How Do I Repair Cracks in the Back Sheet?

SolInvictus
SolInvictus Solar Expert Posts: 138 ✭
in General Solar Power Topics #1
I have several Solarex SXP-44 polycrystalline photovoltaic panels that are more than 20 years old and mounted at a 45 degree angle above a nearly horizontal roof meaning the rears of the panels are accessible and more exposed to the elements than panels mounted parallel and close to a roof. Recently I noticed some cracks in the back sheets of a few of them large enough to let sunlight through. Most of the cracks radiate away from the junction boxes. The back sheets are not peeling up at the edges of the cracks, and I, thus far, can not detect any decrease in current output. They still output the same current as when new. From my understanding, the back sheet serves four major functions:

1. reflect sunlight back to the PV cells to increase power output;
2. prevent moisture from reaching the PV cells;
3. provide electrical insulation; and
4. physical protection of the PV cells.

It seems to me, the most important reason to repair the cracks is to keep moisture out. However, I need to be careful that my repairs do not create thermal stress that expands the cracks, does not short circuit something and hopefully does not add flammable materials to the panels.

My thought is to paint the cracks from the rear of the panels with white spray paint and then cover them with aluminum tape. The sticky side of the tape is white, but I think it best to add an additional layer of insulation by using the paint. The electrical resistance of the sticky side of the tape exceeds 20 Megaohms, but I do not know if the resistance will change over time and the aluminum on the tape being a different material from the back sheet will likely have a different thermal expansion. The temperature of the panels could range from -30 C to ~60 C over the span of a year. The temperature did get close to -30 C last January, the coldest they have ever been, which could explain how the cracks formed.

Another possibility is to put silicone over the cracks.

What do you think of my ideas to repair the cracks?
What is the proper way to repair the cracks?
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: How Do I Repair Cracks in the Back Sheet?

    Welcome to the forum.

    Twenty years is about all you could expect from panels, so you have nothing to lose.

    There are sealants specifically for panels, but personally I'd just use some exterior silicon and expect to be buying new panels soon.

    Others may have further suggestions.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: How Do I Repair Cracks in the Back Sheet?

    Look for "neutral cure" or Silicon II (2) for the patches...

    Original silicon is an "acetic acid" cure (smells like vinegar--which it is) and may cause corrosion issues with the conductors/cells.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • n4wff
    n4wff Solar Expert Posts: 48 ✭✭
    #4
    Re: How Do I Repair Cracks in the Back Sheet?

    Sashco makes 2 products which would be excellent to repair the panels. Lexel is clearer than silicone, paintable, non conductive, non corrosive, and will outlast silicone. Through the Roof is Lexel on steroids. It contains UV inhibitors to withstand roof applications. Either one will last for years in your application. Both are available in caulk tubes, but Through the Roof comes in quarts and gallon cans also.
    http://www.sashco.com/hi/lexel.html
    http://www.sashco.com/hi/through-the-roof.html
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  • SolInvictus
    SolInvictus Solar Expert Posts: 138 ✭
    #5
    Re: How Do I Repair Cracks in the Back Sheet?

    Thank-you for the replies.

    So you guys think I should use a sealant that will fill the cracks and in some places touch the silver traces and the inside surface of the glass on the front. If the cracks were caused by thermal expansion, I am concerned that filling the cracks may increase the stress and expand them when the temperature makes them narrower.

    I have silicone II, will get Sashco's Through the Roof and try various methods to see what works best.

    It is odd that all 4 of the panels in one series string have cracks in their back sheets, but none of the panels in the other string have any.
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  • SolInvictus
    SolInvictus Solar Expert Posts: 138 ✭
    #6
    Re: How Do I Repair Cracks in the Back Sheet?

    A few days after I repaired the cracks, the power output of the repaired panels has decreased. Their open circuit voltages are a little low, short circuit currents are ok and the operating currents are variable, sometimes low and other time ok. Until I better understand what is happening, I advise people not to repair cracks in the back sheet.
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  • n4wff
    n4wff Solar Expert Posts: 48 ✭✭
    #7
    Re: How Do I Repair Cracks in the Back Sheet?

