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.
Panel cleaning
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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.
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.Leave a comment:
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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.
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.Leave a comment:
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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.Leave a comment:
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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, 12:28 AM.Leave a comment:
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There is a lot of variation of people's experience to make me suspect that the kind of panels also plays a part.Leave a comment:
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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.
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, 06:44 PM.Leave a comment:
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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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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.
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.Leave a comment:
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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.
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Update, 8:00 p.m. Today's total is 27.86 kWh. Much better than yesterday's 24.369 kWh and better than the previous highest August day so far this year of 26.397 kWh. Methinks washing the panels did some real good. As for the weather, it has been dry and hot this month so far with quite a few days over 100 degrees. Today peaked at 106 here in Bakersfield according the NWS and yesterday was "only" 101, so all things being equal, today's power generation should have been a bit less than yesterday.
KirkLeave a comment:
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Update, 8:00 p.m. Today's total is 27.86 kWh. Much better than yesterday's 24.369 kWh and better than the previous highest August day so far this year of 26.397 kWh. Methinks washing the panels did some real good. As for the weather, it has been dry and hot this month so far with quite a few days over 100 degrees. Today peaked at 106 here in Bakersfield according the NWS and yesterday was "only" 101, so all things being equal, today's power generation should have been a bit less than yesterday.
KirkLeave a comment:
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I washed my panels last evening before it got totally dark using Windex Outdoor Window Cleaner (the kind that screws onto the end of a hose) and my output at peak today was about 11% higher than yesterday (same weather conditions). It'll be interesting to see the total kWh I get at the end of today compared to yesterday. My system is fourteen 327 watt SunPower SPR-327NE panels and a SolarEdge SE5000, facing 110 degrees (East Southeast). They were installed at the end of April and have gotten some rain maybe twice. We have new homes going up near us and there is quite a bit of blowing dust from the empty lots. The panels had a very noticeable layer of dirt on them, and some spots of bird poop, and now they are actually blue/black again. I was thinking the lower output I was noticing over "just-installed" was due to the high temperatures we've been having lately (100+ degrees), but now that the panels are clean, I'm thinking dirt was the main culprit. Since I only have to stand on a ladder to get above a patio awning, and don't have to get on the roof, I believe I'll be hosing them off every other month or so now. I checked with SunPower before using the Windex and they said it was fine.
Kirk
I've no problem believing the # you quote, but without an on site weather station (or even with one for that matter), its hard to separate weather effects from dirt. I'm SWAGing ~~ 0.75% - 1.0% performance degradation/week for my location with no rain, and I suspect that rate may become noticeably asymptotic at some point beyond 6-8 moisture free weeks, but I've got no data to support that suspicion. Your #'s seem to sort of agree on the rainless rate. This fouling business is far from an exact and predictable sort of thing anyway.
FWIW, I'm in the 10th day of a 13 day series of very thorough, daily (early A.M.) array cleanings, with instantaneous measurements/recording of irradiance and weather data, and important array parameters taken at solar noon on each day at the inverter. I do this around April 25 or so and also around August 11 or so when the incidence angle is closest to zero on the array and check performance degradation when glazing reflectance is probably at a minimum and approximating something perhaps close to STC incidence angle. I adjust for (average) array temp. as calculated using comparison of array voltage with measured array temps. done in the past, calc the POA irradiance using a consistent HDKR model, throw out the high and low resultant calculated % fouling number, average the rest, and call that the clean array performance. That ave. calculated fouling (assuming my 13 consecutive days of thorough cleaning gets things about as clean as they're likely to get) then becoming the cumulative performance degradation of the system. This year, I intend no cleaning until the winter solstice, but will continue to monitor instantaneous output in the usual fashion on very clear days as I've been doing for over two years.
One advantage of a micro inverter system for me might have been that it would be a whole lot easier to get a number for array fouling by daily or somewhat regular cleaning of just one panel. Too soon old. Too late smart. Just sayin'.Leave a comment:
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