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  • #16
    Originally posted by Ward L View Post
    After my panels were up for a year I washed two of the 40 panels. The two panels were on the ground at the time and I made sure I did a very good job cleaning them. I have micro-inverters and after the cleaning of the two panels, I could not detect a difference in the power output between the clean panels and the other panels. I live in a dusty area, but my panels are on the top of a 2nd story house. In my situation, I don't think cleaning my panels very often is worth it. When I look at the average peak power decline and the total power decline over the last year, I get a -1.3% change in both. It seems reasonable that each installation has different payout for cleaning panels. I can't see, but I expect there is some heavy dew on my panels occasionally that might help clean them.

    ​The other thing to watch out for is making sure you don't let hard water dry on the panels and leave calcium deposits to block the energy.
    As for hard water stains blocking irradiance, as a subset of my array fouling estimates, I cleaned my panels on 2 consecutive "clear" days on a couple of separate occasions. The first day with tap water which does leave a residue, and the next day with distilled water, using a cloth to remove water drops for the distilled water cleaning which resulted in no visible residue. I was unable to detect a difference in the array's instantaneous performance from measurements taken at the 12 min. around the time of min. array incidence angle.That was done by measuring individual panel temps. as described in previous posts, measured GHI at the array, wind velocity at the array and air temp. at the array, converting the GHI to plane of array irradiance and calculating measured efficiency against theoretically clean efficiency. The difference in measured efficiency one cleaning method to the other - one with hard water spots and one without hard water spots was about .003, or 0.3%. Well below my instruments and methods precision of ~~ +/- 0.5% - 0.75% or so if I'm careful and lucky. If water spots impair performance, I can't measure it.

    Not claiming perfection, but what I measured may me more than anecdotal.

    Also, as it turns out, and depending on the nature of the light, and its frequency, the human eye is probably not a very good instrument to judge the transmittance or reflectance characteristics of various films with respect to solar radiation frequencies. In other words, hard water spots may very well look worse than they are with respect to how much they impair solar PV production. Sometimes things are not what they seem.

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    • #17
      Just to add another data point, I hosed off my panels the evening of July 8th and noticed a roughly 8% improvement based on looking at pvoutput data => http://pvoutput.org/list.jsp?userid=44718 The last time it rained was May 6th and the last time I cleaned them was April 2nd. When I cleaned them in April, it looked like recent rains had kept them fairly clean, they mostly had the yellow bee dropping spots on them that I scrubbed off.

      My panels most likely soil faster than most in my area since they are horizontal and some neighbors have taken to drastic water saving measures that still get mowed each week by the gardeners creating a lot of dust.

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      • #18
        Originally posted by Ward L View Post
        After my panels were up for a year I washed two of the 40 panels. The two panels were on the ground at the time and I made sure I did a very good job cleaning them. I have micro-inverters and after the cleaning of the two panels, I could not detect a difference in the power output between the clean panels and the other panels. I live in a dusty area, but my panels are on the top of a 2nd story house. In my situation, I don't think cleaning my panels very often is worth it. When I look at the average peak power decline and the total power decline over the last year, I get a -1.3% change in both. It seems reasonable that each installation has different payout for cleaning panels. I can't see, but I expect there is some heavy dew on my panels occasionally that might help clean them.

        ​The other thing to watch out for is making sure you don't let hard water dry on the panels and leave calcium deposits to block the energy.
        I thought I saw my performance dropping and I have more of a farm atmosphere around my house than most anyone here. We dont have the horse any more but the emus and tortise keep much of the backyard 1/2 acre a dust bowl. Also the driveway is dirt and ours is a very fine powdery dust. Between cars and animals the dust is pretty thick in a month. I'd guess I have more dirt in a month than most standard suburban homes have in a year.
        So I went up recently and hosed the panels down at dusk. I figured dusk as the best so we dont crack anything while hot and allowing slower drying. Hopefully having the hard water be partially wiped down overnight with a dew layer before the hard water was baked on during daylight. So I cleaned up 2 1/2 months of seriously dirty looking dust but watching output levels over the next week I didn't notice any noticeable output change. I suspect a 1-2 % change but without any kind of accurate automated constant monitoring I can't tell. And it seems for me the panels didn't look any more dirty after 1 month than 2 1/2 months.
        I suspect I'd notice 5% and certainly 10% but it just isn't there. I may still clean them a few times a year just as a reason to check on them and keep my caustic dirt from pitting the metal and or maybe even the glass.
        Last edited by nomadh; 08-17-2016, 07:44 PM.

