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  • Solar Pergola Racking Recommenation

    I am going to have a solar pergola built, approximate dimensions 75 x 27ft with a 3/12 or 4/12 pitch roof substructure. Any recommendations for racking for a solar pergola? I want to hide the aluminum frames as much as possible, as I am planning to use the LG 400 watt bifacial panels. I have a general contractor who is handling the structural build.

  • #2
    Check local building code requirements before you start including any wind and seismic requirements from increased loadings due to the panels.

    Also, and as a rough approx. and depending on site particulars: If mostly equator facing, a panel pitch (tilt) closer to you local latitude will be more productive with respect to annual production.

    I'd also check prices on bifacials vs. std. panels. Know this: Bifacials won't get you as much additional production as the advert. hype would like you to believe (the "up to X % more output" crap), particularly after support structure shading on what's already not much albedo sourced irradiance. Bifacial enhancement claims of production are a slippery issue that is mostly full of B.S. added by mfgs./peddlers. You might get 5% enhancement if you're lucky - real lucky.

    LG is good stuff, but panels being a commodity these days, other good - but much less expensive panels will probably be more cost effective and about as reliable.

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    • #3
      I am not counting on the bifacial output. The price I had for the bifacial panels was around $300 per 400 watt panel, which is around $0.75/watt. I've looked at the REC alpha panels which seemed to be around the same price point / watt. I wish I could get closer to 32-34 degrees pitch, but from a visual perspective and HoA perspective that would be pushing my luck.

      Definitely going to verify everything with the building department for their requirement since with the panels on it, it is pretty much a large sail.

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      • #4
        Originally posted by heimdm View Post
        I am going to have a solar pergola built, approximate dimensions 75 x 27ft with a 3/12 or 4/12 pitch roof substructure. Any recommendations for racking for a solar pergola? I want to hide the aluminum frames as much as possible, as I am planning to use the LG 400 watt bifacial panels. I have a general contractor who is handling the structural build.
        Check out this thread.
        https://www.solarpaneltalk.com/forum...wooden-framing

        @RShackleford did something similar although he built it himself. There is a lot of extraneous conversation about the pros and cons of a wooden structure that you can ignore.

        There was a good conversation about the racking itself. My only suggestion is that your contractor be aware of the potential of solar panels so his structural engineer can make sure he accounts for the uplift loads on the structure by the panels since that is what the building authority will want to see. The actual weight of the panels is not as much a concern as the uplift loads. I am not an engineert but just speaking about practical owner, contractor, structural engineer communication issue. That could save you an extra iteration with the structural engineer. You can go to UniRac or IronRidge sites for racking and they will provide calcs and recommended fastener spacing for the racking. You can give that to your contractor for the structural calculations. All you need to decide is how many rows of panels and how long each row should be. That is 2,000 Sq. feet of potential panels. Using an old rule of thumb that is potentially 30 or 40 kWs of panels. Use your own panel selection to get a more accurate estimate.
        9 kW solar. Driving EVs since 2012

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        • #5
          > There is a lot of extraneous conversation about the pros and cons of a wooden structure that you can ignore.

          Well, someone thinks you can ignore it. I have a different opinion, and you should be aware wood rots, warps and twists. Depending on wood chosen (treated wood often warps/twists more than other wood because of the porous nature and moisture absorption. And treated wood has reduced strength. In areas with wet winters, your panels will most likely outlast the structure. In desert areas, you may get 30 year life to match the panels.
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          • #6
            Originally posted by Mike90250 View Post
            ...... And treated wood has reduced strength..
            The poster came here looking for advice about putting solar on a structure that he plans to build. Unlike the previous poster he was not asking if he should build a pergola out of steel, aluminum, plastic.

            The comment I have on the above quote is that treating wood does not reduce its strength. It was true many years ago that the only treated wood available was used for mudsill and the structural grade did not matter so the lowest grade was often sent to the wood treating facility. Today treated wood can be ordered in common structural grades because it is more commonly used for pergolas and other exposed architectural designs. The treating process has a similar effect as kiln drying so that the risk of warpage is reduced because treated lumber tends to be dryer than other structural lumber fresh from a mill. There is still an advantage to pick a good grain structure and avoid heart center which is more common these days because the forests are all mostly second growth. The result is that the logs are smaller meaning a high likelihood of heart center which tends to warp and split more than the specification known as FOHC (free of heart center)

            There is no question the quality of wood has declined in the 60 plus years that I have been using that product. It will always be a natural product that will behave as well as it is taken care of like any structural element. Despite my own hijack to respond to the above I hope we can find a way to assist the OP with the kind of relevant answers he is looking for.
            9 kW solar. Driving EVs since 2012

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            • #7
              Originally posted by Ampster View Post
              The poster came here looking for advice about putting solar on a structure that he plans to build. Unlike the previous poster he was not asking if he should build a pergola out of steel, aluminum, plastic.

              The comment I have on the above quote is that treating wood does not reduce its strength. It was true many years ago that the only treated wood available was used for mudsill and the structural grade did not matter so the lowest grade was often sent to the wood treating facility. Today treated wood can be ordered in common structural grades because it is more commonly used for pergolas and other exposed architectural designs. The treating process has a similar effect as kiln drying so that the risk of warpage is reduced because treated lumber tends to be dryer than other structural lumber fresh from a mill. There is still an advantage to pick a good grain structure and avoid heart center which is more common these days because the forests are all mostly second growth. The result is that the logs are smaller meaning a high likelihood of heart center which tends to warp and split more than the specification known as FOHC (free of heart center)

              There is no question the quality of wood has declined in the 60 plus years that I have been using that product. It will always be a natural product that will behave as well as it is taken care of like any structural element. Despite my own hijack to respond to the above I hope we can find a way to assist the OP with the kind of relevant answers he is looking for.
              Most automobile frames and most PV support frames, including those over parking lots are metal for several reasons and probably some of the same considerations that apply to parking lot PV support structures also apply to the OP's situation.

