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  • Bifacial solar panel pergola

    I've been researching solar pergolas as much as possible, and there are quite a few examples online. The concept at a basic level seems pretty simple and very intriguing, and even has some side benefits (like being able to count the cost of the pergola itself for tax credit purposes since it's effectively serving as the racking for a ground-mount solar array).

    The one thing I don't feel like I've fully figured out yet though is how people design for rain. Many of the pictures I've seen would seem to obviously imply a structure that's meant to have no water leaking through the top (though it's an outdoor structure, so it's not at the same critical level as it would be on a home roof, for example). Several companies doing custom design even explicitly call out the "roof" being weatherproof.

    The basic overall idea is using bifacial solar panels on top of a "trellis style" (for lack of a better term; i.e., not something with a traditional roof) wooden pergola structure that would serve dual purpose as both solar generation as well as some shade protection (though the bifacial panels let in some light) and also some cover from precipitation.

    I suppose the solution might be different between standard framed and frameless panels, and I've been contemplating anything ranging from:[LIST][*]Using some kind of channel system to act as a "gutter" underneath each panel intersection, to[*]Using a UV rated weatherproofing tape to bridge from the panel to the structure, to[*]Using roof-grade rubber to act as a buffer between the panel (either on the side or on the bottom) and the structure, and rely on the clamping to create a seal with that rubber.[/LIST]
    Does anyone have any ideas on what might work or experience on this kind of project?

  • #2
    The easiest way to get building code approval is to use some of the racking like Iron Ridge or Unirac who provide engineering diagrams. In my jurisdiction a shade structure does not require a building permit but the solar panels do require a permit. I would start with the code requirements for your jurisdiction. There may be grounding requirements to consider as well.

    I am not sure what the benefits are of bifacial panels vs the expense and why you are concerned about rain?
    Last edited by Ampster; 08-17-2019, 03:41 PM.

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    • #3
      I did that very thing, put the panels on normal racking, and then used 2" wide aluminum tape (not half mill thick foil, but real aluminum) and using a pencil as a form for an expansion channel, taped the joints. Been holding for 3 years now.

      Un taped:
      20160821_090914.jpg


      post tape:
      20160821_122339.jpg

      You can see in the shadow, there is no stripe of light. I just didn't bother to take a pic of the tape

      The tape goes on the top side, only on the frame bezel, not on the glass.
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      • #4
        1 the easiest way is to use a under the deck draining system like Trex. https://trexrainescape.com/
        2. alternaively you can use TPO roof membrane which is white and help with bifacial production.
        3. pay the big bucks and get a dedicated bifacial racking solution: https://opsun.com/mounting-solutions...al-pv-racking/
        I am sure there are others, that is just a link that showed up when googling bifacial solar racking
        4. I have not tried this but Use a normal pv racking system and then use a gutter cap between the rows to divert rain like this product: https://www.caddetails.com/Main/Comp...Featured=False

        5. install a metal roof over the pergola (make sure you do not include the cost of the roof in your tax deductions because it is not part of the solar installation, unless you maybe use a white roof so reflect onto the bifacials. please note if you use a roof than your local permit company will almost always require a permit and stamped engineering and that will cost at least $600 to $1200 more.
        Bifacial looks neat, but honestly probabaly not worth the 3x cost. Go with regular 35cents/watt panels.

        send me a pm and I can give you a 24x36 treated wood pergola engineered drawing. Depending on wind loads you will need about 3 to 6 yards of cement around the posts.

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        • #5
          low slope pergolas less than 5 degrees require more cleaning since gravity doesn't help rain wash away the dirt as easily.

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          • #6
            Originally posted by Ampster View Post
            I am not sure what the benefits are of bifacial panels vs the expense and why you are concerned about rain?
            Because I want to use it as a pergola as well

            So in this case the benefits of bifacial panels isn't so much additional production but rather allowing some light through as much as the aesthetics of appearing more like it's a standard glass-roof pergola when you sit under it.