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        • #19
          There is a lot of variation of people's experience to make me suspect that the kind of panels also plays a part.

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          • #20
            Originally posted by huge View Post
            There is a lot of variation of people's experience to make me suspect that the kind of panels also plays a part.
            Some, but not much. Sunpower panels get just as dirty as anyone elses. More of the variation is probably due to local or micro climate effects as well as panel tilt. Overall, trying to get some quantitative grip on what's usually a small number (several % or so) is difficult even under controlled conditions with decent equipment. This is a task that's about halfway between qualitative and quantitative and my opinion is that's probably good enough. As in : Do the panels look dirty ? If so, hose'em off about 1X/month or so if it doesn't rain. Commercial cleaning is a rip off.

            Bottom line for measurements I've done: My array fouls at a rate of about 0.75% - 1.0%/week without rain. That's somewhat variable and non linear. That rate may decrease as the dry spell continues but I'm not sure about that. And, a decent rain removes ~ 1/2 to 3/4 of the effects of dirt.

            That's probably about as precise as I can get and my guess is that rate is somewhat typical in So. CA, but by no means universal. It's probably an adequate description and about as precise or more precise than needed for the task.

            Thanx everyone for the information/opinions. I'll report my results from ongoing measurements from time/time.
            Last edited by J.P.M.; 08-18-2016, 01:28 AM.

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            • #21
              I've had my panels since Feb this year and have cleaned them with soft water since the beginning. I bought a small portable softener made for using on RV's and my results have been positive. I wanted to figure how often I should wash my panels since "it never rains in So Cal". I have the luxury of Enphase micro inverters so I washed half of my panels after 2 weeks of prior cleaning. What I discovered was the dirty panels had lost ~ 1.5% power over those 2 weeks. To verify, I then did a total cleaning and all panels returned to equal output. So I now feel I'm justified in washing them very two weeks. It's easy because I'm single story and can wash them from ground level.

              I'm researching soft bristle brushes and long handles so I can do a more thorough cleaning perhaps annually. I see a lot of dirty panels in my neighborhood and they have to be down 10-20 % production.

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              • #22
                Originally posted by SanteeFJ View Post
                I've had my panels since Feb this year and have cleaned them with soft water since the beginning. I bought a small portable softener made for using on RV's and my results have been positive. I wanted to figure how often I should wash my panels since "it never rains in So Cal". I have the luxury of Enphase micro inverters so I washed half of my panels after 2 weeks of prior cleaning. What I discovered was the dirty panels had lost ~ 1.5% power over those 2 weeks. To verify, I then did a total cleaning and all panels returned to equal output. So I now feel I'm justified in washing them very two weeks. It's easy because I'm single story and can wash them from ground level.

                I'm researching soft bristle brushes and long handles so I can do a more thorough cleaning perhaps annually. I see a lot of dirty panels in my neighborhood and they have to be down 10-20 % production.
                Thank you. That 1.5 % over ~ 2 weeks you report seems about in line w/ my 0.75 % - 1.0 %/week. Seeing as how most of this business on panel fouling is probably quite variable and not very predictable as to location, season, rainfall, and type of stuff floating around in the air, my opinion, FWIW, is settling in on an ~ 1X/month hose down of an array as the best tradeoff between a reasonably clean array that doesn't have a large performance penalty due to fouling, and getting on a roof with a bucket, etc. I'm still of the opinion that commercial cleaning of an array is a rip[ off and a waste of money.