              Overall, and from an engineering design standpoint, metal is a more fit for purpose material choice for most of those applications.

              Wood is a fine material for pergolas until the pergola becomes a support frame for a PV array. Then, other considerations enter the design picture - a few of which most folks don't know much less consider or think about.

              Seems to me the structure the OP planning is dual purpose, pergola and PV support frame. If so, from an engineering design standpoint, the pergola has some of the design constraints associated with a PV support frame and from a design engineering standpoint those constraints need to be considered along with the usual pergola design criteria.

              FWIW, my non PV support frame pergola is maint. free aluminum and looks as good as the day it was erected. The one it replaced was 10 yrs. old, needed maint./painting and looked like crap. The new one cost a lot less than the estimates I got for a wood replacement.

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              • #8
                Originally posted by J.P.M. View Post
                .....
                ..........

                FWIW, my non PV support frame pergola is maint. free aluminum and looks as good as the day it was erected. The one it replaced was 10 yrs. old, needed maint./painting and looked like crap. The new one cost a lot less than the estimates I got for a wood replacement.
                No doubt you are correct from an engineering standpoint. Seems to me you want to beat the same horse that you did in the thread of @RShackleford. As the OP noted the HOA has already expressed an opinion from a "visual perspective in his post #3.

                I don't think the pergola design is up for debate. Feel free to start a new topic if you want to debate wood vs aluminum from an engineering viewpoint. My point was not to argue for wood but clear up a misunderstanding about wood treating and I was simply trying to give the OP some advice to improve the quality of the wood that he or his contractor use based on my experience in the building materials business.
                9 kW solar. Driving EVs since 2012

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                • #9
                  Appreciate all the feedback and insights. I am talking to multiple contractors about different options, so it could be a metal structure or a wood structure. As mentioned wood is an organic and weather impacts its lifespan. I imagine choosing the right wood, staining/sealing it, and having the panels as a pseudo-roof structure, would impact its lifespan.

                  I did find this image that would be the shell of the design. My plan is to use the IronRidge uplift/loading data and let my general contract do what he does... build things.timber.jpg

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                  • #10
                    [QUOTE=Ampster;n419220]
                    No doubt you are correct from an engineering standpoint. Seems to me you want to beat the same horse that you did in the thread of @RShackleford. As the OP noted the HOA has already expressed an opinion from a "visual perspective in his post #3.

                    Seem what you want. But since, among other things, this is a place to express opinions, based on my experience, education and training, one of mine is that wood is not the most fit for purpose material for PV structural support for most applications.

                    Most of what I write is meant as much for others with similar applications/questions as for an OP and meant as a more sane contrast to a lot of what shows up around here.

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                    • #11
                      I will be interested to see how the value engineering comes out. Taking another look at the size of the pergola the 27 foot span is going to be difficult to do as a clear span with wood, unless you go with Glue Laminated beams, Posts on 13.5 foot centers would allow you to use traditional framing lumber but the aesthetics and preference for more or less posts my be the driver of the main structural element. If you are in California, seismic calculations will also influence costs.
                      At any rate keep us informed as your work through the process.
                      9 kW solar. Driving EVs since 2012

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                      • #12
                        supportstructure01.jpg
                        For a steel structure, that is about 80ft x 26ft, the price (minus installation) is around 42k. So figure 8k, that's 50k - 26%, around 37k. Still have multiple quotes and pricing exercises to go. Just wanted to share as I go through this process. This design features 5 degree angle, but ideally that will be close to 10-15 degrees in the end.
                        supportstructure01.jpg
                        Attached Files
                        Last edited by heimdm; 09-03-2020, 09:59 PM.

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                        • #13
                          Today I received a preliminary estimate from Florian Solar. Based on a coverage area of 75 x 27, with just the structure and integrated racking, sides, inverters, panels would be extra. the cost was over $100,000. To say the least, that is definitely not in the running for this project.

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                          • #14
                            Originally posted by heimdm View Post
                            Today I received a preliminary estimate from Florian Solar. Based on a coverage area of 75 x 27, with just the structure and integrated racking, sides, inverters, panels would be extra. the cost was over $100,000. To say the least, that is definitely not in the running for this project.
                            I believe you could do a lot better if the idea of a pergola was scrapped in favor of a more simple ground mount. Lots of options available. See some of Bruce Roe's stuff. While I don't agree with all of his thinking, he's the go to guy around here for ground mounted PV in cold/snow. You could do a lot worse than look/see/learn what he's got to share.

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                            • #15
                              Originally posted by heimdm View Post
                              Based on a coverage area of 75 x 27, with just the structure and integrated racking, sides, inverters, panels would be extra. the cost was over $100,000. To say the least, that is definitely not in the running for this project.
                              I am trying to figure out if you are trying to build a big Pergola that can support
                              some solar panels, or if you are trying to build a 35KW ground mount solar system
                              that can be used as a Pergola. If the latter, numbers like $100K are entirely normal.
                              Ground mounts cost more, I spend more on substantial mounting than on panels.
                              However you seem to forfeit some of the potential advantages of a ground mount
                              (such as best angle, variable tilt, snow rejection) which is OK for the former.
                              Bruce Roe

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