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Cleaning panels doesn't really matter - measurement and photo

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  • Cleaning panels doesn't really matter - measurement and photo

    I'm in Oakland California. Did you hear that CA is burning? We've had 4 days of ash fall and blocked sunlight. Today we got some sunlight, so I cleaned the ash of my roof top panels to see what difference it made. Used a hose and a brush and I cleaned 10 out of 14 rooftop panels. See below photos of the panels before cleaning so you can see how dirty, and the energy generated graph. It's not very a scientific test, but I am surprised that dirty panels work almost as good as clean ones.

    graph.jpg

    dirtyPanels.jpg




  • #2
    Glad the ash is not bad on your panels. But a single days output comparison may not show you the real improvement of having clean panels.

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    • #3
      I read an article at Sandia that said in their testing cleaning panels made about 5% difference. I suspect the folks who say "OMG cleaned my panels and output jumped!!!" are seeing the effects of cooler cells (reduced temperature derating).

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      • #4
        Originally posted by secessus View Post
        I read an article at Sandia that said in their testing cleaning panels made about 5% difference. I suspect the folks who say "OMG cleaned my panels and output jumped!!!" are seeing the effects of cooler cells (reduced temperature derating).
        Are you writing that you believe cleaner panels have improved performance because they are running cooler ?

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        • #5
          Originally posted by J.P.M. View Post

          Are you writing that you believe cleaner panels have improved performance because they are running cooler ?
          The panels are cooler because they're wet and that's why the power output increases not because they're cleaner.

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          • #6
            Originally posted by nwdiver View Post

            The panels are cooler because they're wet and that's why the power output increases not because they're cleaner.
            You may have worked in nukes, but you don't seem to know much about testing of solar equipment.
            Panels are most probably not wet when having their levels of fouling evaluated.
            I'm pretty sure most panel fouling measurements of the before/after cleaning type are of the instantaneous or at least very short period duration, on site and under in situ conditions of semi or quasi steady state irradiance. That means temperatures and other variables are not changing much, if at all, including how wet things are.

            Also, under any reasonable insolation where the more reliable measurements can be easier to obtain, panels will usually be well above the dew point temp. meaning any water on a warm panel water will have evaporated long before any measurement.

            Also, panels are rarely, if ever cleaned when they are warm or hot as would most likely be the case for testing in situ.

            In short, panels being tested or their outputs measured clean and dirty are in all probability not wet as the variability introduced by the moisture would invalidate the testing.

            Doing an energy balance on a panel can provide some insights.

            Depending on the nature of the fouling and it's characteristics, the clean(er) panel will probably run a bit hotter than a fouled panel.
            Reason: For most situations more energy gets into a clean(er) panel.
            If heat transfer correlations and rates of heat transfer per degree of temp. difference, panel <-> environment, don't change too more energy into a panel means higher temp. differences will be required.
            Yes, more sun turned into electricity by a clean panel means less heat rejection but about 4 times as much excess heat will be generated internally that'll need to be rejected. That's also before decreases in panel efficiency from higher operating temps. are considered. All that will necessitate an increase in the temp. of a clean panel to maintain the energy balance. The net result in most every case will be that a cleaner panel will run hotter.
            There are some considerations that may modify the above situation such as a very light colored fouling layer that may also reflect some of the longer IR frequencies while passing some of the UV stuff below the cutoff wavelength for usefulness by silicon solar devices, but that's not too likely unless by design.
            I worked a bit with some folks a long time ago on a type of surface coating that passed wavelengths below the useful PV cutoff range and reflected everything above that wavelength. Lots of problems that had no workable/practical solutions at the time. Maybe things have changed.
            Last edited by J.P.M.; 09-13-2020, 05:18 PM.

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            • #7
              Originally posted by J.P.M. View Post

              Probably not. I'm pretty sure most panel fouling measurements are of the instantaneous or at least very short period duration, on site and under in situ conditions of semi or quasi steady state irradiance. That means temperatures and other variables are not changing much, if at all, including how wet things are.
              Evaporative cooling is pretty effective. I sprayed my first array in TX once on a hot day (terrible idea I know). The increase in power was enough that I could HEAR the inverter output increasing.