                On the idea of appearance being a valid indicator of fouling penalty, last year, my array, after about 7 weeks with no rain looked very much like most neighbors' arrays - dusty with the bathtub ring on the bottom 12" or so of all the panels. My performance at that point had dropped about 4-5 % from the clean condition.Then it rained. Performance came back to ~ 97-98 % of clean. Arrays in my neighborhood mostly never get anything but rainwater. If array fouling rate is not asymptotic at some point, at 1%/week, an array would cease to function at some point, and altogether at 100 weeks if the fouling rate was 1%/week. Skylights don't seem to block a lot of light. Of course, it does rain. And to perhaps belabor a point, this is not a precise endeavor.

                Also to repeat, the eye is not necessarily the best tool to use to gauge how much an array's performance has deteriorated because of dirt/dust/water spots, etc. Again, qualitative info is scarce and people sometimes do no more than repeat what they hear or what seems common sense to them, but is not necessarily good/correct info. Looks can be deceiving.

                Be careful of bristles/chemicals. They may interact with a panel's ARC coating and hurt performance by increasing panel reflectance. Such actions may also cause warranty claim denials or hassles. I use a soft cloth or microfiber and a light touch.

                Thanx again for the info.

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                • #23
                  Originally posted by SanteeFJ View Post
                  I wanted to figure how often I should wash my panels since "it never rains in So Cal". I have the luxury of Enphase micro inverters so I washed half of my panels after 2 weeks of prior cleaning. What I discovered was the dirty panels had lost ~ 1.5% power over those 2 weeks. To verify, I then did a total cleaning and all panels returned to equal output. So I now feel I'm justified in washing them very two weeks. It's easy because I'm single story and can wash them from ground level.

                  I'm researching soft bristle brushes and long handles so I can do a more thorough cleaning perhaps annually.
                  fwiw, I got a long window washing tool from the cleaning supply store up the street for $80. Squeegee on one side and soft fuzzy cloth on the other. It did a pretty good job cleaning one panel with just the dew, but I haven't done any measurements.

                  You might consider a longer term experiment where you never clean one row of panels.
                  JPM's results make me think that no-clean might be an acceptable option; if each rain resets to 98% output, it's hard to justify the effort.

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                  • #24
                    Originally posted by DanKegel View Post

                    fwiw, I got a long window washing tool from the cleaning supply store up the street for $80. Squeegee on one side and soft fuzzy cloth on the other. It did a pretty good job cleaning one panel with just the dew, but I haven't done any measurements.

                    You might consider a longer term experiment where you never clean one row of panels.
                    JPM's results make me think that no-clean might be an acceptable option; if each rain resets to 98% output, it's hard to justify the effort.
                    As for rain, my (maybe) slightly better than SWAG on cleaning is : Depending on how much and how long it rains, a "decent" rain will remove about 1/2 - 2/3 or so of the fouling. Thus, a, say, 8% fouled panel will be very approx. 3% to 4% fouled after a decent rain, +/- some depending on how hard and how long it rains. Additionally, how much stuff the rain removes is also f(what the crud actually is and its' composition/particle size distribution and a bunch of other stuff.)

                    As for trying to get some quantitative estimate of how fouled an array is (or clean performance, which will be needed before you'll know how dirty it is) partly because of the small %age that fouling represents relative to the rest of the variablers involved and the uncertainty/accuracy of measuring those variables, without a pyranometer that's pretty close by, and a way to convert its output to plane of array irradiance, as well as some reasonably consistent way to determine an "average" or representative array temp., your wasting your time thinking you will be able to get a reliable quantitative result for how dirt is affecting array performance.

                    Even with a pyranometer and a way to estimate array temps. and a lot of very careful and precise/consistent procedures, corralling all the variables is a lot like herding cats. I've done it a few hundred times at my array's min. daily incidence angle. According to SWMBO, the result of having too much time on my hands.

                    Don't shoot me, I'm only the piano player.
                    Last edited by J.P.M.; 08-22-2016, 08:19 PM. Reason: corrected "after" fouled % estimate.

