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  • #16
    Originally posted by SunEagle View Post

    I would expect you would see more production losses due to dirt or foreign matter build up on the panel surface then from cell degradation.

    Then again cheap cells having a shorter life then A grade cells is probably a safe assumption.
    according to research done by JPM and his data are the most scientifically strict measurements on the matter I came across so far indicate dirt or other build up as insignificant contributors to the panel's degradation in performance and tend to 'level out'- stuff sticks on-stuff washes out. Cell degradation is unfortunately cumulative process and adds up with time exceeding dirt induced one after few years.

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    • #17
      Originally posted by kb58 View Post
      That's pretty fantastic sw, thank you very much for showing its abilities. On the one hand I'm surprised that Trise is only ~10C but on the other hand, if it was otherwise, premium panels would have more of a foothold here. I played around with SAM a little and understand having to step lightly to get good data.

      As for the big picture, I'm also going to improve things the old-fashioned way, conservation. The main pond pump and UV lamp will be put on a timer to turn them off between 4-9pm. That'll save roughly 0.5KWh, which adds up. The biological filter aeration and pond returns will be left enabled to ensure good oxygenation, but they don't take nearly as much power.

      My end goals is to determine both the required panel count, and what I can expect for a yearly electric bill as a result - assuming TOU 2.0. To zoom out even further, I guess the ultimate goal is to see whether through some reasonable conservation, can we achieve a low enough bill that solar ends up being more trouble than it's worth, and right now I don't know.

      (EDIT: This is now way off topic... I should probably start a "blog" thread on our installation)
      I can somewhat confirm with measured data what Sensij's submitted. For one thing, my roof mounted ( w/~ 11" of clearance under the array to the roof deck) array's clean array cell temps. are, under bright sun (between 930 and 1,010 W/m^2 P.O.A., on average, 28.5 C (min., 23.9 C, max, 31.6 C., std. dev. 1.8 C., N = 36) above the roof ambient air temp., all as measured.

      Those numbers seem to agree reasonably well with the Sandia method of calc'ing the difference (cell temp. - amb. temp.).The Sandia method estimates cell - amb. temps. for the same conditions as: Mean = 28.3 C., min.= 26.8 C., max. = 30.1 C., std. dev. = 0.9 C.

      What the Sandia method estimates for cell to amb. temp. difff. has a smaller spread than my measurements, probably, and I'd suggest mostly, due to the variation in wind vector and how I account for it.

      Roof amb.air temps. for the same 36 measurement trials: Mean = 28.6 C., min. = 20. C., max. = 36.8C., std. dev. = 4.8 C.

      Cell temps: By measured method: Mean = 57.1 C, min = 47.0 C, max. = 66.1 C, std. dev. = 5.1 C.
      By Sandia estimated method: Mean = 57.0 C, min.= 48.0 C, max. = 65.3 C, std. dev. 5.0 C.

      Cell temps. measurements taken from array Voltage measurements as correlated with actual panel back plate temp. measurements and adjusted per the Sandia method (add 3 C. to back panel temps.)

      All data taken at the time of min. angle of incidence ( and therefore quasi/semi instantaneous) on clear days only between 04/06/2017 and 06/20/2017.

      As you say you'll be doing, conservation is still, far and away the first thing(s) to do before considering PV. I strongly encourage such measures as a fist step.
      Last edited by J.P.M.; 09-21-2017, 05:14 PM.

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      • #18
        Originally posted by max2k View Post

        Well, like with any model it makes certain assumptions and reflects only portion of what is going on. For example, how these 2 panels age? It wouldn't be so great anymore if Silfab loses its performance significantly faster than Panasonic or would require replacement somewhere after 10 years in service due to cheaper sealant used. That's not to say I recommend either I only trying to be level headed about this. From installer's point of view Silfab option is no brainer based on this analysis: higher initial output, more competitive price and if it fails it would lead to new sales.
        The OP's hypothesis in the first post was "the more expensive panel generates $627 more in electricity due solely to better thermal performance". I used SAM to investigate that. Extending that investigation to a conclusion of "Lowest panel cost per watt (assuming the panel isn't crap) really is the way to go" is a leap, but even with more much more detailed analysis, it probably isn't far from the truth. The trick is to figure out how to recognize "crap".