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              • #8
                Originally posted by nwdiver View Post

                Evaporative cooling is pretty effective. I sprayed my first array in TX once on a hot day (terrible idea I know). The increase in power was enough that I could HEAR the inverter output increasing.
                Probably more sensible cooling than evap. cooling, at least if it was noticeable enough to be heard by changes in equipment noise.
                Also, given the thermal time constant of most PV panels the cooling effect lasts only about as long as the water is running.
                Keeping water running uniformly over an array will cost money for equip., maint. and pumping costs besides the cost and use of the water. If nothing else, careful cost analyses may be a good idea.
                Continuous tap water on glazing, hot or otherwise, will increase the buildup of scale on the glazing. That will increase panel fouling and decrease performance. Sounds like the possible makings of a foot shooting party to me.
                All in all, spraying water on PV panels seems to have been found about as sensible as spraying water on residential A/C condensers to decrease the working fluid temp. to the coolant before expansion.
                Such things don't usually come to a good end for reasons most folks are clueless about.

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                • #9
                  The best way to actually measure the difference would be on an array with individual panel monitoring, cleaning half of them only and looking at the panel production of both side at the same time.
                  From what I have read, people report for 2% to 10% depending how dirty the panels were.

                  A thin layer of dust probably does not make a huge difference. A thick layer of ashes like we have had recently in CA and or lots of bird dropping probably creates a bigger difference.
                  It will all depend on the opacity of the layer.
                  Last edited by scrambler; 09-15-2020, 03:14 PM.

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                  • #10
                    Originally posted by scrambler View Post
                    The best way to actually measure the difference would be on an array with individual panel monitoring, cleaning half of them only and looking at the panel production of both side at the same time.
                    From what I have read, people report for 2% to 10% depending how dirty the panels were.

                    A thin layer of dust probably does nor make a huge difference. A thick layer of ashes like we have had recently in CA and or lots of bird dropping probably creates a bigger difference.
                    It will all depend on the opacity of the layer.
                    Agreed. If I was of a mind, I'd engineer a PV fouling measuring gadget consisting of 2 PV panels, a pyranometer, a data logger and some wiring/ancillary equipment.

                    A workable alternative might be a pyranometer and a fixture that allows two glass lites' transmission to be compared, one constantly exposed to the atmosphere and not cleaned with the other cleaned before measuring how much sunlight gets transmitted through it.

                    As for the rate of panel fouling, it varies a lot by location and application. For my location and application only, after measuring a bunch of stuff for 6 years, including irradiance, and other weather variables w/a Davis Pro II + weather station, under normal conditions, without rain or cleaning, my array fouls at a rate such that the output drops somewhere between about 0.75% and 0.90%/week. I believe that rate may become asymptotic after 8 weeks or so, but I've not been rain free long enough to get data to confirm/refute. The max. fouling I've measured doing daily readings on days under clear skies is ~ 6+ % or so and the rate of fouling seemed to be leveling off the few times I had rainless periods that long. More anecdotal than provable. I suppose I could check windshields in junkyards with a handheld pyranometer.
                    Last edited by J.P.M.; 09-13-2020, 08:35 PM.

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                    • #11
                      I live in a dusty semi-rural area. I cleaned my panels once, only noticed a nominal increase in production. Within a week or so my panels were dirty again. I have't bothered with cleaning again.I believe this was discussed in an earlier thread and the general consensus was that cleaning panels is not necessary unless they are REALLY dirty to the point where they are completely obstructed from sunlight.
                      Last edited by PugPower; 09-15-2020, 08:08 PM.

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                      • #12
                        Originally posted by PugPower View Post
                        I live in a dusty semi-rural area. I cleaned my panels once, only noticed a nominal increase in production. Within a week or so my panels were dirty again. I have't bothered with cleaning again.I believe this was discussed in an earlier thread and the general consensus was that cleaning panels is not necessary unless they are REALLY dirty to the point where they are completely obstructed from sunlight.
                        For my array, I've got numbers that point to a 5 minute or less simple hosing restoring about 2/3 to 3/4 of the array output that was lost to fouling. If you or anyone else can get at their array with a hose, it might be worth considering.

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                        • #13
                          Originally posted by J.P.M. View Post

                          For my array, I've got numbers that point to a 5 minute or less simple hosing restoring about 2/3 to 3/4 of the array output that was lost to fouling. If you or anyone else can get at their array with a hose, it might be worth considering.
                          Your output increased 3x by cleaning them? Were your panels covered in 3 layers of water soluble paint? I’ve cleaned panels so dirty they were more brown than blue and got <10% increase. And unless you have REALLY good water you need to squeegee the water off to avoid scale buildup.

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                          • #14
                            Originally posted by nwdiver View Post
                            Your output increased 3x by cleaning them?
                            That is not what he wrote. To say it another way, he got back 2/3 to 3/4 of the few percent of production he was down due to dirty panels. With a simply spray of a garden hose he recovered nearly all of the partial production that was lost to dirt on the glass. A more through scrubbing and rinse gets you back to the best output possible.