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                    • #25
                      Originally posted by J.P.M. View Post

                      As for rain, my (maybe) slightly better than SWAG on cleaning is : Depending on how much and how long it rains, a "decent" rain will remove about 1/2 - 2/3 or so of the fouling. Thus, a, say, 8% fouled panel will be very approx. 3% to 4% fouled after a decent rain, +/- some depending on how hard and how long it rains. Additionally, how much stuff the rain removes is also f(what the crud actually is and its' composition/particle size distribution and a bunch of other stuff.)

                      As for trying to get some quantitative estimate of how fouled an array is (or clean performance, which will be needed before you'll know how dirty it is) partly because of the small %age that fouling represents relative to the rest of the variablers involved and the uncertainty/accuracy of measuring those variables, without a pyranometer that's pretty close by, and a way to convert its output to plane of array irradiance, as well as some reasonably consistent way to determine an "average" or representative array temp., your wasting your time thinking you will be able to get a reliable quantitative result for how dirt is affecting array performance.

                      Even with a pyranometer and a way to estimate array temps. and a lot of very careful and precise/consistent procedures, corralling all the variables is a lot like herding cats. I've done it a few hundred times at my array's min. daily incidence angle. According to SWMBO, the result of having too much time on my hands.

                      Don't shoot me, I'm only the piano player.
                      If you have individual panel monitoring and they are all on the same plane, you can measure performance improvement by cleaning only a portion of the panels and do a reasonable comparison of the clean and dirty panels.

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                      • #26
                        Originally posted by SanteeFJ View Post

                        If you have individual panel monitoring and they are all on the same plane, you can measure performance improvement by cleaning only a portion of the panels and do a reasonable comparison of the clean and dirty panels.
                        Correct. I mentioned as much in a prior post - see the last paragraph of my 08/14/20106 missive posted at 1435 hrs.

                        Just get a good, clean baseline for comparisons to be able to correct/adjust for small differences between individual panels. After that, I'd respectfully suggest monitoring one cleaned and one noncleaned adjacent panel that have the closest performance and obviously the same shading characteristics, and avoid inintentional cleaning of the "dirty" panel w/H2O overspray or anything that the "clean" panel might get.

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                        • #27
                          My production went from 43 to 35 kWh per day over the last 2 months. At first I thought it was high temperatures, less sun hours or most likely increased shading, as I thought. Recently I went to the neighbor's house and looking down to my house I was shocked at how dirty the panels were. Another unexpected thing I noticed was that there was actually less shading of the south panels than 2 months ago. After reading this thread, I was sure I found the culprit.

                          The next day, what I did was hose down half of the panels for about a minute. The panels were much cleaner looking. I monitored the panels for the next 2 days and here is what I found. The production for both those days was the same as the previous few days prior to hosing, at 35 kWh. The temperature was about 5-10 degrees F hotter on those 2 days, so even though the production did not change, we can assume hosing the panels did make a small difference, which I would estimate at 1-2%, otherwise there would have been a drop in production from the higher temperature.

                          Here is the real interesting point. I did not find a statistically significant difference between the clean and dirty panels' production. I proceeded to hose down the other half, and for the last week, production has been static around 35 kWh on a clear day. I'm not sure what to make of this. The panels look much cleaner but the production did not change. My hypothesis is that there are other environmental factors not accounted for. The only thing I can think of is humidity. It has been more humid lately. I only considered clear days into my calculations and ignored days with scattered clouds.

                          Edit for the scientists: the west panels were the control group. Half of the south and half of the East side by side panels were the experiment groups.
                          Last edited by huge; 09-06-2016, 01:00 AM.

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                          • #28
                            Originally posted by huge View Post
                            My production went from 43 to 35 kWh per day over the last 2 months. At first I thought it was high temperatures, less sun hours or most likely increased shading, as I thought. Recently I went to the neighbor's house and looking down to my house I was shocked at how dirty the panels were. Another unexpected thing I noticed was that there was actually less shading of the south panels than 2 months ago. After reading this thread, I was sure I found the culprit.

