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what inverters will work with an older positive ground panel array?

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  • what inverters will work with an older positive ground panel array?

    Hello, this is my first post here. I have been lurking on solar forums on and off for several years, and have helped install a few systems, so I am a little bit familiar with this..
    My wife and I had been intending to do a grid tied system, but with the current state of the economy and the job market, we made the call to do it now, and lower our monthly expenses as soon as possible. I am putting a system together of used components, some of which I know are not the ideal choice but it's how I can afford to do it now.
    What I have currently, are 24 sun power 305 panels.
    I have access to a NOS, SunnyBoy 7000us transformer type inverter, that has positive ground capability to work with these panels. Going this route costs me about a quarter of what it would cost with new panels, which is nice as work has been sporadic for the past several months. My issue is, the SB 7000 only appears to have one MPPT input, that supports one string. I can do it, and wire the panels to be below the max voltage and amperage.... but the barn roof has two sections facing slightly differently. Ideally I would have two strings and two MPPT inputs. So, is there any newer inverter, that can be made configured to run a positive ground array of panels? Or, is there a way to add more MPPT inputs to the SB 7000?
    I have found someone locally, that has supposedly found a way to use these same panels with a sunny boy island, and is grid tied somehow, but I have yet to connect with him.
    I've read a ton, and come up with nothing positive, but I'll keep asking before giving up completely.
    Thanks in advance, SD

  • #2
    If both arrays are sized the same and BOTH have no shade or shadow issues, simply putting them in parallel will only have a slight amount of loss. At least that's what I've heard.

    Now there may be a problem getting used gear certified to work with your energy provider, they may insist on new gear with smart grid standards
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister

    Comment


    • #3
      Originally posted by Mike90250 View Post
      If both arrays are sized the same and BOTH have no shade or shadow issues, simply putting them in parallel will only have a slight amount of loss. At least that's what I've heard.
      Correct. The key fact is that even though two strings with different facing directions will produce peak power at different times of day, the Vmp voltage will be almost independent of the insolation on the string, Imp will vary greatly, but that will not be a problem with strings in parallel.
      If one of the strings is partially shaded, that will activate bypass diodes which will change the voltage and therefore Vmp of that string.
      Unless the Vmp values are within 5% or so, putting two strings with (momentarily) different Vmp values into a single input will cause significant power loss. Even then you need to decide whether or not the lost power outweighs the cheaper system cost.
      SunnyBoy 3000 US, 18 BP Solar 175B panels.

      Comment


      • #4
        Originally posted by inetdog View Post
        Correct. The key fact is that even though two strings with different facing directions will produce peak power at different times of day, the Vmp voltage will be almost independent of the insolation on the string, Imp will vary greatly, but that will not be a problem with strings in parallel.
        If one of the strings is partially shaded, that will activate bypass diodes which will change the voltage and therefore Vmp of that string.
        Unless the Vmp values are within 5% or so, putting two strings with (momentarily) different Vmp values into a single input will cause significant power loss. Even then you need to decide whether or not the lost power outweighs the cheaper system cost.
        While agreeing with most of that, I'd note that using most inverter methods, and for this application a string inverter, a panel/cell's temp. is a f(energy balance) on the cell/panel. That makes the cell/panel temp. at least partially dependent on irradiance. The more irradiance a panel sees, the hotter it runs. If the two orientations are close to one another for azimuth and to a lesser degree tilt, there most likely won't be a "whole lot" of difference in temp. How much a "Whole lot" needs to be before it starts to cause problems is the question.

        Those older S.P. 305's have voltage and current characteristic similar to the 327's on my roof, with the published coeff. of voltage w/respect to cell temp. being an identical -0.1766V/deg. C for either panel.

        Because I've got more time than brains, after measuring quasi instantaneous voltages and all 16 panel temps 60 times on clear days at the time of min. solar incidence angle, I've come up with an empirical correlation for voltage as f(array temp.) for my array.

        For 8 panel strings, the variation with string voltage as f(cell temp.) that I measured was -1.476 V/deg. C (or 0.185 V/panel). Sample std. dev. = 0.068. N = 60 (34 in winter, 26 in summer).

        The short description of all that is that the array voltage/temp. change is pretty linear and follows the published spec sheet voltage change/deg. C of cell temp. per panel when field wiring losses are considered/thown in. Not a big surprise there.
        The other blinding flash of the obvious piece of information I conjured up is that my array overall temp. differential Cell to ambient temp. pretty much follows other stuff published by NREL and others for PV panels operating under field conditions. One example being: T,cell ~ = 0.943*T,a + 0.028*(I) - 1.528*(W) + 4.3 where:T, cell = cell temp. in deg.. C, I = P.O.A. irradiance in watts/m^2, W = wind velocity in m/sec. All that also jibes well with published NOCT temps.