                            Dave W. Gilbert AZ
                            6.63kW grid-tie owner

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                            • #15
                              Originally posted by nwdiver View Post

                              Your output increased 3x by cleaning them? Were your panels covered in 3 layers of water soluble paint? I’ve cleaned panels so dirty they were more brown than blue and got <10% increase. And unless you have REALLY good water you need to squeegee the water off to avoid scale buildup.
                              I wish. No. Reread what I wrote. Note the part about restoring ~ 2/3 to 3/4 of the output that was lost to fouling.

                              A (recent) example may help make things clearer.

                              On 09/04/2020, after my summer session of ~ 65 daily cleanings for reasons loosely related to this topic, I gave my array what I thought would be one final cleaning until next May - and I mean a very thorough one, soaping 2X, rinsing 3X and squeeging. Details on why the 65 daily cleanings if you want.

                              Anyway, smoke/haze started for me on 09/06/20. I left the array alone for a few days but keeping an eye on input, output and array performance, on 09/11/2020 I noticed a lot of visible fouling and measured the fouling to have caused a performance deterioration of ~ 0.054 or ~ 5.4 % from what I believe were the clean conditions of 09/04/2020.

                              Under normal conditions, I expect a performance deterioration of ~ 0.75% to maybe 0.90% fouling penalty/week. A blinding flash of the obvious led me to conclude things are not normal.

                              The next morning, 09/12/2020, at 0800 P.D.T., I got up on the roof with my trusty hose and hosed off the array at a rate of ~ 3/4 gal./panel. No wiping/scrubbing/squeeging. Just a hosing.
                              On 09/12/2020 on the 9 minutes centered around the time of minimum incidence angle on the array for that day, I took measurements in my regular way and usual time that allow me to estimate array fouling.
                              The fouling rate I measured for that day was 0.012 or a bit more than 1 %.

                              So, the percentage improvement in lost output, or restoration of output that was lost due to fouling amounted to (0.054-0.012)/0.054 or ~ 78 %.

                              A comment on how accurate I think those fouling numbers might be: Having done this (measured array performance - both clean and fouled) several hundred times for going on 6+ years, I estimate the methods I use and the numbers I get for fouling are probably good to +/- 0.90 % or so at the 95 % confidence level. So, for this example, I would say my estimate of the dirty fouling to be between maybe 4.5 % and 6.3 %, and the post hosing fouling between maybe 0.3 % and 2.2 %.

                              A final comment: When I began all this array performance measurement/cleaning stuff in 2014, I compared before/after performance for my array cleaned in 4 different ways:
                              - Soaped, brushed, rinsed, wiped w/distilled water and then squeeged.
                              - Soaped/brushed/rinsed w/tap water but not wiped w/distilled water and not squeeged, just air drying.
                              - Not soaped but hosed w/ tap water, brushed and hosed again w/ tap water, no squeeging, and air drying only.

                              - Simple hosing from above the array at a rate of ~ 3 l/panel, with no other treatment. That method seems to restore ~ 2/3 to 3/4 of the array output that was lost to fouling as I wrote.

                              After doing each method at least 30 times (which took about 18 months or so) and measuring performance on most days when the sky was cloudless for at least an hour or so before the 9 minutes around the minute of minimum solar incidence angle for that day (which is the time when I take irradiance, weather and array measurements), I was unable to measure any significant difference in the level of post cleaning fouling attained by any of the first 3 methods described.

                              My conclusions:
                              - While there may be a difference in % of performance restoration after each of the first 3 thorough cleaning methods described above, the methods I use to measure array fouling and its effect on array output show little if any difference in clean performance. If there is a difference, which I don't doubt, I can't measure it and it's masked by the variance in my measurements. If I can't measure it, and given the accuracy of most every method I know of or have read about, I'm not worrying about it. Hell, even pyranometers kept in very tight calibration are +/- 1 % if you're lucky.
                              - Water spots left from not squeeging an array after cleaning make no difference in clean output using the methods I describe.
                              - If I ever stop fooling around w/my array, if I want to keep the array's fouling penalty below about 3 % or so average, the easiest way for me to do so will be to get up on the roof ~ 1 X/month if it doesn't rain and hose the 16 panels with about 4-5 minutes of simple tap water.

                              Take what you want of the above. Scrap the rest.
                              Last edited by J.P.M.; 09-16-2020, 12:36 PM.

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