        There is a lot that can go wrong over 10 or 20 years, and not much to go on today to distinguish today good from bad. I'm not sure the installer's calculus is as simple as you make it out to be... otherwise, there would be many more Silfab installations and fewer with LG and Sunpower. Marketing and branding works.
        CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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        • #19
          Originally posted by sensij View Post

          The OP's hypothesis in the first post was "the more expensive panel generates $627 more in electricity due solely to better thermal performance". I used SAM to investigate that. Extending that investigation to a conclusion of "Lowest panel cost per watt (assuming the panel isn't crap) really is the way to go" is a leap, but even with more much more detailed analysis, it probably isn't far from the truth. The trick is to figure out how to recognize "crap".

          There is a lot that can go wrong over 10 or 20 years, and not much to go on today to distinguish today good from bad. I'm not sure the installer's calculus is as simple as you make it out to be... otherwise, there would be many more Silfab installations and fewer with LG and Sunpower. Marketing and branding works.
          More of the confluence of process economics and the art, more than the science, of engineering. Best information + more experience (including the ability to know B.S when you smell it) = best probability for a safe, fit for purpose, cost effective, good, long term outcome.

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          • #20
            Originally posted by max2k View Post

            according to research done by JPM and his data are the most scientifically strict measurements on the matter I came across so far indicate dirt or other build up as insignificant contributors to the panel's degradation in performance and tend to 'level out'- stuff sticks on-stuff washes out. Cell degradation is unfortunately cumulative process and adds up with time exceeding dirt induced one after few years.
            If the panels are kept clean and cool they will perform better. But IMO most homeowners don't have a clue about how to or when to wash down their panels and rely on the rain to do it.

            If the environment is relatively dirty then there will be dust or dirt buildup that will be ignored since it can't easily be seen. So if performance drops a homeowners first focus is on a panel cell or system equipment issue instead of the more simpler solution to just keep the panels clean.

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            • #21
              Originally posted by SunEagle View Post

              If the panels are kept clean and cool they will perform better. But IMO most homeowners don't have a clue about how to or when to wash down their panels and rely on the rain to do it.

              If the environment is relatively dirty then there will be dust or dirt buildup that will be ignored since it can't easily be seen. So if performance drops a homeowners first focus is on a panel cell or system equipment issue instead of the more simpler solution to just keep the panels clean.
              There's an interesting academic idea that, from an energy balance standpoint, dirty panels will run cooler than clean panels, and clean panels run hotter. Very briefly, the dirt keeps solar energy from hitting the panel and thus lowers the amount of incident radiation that's available for heating the panel. The incident radiation is what causes panel temps to increase, to a fist approx., directly as P.O.A. irradiance. The way I, and because I've not seen it talked about much, I believe probably the way most folks investigating array fouling estimate performance penalties from dirt, the cell temp. depressions caused by dirt get rolled in with and offset to some degree, the performance degradation caused by the dirt, thus masking some of the effect of fouling.

              I'm not sure, however, I follow the idea that dirt on panels will tend to be ignored by virtue of being harder to identify in dirt/dusty environments.

              Anecdotally, the folks at the Furnace Creek Ranch in Death Valley clean their 1.2 MW array every 2 years just to get the starling guano off the panels. They claim a reasonably constant 6 % or so penalty for fouling. Although, after checking in with the operators 1X/yr. in July over the last 5 or 6 years, I kind of doubt they have much of a clue at what their fancy charts and graphs are telling them, but performance seems to not have suffered a whole lot from fouling.

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              • #22
                Originally posted by SunEagle View Post

                If the panels are kept clean and cool they will perform better. But IMO most homeowners don't have a clue about how to or when to wash down their panels and rely on the rain to do it.

