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  • #16
    While I've got it open, I looked at whether reducing the row spacing and lowering the tilt would improve output. The answer is definitely no.

    Another option that looks possible is reducing the array size by 1 panel, and making each row 5 across, with a 9 ft pitch instead of 8 ft. That is a GCR of 0.5977. The loss of the panel hurts more than the better GCR helps (increases optimal tilt to 19 deg), producing 17118 kWh with the 10 kW inverter.
    CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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    • #17
      Originally posted by Techm7 View Post
      I just used this website and put all the proper info and it seems the measurements point to having inter-row spacing of 29.806 inches. I think the plans reflect something very close to that. http://www.rbisolar.com/solar-shading-calc/



      Btw: love this community. So helpful and informative. #Appreciation
      My calculations, in message #7, agree with you.
      There will not be any shading between 9AM and 3PM on DEC 21st.

      Comment


      • #18
        Originally posted by sensij View Post

        Yeah... I was calculating GCR, because it will be the same no matter the tilt angle. That feeds a parametric run in SAM nicely: shading.JPG





        By this analysis, 13 deg tilt looks like the best choice for that particular layout (8 ft pitch).

        Also, back to the original question, here is what it looks like with an 11.4 kW inverter instead of 10 kW... the extra 20-30 kWh is less than $5 / year difference in production. shading2.JPG
        The NOCT Rating of the PV Panels is 236 Watts
        8,496 Watts = 236 Watts x 36 Panels

        Can you please explain the conditions that allow the PV Panels to create more than 10,000 Watts?

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        • #19
          Originally posted by NEOH View Post

          The NOCT Rating of the PV Panels is 236 Watts
          8,496 Watts = 236 Watts x 36 Panels

          Can you please explain the conditions that allow the PV Panels to create more than 10,000 Watts?
          Any set of hourly conditions, starting most likely when P.O.A. irradiance is > ~ 850 - 900 W or so may well allow an 11.5 kW array to operate at 10 kW output. But that's an estimate.

          For starters, models like PVWatts do not create power (Watts). They give estimates of system output in kWh (energy).

          Aside from that, the NOCT is not much use for anything beyond a way to compare possible/likely performance estimates among and between panels at a particular and very specific set of conditions. 236 Watts may (or may not) be the power a panel will produce at 800 Watts P.O.A. irradiance when the amb. temp. is + 20 C with a steady wind of 1 m/sec. Operating conditions will almost always be different.

          Any other set of conditions a panel operates in will produce a different hourly output, maybe more, maybe less. and the total energy produced in kWh over any period will be more or less than that 236 Watts produced at NOCT conditions X the number of hours in the measurement period.

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          • #20
            Originally posted by NEOH View Post

            My calculations, in message #7, agree with you.
            There will not be any shading between 9AM and 3PM on DEC 21st.
            SAM agrees that there is no self-shading from *beam* irradiance. However, not all irradiance is direct, and the model is accounting for effects in received ground and sky diffuse irradiance as well.
            CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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            • #21
              Originally posted by J.P.M. View Post

              Any set of hourly conditions, starting most likely when P.O.A. irradiance is > ~ 850 - 900 W or so may well allow an 11.5 kW array to operate at 10 kW output. But that's an estimate.

              For starters, models like PVWatts do not create power (Watts). They give estimates of system output in kWh (energy).

              Aside from that, the NOCT is not much use for anything beyond a way to compare possible/likely performance estimates among and between panels at a particular and very specific set of conditions. 236 Watts may (or may not) be the power a panel will produce at 800 Watts P.O.A. irradiance when the amb. temp. is + 20 C with a steady wind of 1 m/sec. Operating conditions will almost always be different.

              Any other set of conditions a panel operates in will produce a different hourly output, maybe more, maybe less. and the total energy produced in kWh over any period will be more or less than that 236 Watts produced at NOCT conditions X the number of hours in the measurement period.
              OK, so lets use 900 W POA irradiance vs 800 NOCT
              9,515 Watts = 8,496 Watts x 112 %

              How are we getting above 10,000 Watts ?