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            • #7
              Originally posted by khanh dam View Post
              Bifacial looks neat, but honestly probabaly not worth the 3x cost. Go with regular 35cents/watt panels.
              Yes, the reasoning on bifacial panels is largely aesthetic. I am curious where you're getting 35c/w panels though; I've generally seen about 50c/w at the low end, and that's on the cheapest poly panels. I've seen some bifacial panels that rival the cheaper mono panels at 66c/w.

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              • #8
                Originally posted by khanh dam View Post
                send me a pm and I can give you a 24x36 treated wood pergola engineered drawing. Depending on wind loads you will need about 3 to 6 yards of cement around the posts.
                I don't appear to be able to PM you...unsure if that's because I'm a new user or some other reason.

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                • #9
                  [COLOR=#FF0000][FONT=comic sans ms][I][B]
                  Originally posted by jtc331 View Post
                  allowing some light through as much as the aesthetics of appearing more like it's a standard glass-roof pergola when you sit under it.
                  [/B][/I][/FONT][/COLOR]
                  sorry but that is not a good reason to use bifacials. put polygal or clear corrugated roofing panels on top if you want light to come through. add a foot space between panels, or just be practical and put some led ceiling lights under it. Tounge and groove ceiling planks look nicer than the backs of solar panels anyways.

                  . Most pergolas don't have glass roofs, not sure where you got that idea.

                  sunpower p17 340w panels are sold by a few different vendors 1 year warranty $110, so about 33 cents a watt, can only be used with grid tie inverters.

                  main benefit of a pergola is you do not need module level shut down which can save you about $2000 to $3000 dollars in electronics. solar panels on homes have to have that safety feature, which is a joke IMHO. firefighters aren't going to be smashing up solar arrays.

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                  • #10
                    downside is a 8kw pergola might cost $8000 just for the lumber and kit materials. If you are handy you can build it for about half that amount. https://pergoladepot.com/product/big...SABEgLUifD_BwE
                    and if you go with an aluminum bifancial it is probabaly closer to $15 to $20,000 just for pergola
                    https://solarpergolas.com/get-a-quote/

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                    • #11
                      Originally posted by khanh dam View Post
                      sorry but that is not a good reason to use bifacials. put polygal or clear corrugated roofing panels on top if you want light to come through. add a foot space between panels, or just be practical and put some led ceiling lights under it. Tounge and groove ceiling planks look nicer than the backs of solar panels anyways.
                      Respectfully I don't think "aesthetics isn't a good reason" is something you can make the call on for me; that's a personal decision. One of the big reasons for using bifacial panels in many higher-end installations is precisely that, like solar overhands over walkways etc.

                      Originally posted by khanh dam View Post
                      Most pergolas don't have glass roofs, not sure where you got that idea.
                      I didn't say they did; "standard glass-roof" not "standard pergola". It looks more like using a glass roof, should one desire that look or function.

                      Originally posted by khanh dam View Post
                      sunpower p17 340w panels are sold by a few different vendors 1 year warranty $110, so about 33 cents a watt, can only be used with grid tie inverters.
                      I guess I'm not sure how comparing prices to bargain basement panels with no manufacturer warranty and an extra warrant that's 5 to 10% of standard panels is particularly useful. By that measure virtually all standard panel sales would be 2 to 3x more times cost.

                      Originally posted by khanh dam View Post
                      main benefit of a pergola is you do not need module level shut down which can save you about $2000 to $3000 dollars in electronics. solar panels on homes have to have that safety feature, which is a joke IMHO. firefighters aren't going to be smashing up solar arrays.
                      I already have a SolarEdge system on the roof, and since it's configured with a battery for TOU shifting, it kinds makes sense for me to use SolarEdge for this also so the inverters can coordinate. Given that I'll have per-module optimizers anyhow, so that's not a savings I can as easily make here.