                If the environment is relatively dirty then there will be dust or dirt buildup that will be ignored since it can't easily be seen. So if performance drops a homeowners first focus is on a panel cell or system equipment issue instead of the more simpler solution to just keep the panels clean.
                you're about to provide basis for panel cleaning businesses and my recollection was the JPM's results sharply indicated the opposite, like the rain would take care of it at his location. BTW, I'm on the lazy side of this 'dirty panels' problem but since my system is only 2 months old I don't have much of reference data. Currently my panels look dirty but their performance correlates more with clouds than anything else, time will tell.

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                • #23
                  Originally posted by max2k View Post

                  you're about to provide basis for panel cleaning businesses and my recollection was the JPM's results sharply indicated the opposite, like the rain would take care of it at his location. BTW, I'm on the lazy side of this 'dirty panels' problem but since my system is only 2 months old I don't have much of reference data. Currently my panels look dirty but their performance correlates more with clouds than anything else, time will tell.
                  Yea, commercial panel cleaning is mostly a rip off to separate the solar ignorant from their assets.

                  Max: If you're in SO. CA, at 2 months new, and if you're not living next to, or of downwind a freeway, you're probably fouled at ~~ 4 - 6 % or so, probably less if you've received some precip.
                  The eye is a poor tool to gauge panel performance degradation. Mine look filthy, bathtub ring and all, but performance measurements indicate a roughly 4 - 6 % performance loss. I've had ~ 0.08 " precip in the last 2 weeks, caking things up nicely on the array.
                  Performance better correlate pretty well with cloud presence. That's how it works.
                  Last edited by J.P.M.; 09-21-2017, 10:17 PM.

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                  • #24
                    Originally posted by kb58 View Post
                    I know the general rule is "no" but I thought I'd work through the numbers because one factor made me wonder: thermal derating.

                    For "fun" I picked two panels of similar output, with the main difference being their thermal derating. For some of the following calculations I didn't have hard numbers and guessed, so feel free to correct me.

                    The contenders:

                    Panel A: Silfab Solar SLG-345M, $267, thermal derating -.38%/C
                    Panel B: Panasonic VBHN330SA16, $398, thermal derating -.26%/C
                    Price difference, $131

                    Assumptions:
                    Panel runs 50C above room temp
                    Runs this way 4 hrs a day, for four months of the year.
                    Average power cost of $0.25/KWh

                    Power output at 25C:
                    Panel A: 345W
                    Panel B: 330W

                    Power derating due to 50C rise:
                    Panel A: 0.38% * 50 * 345 = 66W loss
                    Panel B: 0.26% * 50 * 330 = 43W loss

                    Power loss over time:
                    Power loss difference, 66 - 43 = 23W
                    0.023KW * 4hrs/day * 30 days * 4 months = 9.6KWh a year of lost power

                    Assuming an average cost of $0.25/KWh for electricity, that's a loss of roughly $38 a year

                    Panel B costs $131 more than Panel A, so it'll take $131 / $38, or roughly 3.5 years to break even on.

                    Assuming (again, roughly) a 20 year lifespan, the more expensive panel generates $627 more in electricity due solely to better thermal performance.

                    "If" I'm close on these numbers, it makes considering the premium panels more realistic.

                    Do I have any gross errors above?

                    How did you compute $38.00 per Year ?
                    9.6 KWh per year x $0.25 per KWh = $1.65 per year

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                    • #25
                      KB: Back on the original subject of this thread, you're now in a position to estimate the difference in revenue caused by changing panels. Run SAM for both panels, plug each panel's hourly output into your spread sheet, get an annual total for each array, adjust each for a per STC kW output and compare. If you've got SAM dialed in and your rates dialed in, you'll get a difference form one panel to the other that's maybe a decent 1st approx. of which panel will produce more output per STC kW.

                      One other, somewhat related point: On your cost/kWh spreadsheet, I forgot to note that any array output values need to be adjusted for daylight saving time. The SDG & E rate times are for local "clock" time, meaning daylight saving time or not as applicable. The model's times are all standard time. I apologize for the omission. Not adjusting for std. - D.S.T. as appropriate will change tings in a small but noticeable way.

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                      • #26
                        Originally posted by NEOH View Post

                        How did you compute $38.00 per Year ?
                        9.6 KWh per year x $0.25 per KWh = $1.65 per year
                        Good catch, and fixed!

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