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              • #22
                Originally posted by NEOH View Post

                OK, so lets use 900 W POA irradiance vs 800 NOCT
                9,515 Watts = 8,496 Watts x 112 %

                How are we getting above 10,000 Watts ?
                I don't have SAM in front of me now and can't provide the specific conditions it is using until later. However... Just look at real world data on PVOutput. It isn't hard to find data points with >85% kW/kWp, especially when boosted by edge of cloud reflections, but the amount of energy lost by clipping at that level is inconsequential.
                CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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                • #23
                  Originally posted by NEOH View Post

                  OK, so lets use 900 W POA irradiance vs 800 NOCT
                  9,515 Watts = 8,496 Watts x 112 %

                  How are we getting above 10,000 Watts ?
                  It looks to me like you do not understand the uses and limitations of the NOCT information .

                  I'll make it simple for you : IGNORE IT ! IT MEANS NOTHING ! IT WILL ONLY CONFUSE YOU MORE.

                  Comment


                  • #24
                    Originally posted by sensij View Post

                    SAM agrees that there is no self-shading from *beam* irradiance. However, not all irradiance is direct, and the model is accounting for effects in received ground and sky diffuse irradiance as well.
                    FWIW, and more of a historical footnote of how it used to be done using the solar profile angle, and perhaps an example of the convenience of SAM and other models, as well as ballpark confirming their output, at the given conditions, location and equipment dimensions, to avoid direct beam shading in the 6 hr. window (0900 - 1500 solar time) on 12/21 will require a row pitch of ~ 98.5 " or so, with a solar profile of ~ 32.7deg. To avoid direct beam shading from 0800 to 1600 solar time on 21/21 will require a row pitch of ~ 117 " or so and a solar profile angle of ~ 23.3 deg.

                    SAM will, as noted, also provide an estimate of a diffuse component that is a real PITA to calc. by hand and most always an estimate. That's a nice bennie, and this isn't a knock, but it's still an estimate whose accuracy is not improved by removal of the drudge of a boatload of calcs. In the distant past, I've done the diffuse adder for array self shading a few times at hourly level for an average day of each of 12 months and convinced myself that adding ~ 2-3 % to the beam component results is probably a practical number to use. But, SAM's a real time saver for those who know the limitations.

                    SAM and the other models are great tools and real time savers. I use SAM and other models a lot, but still feel something is lost except maybe some danger of a type similar to that of giving a loaded pistol to a 2 year old.

                    Comment


                    • #25
                      It might help you to think of STC, PTC, NOTC, and any other 'test condition' as a standardized flashlight. Forget that they start out at some specified spectra of light, at some intensity level, at some temperature, at some air mass index, and some wind speed, etc. Especially forget the ___ W/meter squared of irradiance. As panels roll off the assembly line they roll under the flashlight and their voltage and amperage is measured, power calculated. From the power value, the panel is directed into the 260W, 265W, 270W, or 275W piles and the price tag is stuck onto the panel accordingly. Other manufacturers roll their panels under the same flashlight, so a comparison can be made not only of one manufacturers panels, but of different manufacturers to each other. As you can imagine, managers at each company would love to 'arrange' their flashlight such that their panels measure higher on the scale, thus third party testing is essential for the system to work.

                      As you can see, this is completely disassociated from the level of irradiation they were subjected to. If STC were defined as 1100 W irradiation, or 900, the measurements would still have exactly the same meaning and comparative value. Other people, typically consumer advocates and politicians, favor different test conditions such as PTC and NOTC and that is fine. The more information the better, however the more expensive and harder to reproduce the flashlight becomes, the less likely manufacturers are going to endorse it. Thus test conditions involving set wind speeds, linearity, or awkward mounting, or temperatures difficult to maintain in a factory environment, or ____ are probably not starters for a replacement flashlight.