    Do you think moisture might be trapped inside? Keep us updated.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #8
    Re: How Do I Repair Cracks in the Back Sheet?
    SolInvictus wrote: »
    A few days after I repaired the cracks, the power output of the repaired panels has decreased. Their open circuit voltages are a little low, short circuit currents are ok and the operating currents are variable, sometimes low and other time ok. Until I better understand what is happening, I advise people not to repair cracks in the back sheet.

    Might be a bad contact somewhere.
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  • SolInvictus
    SolInvictus Solar Expert Posts: 138 ✭
    #9
    Re: How Do I Repair Cracks in the Back Sheet?

    It could be trapped condensed moisture or vapors from the silicone causing a short. Removing the silicone would probably fix it in that case. It could be a detached trace making contact with some resistance. I was not able to isolate the panel causing the problem today, so it might be all 4 of them. I detected the problem today, Nov. 11, and I did not detect any problem on Nov. 9. Because it was cloudy yesterday (no rain), I am not sure if the failure began yesterday or today. I repaired them on Nov. 6, so if it is related to the repairs, then it took a few days for something to break.
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  • SkiDoo55
    SkiDoo55 Solar Expert Posts: 414 ✭✭✭
    #10
    Re: How Do I Repair Cracks in the Back Sheet?

    Days are getting shorter and sun angles lower also! Are you tilting to correct the sun angle to compensate, Haze layer can effect also.
    GT3.8 w/4600W Trina 230W, TX5000 w/5000W ET-250W, XW4024 w/1500W ET-250W, 4 L16, 5500W Gen. (never had to use) Yet!!
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  • SolInvictus
    SolInvictus Solar Expert Posts: 138 ✭
    #11
    Re: How Do I Repair Cracks in the Back Sheet?

    The variations in readings on Monday, Nov. 11, were not caused by variations in insolation. I have newer PV panels that measure normally which also suggests that my meters are reading correctly.

    I separated the two series strings of SXP-44's allowing me to measure their electrical characteristics independently. Attempting to minimize the variables I cleaned all 8 of them with window cleaner and a paper towel. Voc of the string that has the cracks increased from 19.0 V to 20.0 V, but the operating current increased by maybe 100 mA. Voc of the series string without cracks increased from 20.0 V to 20.2 V. They are supposed to be 22 V.

    Yesterday the peak operating current from each string was 8.4 A. Today with the sunlight about 5% brighter than yesterday the string with the cracks peaked at 9.0 A and the other one at 8.2 A. Their short circuit currents are about 2 A higher than their operating currents which I am measuring at about 13 V (I adjust loads in the house to keep the battery voltage roughly constant). The operating current from each string should be between 10 A and 12 A like it was for the last 2 decades until last Monday.

    Therefore, dirt caused the low open circuit voltage on the string with cracks but does not account for the sudden decrease in current output to less than 50% on Nov. 11 and to about 70% the last two days. I do not think the decrease in power was caused by repairing the cracks in the back sheets because both series strings are showing problems.

    When I was cleaning the PV panels, I noticed a film on the glass along the lower rim of all 8 panels which shades the edges of some of the cells. It is not on any of my newer panels. Because I was unable to remove the film using the window cleaner and paper towel, I initially thought it might be sediment from condensation on the inside surface of the glass. However, after scrubbing on them today with a toothbrush, window cleaner, water, hydrogen peroxide and vinegar, I was able to remove some of it proving it is on the outside. This likely sediment composed of dirt and bird droppings is very difficult to remove from the glass with neither the water nor vinegar being helpful. Those with old PV panels might want to examine them to determine if they have a similar film that is degrading their power output. Removing the film did not increase my power output, but that may have been due to the glass on Solarex panels having a microscopic polygon pattern (you can see it using a 10x magnifier) that spreads the light around.
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