The dirt on my panels after 6+ months...

[Bostonfan49]

So, 6 plus months in and I am totally pleased with all facets of my Solar especially after getting my Credit situation cleared up with NStar now EverSourse. With a 7.23 kW system, the max daily harvest this spring/summer to date is 51.6 kW's with a total of 7 days of plus 50kW's. I kiddingly look at all the 49+ kW days and say....oh, so close! Today we are experiencing a Weather Channel high ozone alert, telling us to stay inside, danger,danger in what otherwise is a beautiful day. So I'm thinking ozone and then air polution and then I take a look at my panels and I wonder, how dirty could they be? Normal rain is supposed to keep them clean, but.....anyway, I take a a damp folded paper towel and wipe about half each of two of the lowest panels and this is what I get. Because they are on our garage I could if I had to, clean them, with some mild board approved solution early in the AM. I also realize that this dirt covering basically one panel is now condensed onto a 3"by4" piece of paper towel. The dirt, spread out over all 24 panels can't hurt the production very much....or could it? Going forward, let's say it cuts the daily production by just 1 kW, that would be over 360kW's a year. 2kW's and now I am looking at a serious drop in production!!! The panels face due south, about 1 mile unobstructed view of the ocean looking down the coast towards NY. The nearest trees are 800 plus feet away. I first thought that a swipe might show up green or brown in the form of pollen, but this must be acid rain? If true, is acid rain going to cumulatively lower my production? Thanks, Bill image.jpg

[inetdog]

So, 6 plus months in and I am totally pleased with all facets of my Solar especially after getting my Credit situation cleared up with NStar now EverSourse. With a 7.23 kW system, the max daily harvest this spring/summer to date is 51.6 kW's with a total of 7 days of plus 50kW's. I kiddingly look at all the 49+ kW days and say....oh, so close! Today we are experiencing a Weather Channel high ozone alert, telling us to stay inside, danger,danger in what otherwise is a beautiful day. So I'm thinking ozone and then air polution and then I take a look at my panels and I wonder, how dirty could they be? Normal rain is supposed to keep them clean, but.....anyway, I take a a damp folded paper towel and wipe about half each of two of the lowest panels and this is what I get. Because they are on our garage I could if I had to, clean them, with some mild board approved solution early in the AM. I also realize that this dirt covering basically one panel is now condensed onto a 3"by4" piece of paper towel. The dirt, spread out over all 24 panels can't hurt the production very much....or could it? Going forward, let's say it cuts the daily production by just 1 kW, that would be over 360kW's a year. 2kW's and now I am looking at a serious drop in production!!! The panels face due south, about 1 mile unobstructed view of the ocean looking down the coast towards NY. The nearest trees are 800 plus feet away. I first thought that a swipe might show up green or brown in the form of pollen, but this must be acid rain? If true, is acid rain going to cumulatively lower my production? Thanks, Bill [ATTACH]7212[/ATTACH]

The main thing you want to avoid is hard water stains and mineral accumulation. The dust will cause some loss by scattering back the incoming light, but since the light normally reflected back from the panel is less than 8%, the dust effect can look a lot worse than it really is.
The true test is to measure the output under identical conditions (including temperature) before and after. Hard to do without lab light sources though.

[Bostonfan49]

The main thing you want to avoid is hard water stains and mineral accumulation. The dust will cause some loss by scattering back the incoming light, but since the light normally reflected back from the panel is less than 8%, the dust effect can look a lot worse than it really is.
The true test is to measure the output under identical conditions (including temperature) before and after. Hard to do without lab light sources though.

Thanks! Our water comes from a large Artesian well and is very hard...We have a good water softener but I would want to verify the post water softness levels before using it on the panels. Not knowing didly about the nature of this black grime, giving our location but is this acid rain or just incoming air polution?
thanks,Bill

[DanKegel]

I don't know, but I think acid doesn't leave smudges, it's probably just dirt and grime.

If you do clean your panels, you probably want to squeegee them and dry them off so they don't get any hard water streaks.

[J.P.M.]

I've been a bit lax on my array fouling updates. After about a year of measuring/recording Global Horiz. Irradiance, wind vector, other meteorological variables, and measuring the temp. of each of 16 panels in 4 random places each about 12 min. on either side of the daily min. incid. angle about 70 times under no cloud conditions., and recording the inverter input and output as close to that min. incid. angle as possible.

The update in sort of abstract form:

Note: All this is for my array and location only. Other locations and methods will, in all liklihood yield different results. That doesn't make #'s right or others wrong, just different. I'd expect, for example, Ian, in Phoenix, to have a higher fouling rate and be of a different nature due to the light transmission characteristics and deposition rates of dust and sand as well as a boatload of other differences. Her Majesty, Amy, the Solar Queen in New England will be different as well, perhaps staying cleaner longer due to more precip. or higher tilt angles or other particulars.

Bottom line stuff first:

1.) As found previously and seeming to continue, without any precipitation, fouling of the array on my roof seems to increase at a rate that causes array output (performance) to decrease about 1% per week. This estimated finding seems consistent over the last year.

2.) Even though we're in a drought, long periods without any precip. are few. There may be some evidence that the rate of fouling may become noticeably asymptotic after about 8-9 weeks, but I cannot say that's more than a tease conjecture by the data at this point. The fouling rate may be asymptotic from day one, but I can't measure things that closely nor claim to.

3.) When rain events do occur, they seem to remove about 2/3 or so of the accumulated fouling since the last rain event or last cleaning without any mechanical action on my part. A slight rain seems to remove slightly less than 2/3. A heavy or consecutive days of rain seems to remove slightly more than 2/3. (Add: so, if the array is, say 6% fouled, rain will knock that back to something like 2%. The fouling will pick up from there and after 4 weeks be the residual 2% + 1%/week for 4 weeks = 6% OR SO.)


