deal if I can get it to last.

Is it possible to buy a vacuum bag of that size?
Cheers

[Sandman57]

It is possible to get a vacuum bag that size though it may be difficult to find material for a "one off" project. Vacuum bag material generally is sold on rolls 52" or sometimes 60" wide. To make a 52" x 96" bag you would roll out 194" of 60" wide material, fold it over and use vacuum sealing tape around the edges to make a bag slightly bigger than the your 52"x96" panel.

Without vacuum sealing you might be able to get the Tedlar to stick to partially cured encapsulant. The purpose of Tedlar is to seal up the back of the panel with a weather resistant material. Most folks seem to have good luck with the liquid encasulate so the Tedlar may be unnecessary if you go the liquid route vs EVA

[conntaxman]

eva/vacuum

The Teddar is bonded to the backs of the solar cells with a sheet of EVA. The EVA bonds when it is heated. Commercially this is done with a vacuum press. If you don't have access to a press you might apply pressure with two aluminum sheets clamped together and heated in an oven. You would have to be careful to get even preasure and not to break the glass. The tough part will be to ensure you get no air bubbles without a vacuum.

.
I have a question.I ordered some eva , Im making 36 cell panels. Im trying for figure out how this eva works.I guess their is a sticky side to it. Now I know that you have to heat it up to bond it together. Im going to build a Box large enough to put in the solar panel.Say 35Lx35W x12 high with an heating element in it,and also a connection for a vacuum pump. Would i first put the glass down then the eva sticky side to the glass,then the face down of the cells onto the eva and then next peice of eva not the sticky side on to the back of the cells,then a back sheet will say of plastic. Now put this in the box and pull a vacuum on it to remove the air,then turn on the heat to about 150 degrees as you mentioned. let it stay in their for about 30 min. DON'T know the time for the heating part. Last let it cool. and then remove it.
would that work?? Please correct me if any part is wrong.
thanks John

[Mike90250]

Heat that layup up BEFORE you pull the vacuum. Pull vacuum slowly, to let parts adjust, or you will crack all the cells. Vacuum conducts NO heat (thermos bottle) so heat & get it all melty before you vacuum.

[conntaxman]

Heat layup

Heat that layup up BEFORE you pull the vacuum. Pull vacuum slowly, to let parts adjust, or you will crack all the cells. Vacuum conducts NO heat (thermos bottle) so heat & get it all melty before you vacuum.

Mike have you ever done a panel like this before?This sound backwards to me.wouldn't it be harder to remove the air if this eva is melting and sticking together,What would happpen if I pulled a vacuum first? and then I still have to find out how much of a vacuum on a gague.
tks for your reply
John
I just dont want to screw it up, cost too much.

[Mike90250]

if you are using 2 hot plates to heat the entire panel, you have to rely on the air inside to move the heat around. if you vacuum first, you will only melt 2 spots. (at each hotplate)

Try it on a sample first. We freeze and bake stuff all the time here at work, and with huge 20x30' Ln2 cold plates and electrical heaters, it takes hours to change the temps in a vacuum chamber, when there is no air, it's real slow.