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  • Longevity experience

    I want to share a real-world case of solar power system longevity –

    Production History 2008-2022.jpg Location is 33.75, -118.33. Azimuth = 155 deg, tilt = 26 deg.
    24 Panels: Kyocera Solar, Inc.:KC175GT
    Inverter: PV Powered LLC:PVP4800
    DC Rating 4.2kW STC, 3.72kW PTC
    The dashed line is geometric computation. Colors are the various years.
    Percentages are of total yearly kWh relative to the geometric computation.
    The graph shows a small loss of performance over the first 15 years.
    2008 was a standout in October, but a slouch in February.
    2022 was variously more, and less, than 2008.
    Overall, year 2008 achieved 75% of the geom calculation; while 2022 achieved 71%.
    The shape of the geom curve is due to solar elevation path over the year.
    Variations at any given date (note, 30-day average) are due to atmospheric conditions.
    Last edited by n6ac; 01-31-2023, 07:04 PM.

  • #2
    Could you define "geometric computation" as you apply that term to solar energy applications? I'm ignorant of that term with respect to solar energy or PV system monitoring.

    Have you done a PVWatts run on your system ?

    If not, check it out. It's a PV model from NREL that's free, user friendly, quite helpful and as I and others have found to be reasonably to surprisingly accurate with respect to stochastic handling of residential array output.
    heck the help screens for a toe dip into probability density functions for array output.

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    • #3
      JPM – My "geometric" calculations were just that, no probabilities of cloud coverage etc.
      I considered only the path (angle) of the sun relative to normal at the panels, calculating the resultant power reduction.
      Numerically integrated. Repeated for various tilts to choose my optimum.
      Looking into my archive, I think the primary tool I used was what you see in this Excel spreadsheet –
      https://www.dropbox.com/s/y8vyrckl6p...onOverYear.xls
      Note – the sheet was created under MacOS 10.6.8, and uses macros. My memory on its use is dim. Pieces may have came from some NASA and NOAA sources.

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      • #4
        Also I guess I could have included the actual kWh per year numbers –
        Geom 9902
        2008 7453
        2009 7398
        2010 7201
        2011 7288
        2012 7419
        2013 7233
        2014 7423
        2015 7299
        2016 7189
        2017 6870
        2018 7072
        2019 6595
        2020 6822
        2021 7072
        2022 7053

        And I did a run of PVWatts for fun, here is the result –
        PVWatts Calculator.pdf
        That 6637 is a bit pessimistic compared with actual performance!
        Attached Files
        Last edited by n6ac; 02-01-2023, 06:41 PM.

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        • #5
          Originally posted by n6ac View Post
          Also I guess I could have included the actual kWh per year numbers –
          Geom 9902
          2008 7453
          2009 7398
          2010 7201
          2011 7288
          2012 7419
          2013 7233
          2014 7423
          2015 7299
          2016 7189
          2017 6870
          2018 7072
          2019 6595
          2020 6822
          2021 7072
          2022 7053

          And I did a run of PVWatts for fun, here is the result –
          PVWatts Calculator.pdf
          That 6637 is a bit pessimistic compared with actual performance!
          I also did a PVWatts run using your location, tilt and azimuth and got 7007 kWh/yr.
          The mean for your data is 7159 kWh/yr.
          That seems reasonable to me.

          Thank you for the explanation. I know that type of data as solar geometry and is the type of data I've programed in FORTRAN back in the day, later in BASIC and the LOTUS and finally in Excel using open literature algorithms and formulas.
          If you're modeling irradiance for PV modeling NREL has "TMY" file (Typical Meteorological Year) data. The irradiance data is mostly modeled for all the TMY locations and originally based on data from 26 U.S. sites. The other climatological data in the TMY is updated once in a while for the several hundred locations in the TMY data base. It's been substantially revised over the years That's what the PVWatts and other models commonly use for irradiance modeling.
          Info on the TMY database, its history an updates is available per www.nsrdb.nrel.gov/data-sets/tmy.
          Last edited by J.P.M.; 02-01-2023, 07:37 PM.

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          • #6
            Thanks!
            My 6637 is lower than your 7159 because I threw in a couple percent of loss for presumed degradation ...

            My programming goes back 60+ years to start with Fortran on an IBM 1620 desk (literally) computer that probably had far less power than our phones today.
            Last edited by n6ac; 02-01-2023, 07:40 PM.

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            • #7
              Here is a version of my first graph, with an overlay of the 6637 PVWatts curve in weird blue-green.
              PVWatts overlay.jpg
              And a year-by-year history (with linear trend line). We did some serious panel washing a couple of years ago. Very hot temps in 2019 & 2020.
              Trend.jpg

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              • #8
                Originally posted by n6ac View Post
                Thanks!
                My 6637 is lower than your 7159 because I threw in a couple percent of loss for presumed degradation ...

                My programming goes back 60+ years to start with Fortran on an IBM 1620 desk (literally) computer that probably had far less power than our phones today.
                Ahhhh !!!, The joys of the 1620, machine language and punch cards.

                Use a 10% system loss parameter and don't try to second guess the model. I adjusted that system loss parameter for my system and site so that the PVWatts annual output matched the annual SAM output for my system including shading and other stuff SAM can handle. That adjusted (PVWatts) system loss parameter (with 3.5 % in it for a shading loss) is 12.8 %.
                The actual system running 365 day output matched that output within ~ +/- about 5% over the 10+ years of operation from 10/13/2013 to the present.
                Separately, from array insitu measurements since then I've estimated the annual panel output degradation at somewhere between 1.0 and 1.3 W/panel-yr. (~ 0.3 to 0.4%/yr. based on STC panel rating) at what seems to be a pretty steady rate.

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