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                      • #12
                        [LEFT][COLOR=#252C2F][FONT=Helvetica][SIZE=13px]I guess I'm not sure how comparing prices to bargain basement panels with no manufacturer warranty and an extra warrant that's 5 to 10% of standard panels is particularly useful. By that measure virtually all standard panel sales would be 2 to 3x more times cost.[/SIZE][/FONT][/COLOR][/LEFT]
                        sunpower is consider one of the top brands in the industry, not cheap panels. the chances of any solar panel maker being in business in 25 years is a total absolute joke. Look at the history of solar panel companies, it's not a small percentage that goes belly up with no warranty cover , it is in fact a huge percentage ,. Combine that with the absolutely horrific resale value and it makes economic sense to self warranty your panels knowing that the warranty is probablay worthless and in 5 years the panels might be worth 1/3rd what you paid. simple math.

                        You are right about aesthetics, what I meant it was not an economical reason to use bifacials. Personally I would rather spend the money on a real metal roof with a "real wood ceiling" with maybe a few ceiling fans underneath. Much more enjoyable to be under in the hot summer and looks better as well to me. Yes the aluminum pergolas with flush panels looks very modern, but when one compares their price & performance vs something more practical it is a tough pill to swallow.
                        Last edited by khanh dam; 08-17-2019, 08:54 PM.

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                        • #13
                          [LEFT][COLOR=#252C2F][FONT=Helvetica][SIZE=13px]it kinds makes sense for me to use SolarEdge for this also so the inverters can coordinate.[/SIZE][/FONT][/COLOR][/LEFT]
                          I've never used solar edge products. does combining two inverters somehow make the system more efficient? By spending thousands more on the module level output you will gain the extra thousands of $$$$ back over the 25 year panel life time? It seems like with panel prices being 50 cents or lower it is almost always easier to just add more panels than to try and tweak the system to be even better.

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                          • #14
                            Originally posted by khanh dam View Post

                            sunpower is consider one of the top brands in the industry, not cheap panels. the chances of any solar panel maker being in business in 25 years is a total absolute joke. Look at the history of solar panel companies, it's not a small percentage that goes belly up with no warranty cover , it is in fact a huge percentage ,. Combine that with the absolutely horrific resale value and it makes economic sense to self warranty your panels knowing that the warranty is probablay worthless and in 5 years the panels might be worth 1/3rd what you paid. simple math.

                            You are right about aesthetics, what I meant it was not an economical reason to use bifacials. Personally I would rather spend the money on a real metal roof with a "real wood ceiling" with maybe a few ceiling fans underneath. Much more enjoyable to be under in the hot summer and looks better as well to me. Yes the aluminum pergolas with flush panels looks very modern, but when one compares their price & performance vs something more practical it is a tough pill to swallow.
                            That's fair; I overstated on the panel quality while mostly thinking about the retail and warranty situation.

                            Some panel warranties are by a third party, which helps, and some are big enough to survive (LG for example). But it's a fair point that if cheap enough that may not add value.

                            As to economics, my price comparison originally was much closer, so that's a factor, and bifacials also aren't affected by the tariff, which helps. I'll chew on the different options a bit more; I do want to understand if the original idea is really feasible. I am trying to avoid the look of panels on top of the roof (as opposed to roughly a continuous top or embedded in the roof) somewhat just to see what's possible and somewhat because I hunk it will look nicer.

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                            • #15
                              Originally posted by khanh dam View Post
                              I've never used solar edge products. does combining two inverters somehow make the system more efficient? By spending thousands more on the module level output you will gain the extra thousands of $$$$ back over the 25 year panel life time? It seems like with panel prices being 50 cents or lower it is almost always easier to just add more panels than to try and tweak the system to be even better.
                              My goal currently isn't so much the best possible economics, but to try to also factor in actual ability to avoid net metering/have a system that makes sense even as those situations change.

                              So the coordination is more so that the existing system knows when to charge the battery. If it's not aware of the second system it could reasonably assume my own demand isn't being met by the solar generation and not charge the battery, even though the other system is already filling the slack, leading to unnecessary export.

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