                      Comment


                      • #26
                        Originally posted by AzRoute66 View Post
                        It might help you to think of STC, PTC, NOTC, and any other 'test condition' as a standardized flashlight. Forget that they start out at some specified spectra of light, at some intensity level, at some temperature, at some air mass index, and some wind speed, etc. Especially forget the ___ W/meter squared of irradiance. As panels roll off the assembly line they roll under the flashlight and their voltage and amperage is measured, power calculated. From the power value, the panel is directed into the 260W, 265W, 270W, or 275W piles and the price tag is stuck onto the panel accordingly. Other manufacturers roll their panels under the same flashlight, so a comparison can be made not only of one manufacturers panels, but of different manufacturers to each other. As you can imagine, managers at each company would love to 'arrange' their flashlight such that their panels measure higher on the scale, thus third party testing is essential for the system to work.

                        As you can see, this is completely disassociated from the level of irradiation they were subjected to. If STC were defined as 1100 W irradiation, or 900, the measurements would still have exactly the same meaning and comparative value. Other people, typically consumer advocates and politicians, favor different test conditions such as PTC and NOTC and that is fine. The more information the better, however the more expensive and harder to reproduce the flashlight becomes, the less likely manufacturers are going to endorse it. Thus test conditions involving set wind speeds, linearity, or awkward mounting, or temperatures difficult to maintain in a factory environment, or ____ are probably not starters for a replacement flashlight.
                        For technical work, my experience is that STC is the most commonly used efficiency number. Other methods simply muddy the water and cause confusion that's not worth the benefits, which benefits seem mostly to those with money to make by the smoke/mirror method.

                        For those interested, the 1,000 W/m^2 used in the STC rating method is not only somewhat convenient for calculations, but as it turns out a pretty good fist approx. for the ~ likely maximum value of direct normal irradiance under a clear, cloudless sky over a good portion of the earth +/- some.
                        Last edited by J.P.M.; 10-13-2017, 03:38 PM.

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                        • #27
                          Originally posted by sensij View Post

                          SAM agrees that there is no self-shading from *beam* irradiance. However, not all irradiance is direct, and the model is accounting for effects in received ground and sky diffuse irradiance as well.
                          So the verdict is to keep things as is? I feel like the contractors are already very annoyed with the hassle we went through to get them to incline the panels. ALSO I noticed system description on the engineering plans say (36) SOLAREDGE P300 POWER OPTIMIZER instead of 320. Something to be concerned about?

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                          • #28
                            Originally posted by Techm7 View Post
                            So the verdict is to keep things as is?
                            As far as the tilt goes, yes I would say that you should leave it as is at 23 degrees and 8' spacing. I do agree that the contractor should have no problem with a 'no shade' clause if that is something resembling standard practice. As far as the reduced optimizer wattages are concerned, I have no idea whether that is tolerable - perhaps that is where he plans to get the money for the tilt sticks.

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                            • #29
                              I think you are leaving some kWh on the table going with 23 instead of 13, or even something in between if you want to speculate on the effect of fouling as a function of tilt, but at some point, it isn't worth stressing about. SAM has great technical documentation and is open source... you can see exactly where the numbers come from and decide for yourself whether the model is sound.

                              Honestly, the LG 320's meet the electrical specifications for the P300's, so it probably wouldn't be a big deal if they went with those. The P320's are the more appropriate choice, though.
                              CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

                              Comment


                              • #30
                                sensij Would you mind taking another look at the tilt? I went to PVWatts for Miami, and changed only the tilt parameter and total energy kept going up as I stepped from 13 degrees to 23 degrees. I originally did this as 13 'sounded' too shallow. I ran it for Roof Mount and Open Rack. I'm wondering if the panels make a difference - aren't his panels the gizmos that collect light from both front and backsides? At any rate, I would appreciate any explanation of the difference.

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