        The bottom line for all this is that for every 30 or so W/m^2 POA irradiance increase on a panel, that panel's cell temp. goes up by ~ 1C and that will change a 305 W S.P. panel's voltage by ~ 0.2 V or so, meaning, for example, that a 10 panel string of 305 W S.P. panels will lose (10panels* 0.2 V/panel) = 2.0 V. for every deg. C. increase in panel temp.

        So, depending on string length, the change in irradiance accounted for mostly by the difference in string orientations, and also any changes caused by changes in wind vector between the orientations will cause changes in string voltages. Depending on how great those changes are, they may/may not have bad or noticeable effects on inverter output and/or operation/service length.

        For this application, if the two orientations are close (and also probably somewhat symmetric about solar noon), the differential irradiance may be small with differential temps. one array to the other corresponding small, and the voltages will probably be "close enough" and so not a problem.

        But I'd suggest we - and including the OP - don't know that with any rough confidence at this time, and may well never know.

        I'm not saying the OP's application won't work, just that that the statement that the MPP voltage will be independent of insolation isn't strictly correct, that thinking so might not be the best way to look at it when considering what might happen, and that some consideration of the differential irradiance inputs and other conditions to different orientations such as different wind vectors might need some further considerations.

        Comment


        • #5
          3 strings of 8 panels into a combiner should serve you well, providing the panels
          of each 8 are facing the same direction, with the same likelihood of shading.

          Here a couple strings of 10 such panels are paralleled to work just fine, 11 might
          work but I do not want to push my luck on the inverter high voltage limit. Check
          your inverter voltage limits and lowest temps before connecting 12 in series.

          Yes power will drop with shade, hopefully it will only hit one string at a time. The
          MPPT inverter will take care of it, you may lose most of the output of the shaded
          string for that time, welcome to renewable energy. Equalizers were invented so
          no outputs except the actual shaded panels would be affected, I prefer a chainsaw
          to that considerable complexity.

          As to partially shaded operation, there are a few different possible modes. In a
          single long string with a shaded panel, the others will force a bypass of it with the
          MPPT ckt in effect operating with one less panel. Here with 6 strings in parallel,
          if a panel disables a string, the other 5 strings will maintain essentially the same
          voltage (at lower total current), so the disabled string will maintain its voltage too
          but the current will be only what the shaded section can deliver, no bypassing.
          With 2 strings, a shaded panel will probably result in operation similar to a single
          string, the fully capable string will deliver more current at less voltage at the
          level of one less panel. Between all these are a whole range of possibilities, not
          to worry too much as the MPPT will make the best of it at that time.

          Here the sun moves to favor some strings over others over the day, depending
          on orientation. MPPT covers it. As for temp rise, I wonder if my 2 sided array
          may have an advantage? They could be regarded as a chimney to encourage
          air rising through the middle from panel heating. Another correction factor,
          maybe I should enclose the ends to help? Bruce Roe

          Comment


          • #6
            Originally posted by bcroe View Post
            As for temp rise, I wonder if my 2 sided array
            may have an advantage? They could be regarded as a chimney to encourage
            air rising through the middle from panel heating. Another correction factor,
            maybe I should enclose the ends to help? Bruce Roe
            Probably not an advantage and quite possibly a disadvantage.
            If there is any wind at all, unobstructed exposure to ambient conditions of free air access is a better convective heat transfer environment.
            Also, the backside of a panel will see a greater temp. difference for more thermal radiation heat transfer when it "sees" the surrounding environment than if all it sees is the backside of a panel that is, in all likelihood, warmer than the surroundings.

            Also, closing up the ends will only reduce any air flow. Not good.

            What you want for the most effective cooling from the backside of a panel is an exposed backside that has as much free air movement as possible to be able to take advantage of any wind . Natural convection can help, but it's effects are dwarfed by just about any breeze. Another consideration is a backside of an array that also "sees" surroundings that are as cool as possible.

            Comment


            • #7
              Thank you all for the thoughts on this. I have a few options: There is a SB-7000 available that would apparently handle all of the panels, so long as they are configured correctly. I have also found some SB-4000, and a 3500; having two inverters would cost a little more, but might allow me some more wiring options. Here is a pic of the barn, so you can see the roof sections. The side pictured faces due south, the upper section is about 25 degrees different than the large face pictured. As for the utility being ok with used equipment, PSEG doesn't seem to care much about the panels, and the inverters are still new, so I think I'm good.
              SD
              IMG_1386.jpg

              Comment


              • #8
                SD: what's your zip?

                Depending on your latitude, the building roof pitches shown probably aren't too far off from ideal summer and winter panel slopes.

                Lots of options made easier with 2 inverters. Things like 14 panels on the steeper (lower) roof section slope and 10 on the more shallow (upper) roof section, or whatever splits the summer/winter load.

                Run PVWatts and optimize that model's output against your monthly use.

                Unless there's more to see off the SE that's out of view of the photo, if it was me, However I split the panels between roofs, I'd located the panels toward the east end of the building with the goal of avoiding late afternoon shade from that tree located to the SW of the SW corner of the building.