                            The next day, what I did was hose down half of the panels for about a minute. The panels were much cleaner looking. I monitored the panels for the next 2 days and here is what I found. The production for both those days was the same as the previous few days prior to hosing, at 35 kWh. The temperature was about 5-10 degrees F hotter on those 2 days, so even though the production did not change, we can assume hosing the panels did make a small difference, which I would estimate at 1-2%, otherwise there would have been a drop in production from the higher temperature.

                            Here is the real interesting point. I did not find a statistically significant difference between the clean and dirty panels' production. I proceeded to hose down the other half, and for the last week, production has been static around 35 kWh on a clear day. I'm not sure what to make of this. The panels look much cleaner but the production did not change. My hypothesis is that there are other environmental factors not accounted for. The only thing I can think of is humidity. It has been more humid lately. I only considered clear days into my calculations and ignored days with scattered clouds.

                            Edit for the scientists: the west panels were the control group. Half of the south and half of the East side by side panels were the experiment groups.
                            Thank you for the info.

                            1.) Without knowing your arrangement, I'd tend to agree with your ideas on temps., less daylight and more shading, as well as the additional factor of less POA irradiance as the sun moves south would account for most of the difference in output.

                            2.) I'd also guess that 2 months of no rain would reduce output on the whole array by very ~~ 6 - 8% or so over that period, such that hosing half an array would remove, again, very approx., about 1/2 of the dirt from less than 1/2 of the array, thereby (maybe) restoring a bit less ~ 1/2 X 1/2 of the fouling caused performance decrease, or something like the 1% - 2% you SWAG.

                            3.) On how the array looks: One more time - The human eye is not necessarily a reliable tool when used to judge how much a PV array's performance is impaired by what look like a bad situation with respect to dirt. My limited experience is that it's not usually as bad a situation as it may appear, in spite of what the panel cleaning companies might want to stampede owners into.

                            None of this fouling business is as exact, precise, or as amenable to analysis as we'd all like it to be.

                            Bottom line: Hose the array about 1X/month in the early A.M. if it doesn't rain.

                            Thanx again for the info.

                            J.P.M.

                            Edit: On the humidity: That would probably not have an impact on the electronics or the panel temps. in a direct way. However, increased water vapor in the atmosphere, as well as other atmospheric constituents like dust, ozone, particulates, mixed gasses, etc. can have an effect in atmospheric transmittance and therefore how much irradiance the array sees. The eye is again, somewhat the culprit. All "clear" days are not the same.

                            Example: Over a recent 17 day period (08/04 - 08/20), around the time of min. incidence angle on my array, which happened between 1315 and 1312 hrs., P.D.T. for those dates, and when the sky was cloudless at that minute (10 days of the 17),the instantaneous GHI at my house, corrected for pyranometer temp., varied from 925 to 970 W/m^2, with the calculated POA values varying from 970 to 1025 W/m^2 respectively. The sky at all those times, on those days appeared "clear" with no clouds. The measured irradiance varied by about 5-6% or so.

                            FWIW, the estimated GHI from the HDKR clear sky model I use for those dates and at that time is about 998 W/m^2 for my location. Most days are not as clear as the model suggests.
                            Last edited by J.P.M.; 09-06-2016, 12:36 PM. Reason: Added edit.

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                            • #29
                              The panels are MUCH tougher than that. A car wash brush won't harm anything. When the snow gets deep on my panels I use a plastic roof rake to clear them off. (Not to make power... almost no sun in the winter, but because 3-4ft of snow is heavy!)

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                              • #30
                                Originally posted by nate379 View Post
                                The panels are MUCH tougher than that. A car wash brush won't harm anything. When the snow gets deep on my panels I use a plastic roof rake to clear them off. (Not to make power... almost no sun in the winter, but because 3-4ft of snow is heavy!)
                                Yea, they probably are, but I'm not sure mechanical cleaning actually gets things that much cleaner. That's part of the reason I've got data from 08/04 - 08/20 - a long story that involves comparison of consecutive daily mechanical cleaning vs. consecutive daily simple hosing. Besides, a water stream is usually easier to get up to a roof than a body.

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