                Comment


                • #9
                  zip is 08043. Theres a few trees, but nothing closer than 150'. The big storm three weeks ago took out the biggest sycamore. Where do I find PVwatts?
                  Thanks SD

                  Comment


                  • #10
                    Originally posted by badwithusernames View Post
                    zip is 08043. Theres a few trees, but nothing closer than 150'. The big storm three weeks ago took out the biggest sycamore. Where do I find PVwatts?
                    Thanks SD
                    You're welcome. Hoping the tree was the only storm loss.

                    WAY back in the day, I dated a very lovely young woman from/near Voorhees township. Thanks for the memory jog.

                    PVWatts is from NREL. Easiest way find it is probably to Google it.

                    PVWatts is a modeling program from the National Renewable Energy Laboratory. Quite useful, relatively easy to learn and mostly user friendly.

                    A respectful suggestion: Read ALL the help screens a couple of times and understand all what's being said there and why before you start. Then, get the inputs as close a you can. GIGO principle applies. Then, do a few runs. Then read the help screens again.

                    You also might benefit from a read of "Solar Power Your Home for Dummies", maybe even before the foray into PVWatts. Lots of stuff in there that can make the PVWatts experience more understandable and productive. A slightly outdated but still useful earlier ed. of the dummies book is a free online PDF download. A revised hardcopy is in bookstores or at Amazon for ~ $25 or so.

                    As you get more familiar w/PVWatts, you'll learn to appreciate ways it can be manipulated a bit to do useful stuff not mentioned in the help screens.

                    Knowledge is power. Get more of the first and any system that makes/generates the second will be better for the effort.

                    After all that, come back and ask questions to fill in the knowledge gaps created by your knowledge quest. Doing it that way will give your questions more meat, and that answers you get will make more sense to you.

                    Good luck.

                    BTW, welcome to the neighborhood.

                    Comment


                    • #11
                      JPM, thank you for the heads up on PVWatts! I had stumbled on it several months ago, and hadn't bookmarked it at the time. Been looking for it ever since. I modeled the system several ways, getting the numbers for different strings on the different roof sections. Even if my 7kw system is 75% as productive as the PVWatts model shows it should come close to covering all of my electric use. And I just pulled the trigger on the two SB-4000us inverters, so I have room to add a few more panels if needed.
                      Thanks! SD

                      Comment


                      • #12
                        Originally posted by badwithusernames View Post
                        JPM, thank you for the heads up on PVWatts! I had stumbled on it several months ago, and hadn't bookmarked it at the time. Been looking for it ever since. I modeled the system several ways, getting the numbers for different strings on the different roof sections. Even if my 7kw system is 75% as productive as the PVWatts model shows it should come close to covering all of my electric use. And I just pulled the trigger on the two SB-4000us inverters, so I have room to add a few more panels if needed.
                        Thanks! SD
                        You're most welcome.

                        Read the book, play w/ the PVWatts model and check in w/the local AHJ to help avoid surprises. They're a bit like Marines in that they can be quite helpful or your worst enemy.

                        Holler back as your needs dictate.

                        Regards,

                        J.P.M.

                        Comment


                        • #13
                          so, a few questions to add here, as the background to what I'm doing is already here.
                          I have 2 new old stock Sunny Boy SB-4000us inverters, and 24 Sun Power 305 panels.
                          Due to the size of the panels, and the size/ shape of the available roof, it works out perfectly to put 8 panels on one roof face, feeding one inverter, and 16 panels (2 strings of 8) on the other roof face, feeding the other. Here is my issue, that is maybe not an issue:
                          The inverter max voltage is 600V. The panel open voltage is 64.2. When corrected for record low temp here, the panel open voltage is 75 volts. 75 volts x 8 panels lands me right at the 600 volt max. Between the fact that these panels are used, and global warming means the record low might be a thing of the past, maybe I'm safe? The inverter max power voltage window is 250-480, would I be better off to cut the strings down to 4 panels?
                          I have been reading and reading. Just not sure if it's ok to be right on the max voltage like that, even though actually getting to that temp (and having full sun at the same time) seems completely unlikely.
                          Thanks. SD

                          Comment


                          • #14
                            8 panels & 1 cold snap and you are buying new gear. Loose 1 panel per string. Also investigate what voltage range your inverter MPPT functions, they usually have a low volt cutoff, and a high limit, where they revert to defaults, instead of MPPT tracking. You don't want to design outside the MPPT range, except maybe the first hour of morning when the panes are icy cold and only produce 60 watts, who cares if it's MPPT'd or default setting for that brief time. And heat, you for sure do not want to have your panel voltage sag below the MPPT range at noon when production is at it's highest
                            Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                            || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                            || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                            solar: http://tinyurl.com/LMR-Solar
                            gen: http://tinyurl.com/LMR-Lister

                            Comment


                            • #15
                              If your panels are that high voltage, better follow Mikes advice and series
                              less than 8. Running on the edge of disaster is a bad idea. Bruce Roe

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