4.) I clean the array several times in succession on/around the solstices. Once or twice with regular soap/plain H2O rinse and measure the performance that day. Then do the same thing the next (clear) day (this takes some planning and waiting for consecutive clear days) using soap, D.I. water and a squeegee. Probably mostly because I can't measure things that tightly, but I have not been able to see a difference, change or improvement using the D.I/squeegee over soap/H2O and simple rinse/air dry. I doubt but can't confirm/refute that one method or the other delays fouling better than the other.

Other stuff if anyone is interested:

5.) The accuracy I ascribe to my methods are such that I'd guesstimate my measurements are accurate to perhaps +/- 0.75% for overall array fouling. That is, If my calcs est. the array fouling to be 1.0%, the best I might claim is an actual fouling of somewhere between 0.25% and 1.75%. That's an estimate, and only then if I'm lucky. That est. mostly comes from my est./SWAG of accuracy/precision of panel temp. measurement, and the Sunpower claim of ~ -0.4% eff. change /deg. C. of panel temp. change and saying I might be within about +/- 2 deg. C. on array temp. if I'm lucky.
If there is any comfort in there, to me it's that the general pattern over time and many measurements usually presents no surprises. That is, for rain free periods, the fouling from one measurement to another, sometimes over consecutive days, sometimes a week or more apart, seem to generally increase in a way that seems to be somewhat linear, or if not in a completely linear, in a way I can generally make sense of that looks like it might be linear (~ 1%/week), and drops in a seemingly consistent way after rain events.

6.) I'd estimate, as have others I've studied who do this sort of thing for a living, that the biggest uncertainty is the panel temp., with a good portion, but not all of that temp. uncertainty due to the variability of the wind vector. As one may suspect, the measured temps. of each panel are slightly different, with "upwind" or leading edge panels being cooler than downwind panels. My IR ther. claims +/- 1 deg. C. accuracy. The panel temps. can vary as much as 5-6 deg. C from one side of the array to the other, less in light wind, more in stronger wind, not much at all in still air and all over the place in gusty/variable wind. Adjacent panel temps. are usually within about 1-1.5 deg. C. of one another.
I measure each of the 16 panels in 4 random places on each panel from under the array - yes, I can get under the whole array - 64 meas. take just about 10 min. both before and after a trip to the inverter to record the display input/output power and voltages of the strings. Lots of asses and elbows for about 25 min. or so. The temp. I define and use for the array is, for better or worse, but at least consistently, the average of the 128 temps. for each efficiency measurement session.
Depending on wind, the array runs about 24-28 deg. C. warmer than roof ambient air temp. in the summer and about 18-20 deg. C.warmer than roof amb. temp. in the winter. Roof ambient air temp. runs about 3-5 deg. warmer than air temp. 6 ft. above ground level all year round.

7.) I also measure the temp. of the roof surface under the array in the same way as the panel temps - 128 times. This is done for radiation heat transfer est. for a heat/energy balance calc. on the array as a check, and, while I'm under the damn thing cooking and sweating my nads off, why the hell not. The under array roof deck temp. tends to run roughly about halfway between the array temp. and the roof ambient temp. as might be expected. The radiant sky temp. needed for the array energy bal. calcs. comes from a couple of empirical correlations that use the precip. water vapor level in the atmosphere as estimated from the dew point. I measure the irradiated roof temp. est. for the roof deck wind heat transfer coeff. and use that in the under deck heat balance of the overall array energy balance. I est. the array wind convec. heat trans. coeff. both top and bottom of array as the average of several empirical calcs. I've used for many years.

Once I give up this measurement business, I plan to hose down my array about every six weeks when it doesn't rain or 6 weeks after a decent rain. From what I think I've learned so far, that may, over time, hold the average performance penalty due to fouling for my array and location to something like about 3% or so.

[DanKegel]

Sounds like you're serious about measurement. Thanks for posting that update.

[Bostonfan49]

I don't know, but I think acid doesn't leave smudges, it's probably just dirt and grime.

If you do clean your panels, you probably want to squeegee them and dry them off so they don't get any hard water streaks.

The main thing you want to avoid is hard water stains and mineral accumulation. The dust will cause some loss by scattering back the incoming light, but since the light normally reflected back from the panel is less than 8%, the dust effect can look a lot worse than it really is.
The true test is to measure the output under identical conditions (including temperature) before and after. Hard to do without lab light sources though.

***So the dirt and grime that I am getting, is it in the form of air polution or is it polution in the rain itself? My installer does a free one year check-up on the install with the owner(s) to check the whole panel mounting and all the electrical hardware ect...I am going to ask him what he would charge to clean the panels (out of curiosity) and to get his explanation of the dirt. Short term I don't see any problem but after a couple of years I would think could affect production.
thanks, Bill

[DanKegel]

The dirt and grime are not from the rain. They seem to be normal during dry times in Los Angeles - the air is just full of dust, and near busy roads, the dust may be grimier and/or there may be more of it.

JPM said when it rains, it cleans off about two thirds of the crud on his system.

[J.P.M.]

***So the dirt and grime that I am getting, is it in the form of air polution or is it polution in the rain itself? My installer does a free one year check-up on the install with the owner(s) to check the whole panel mounting and all the electrical hardware ect...I am going to ask him what he would charge to clean the panels (out of curiosity) and to get his explanation of the dirt. Short term I don't see any problem but after a couple of years I would think could affect production.
thanks, Bill

I'd try hosing them off (in the early morning - BEFORE 8 A.M. or so solar time ) and see if it makes a diff. For very dirty panels, if the array looks cleaner after hosing, it probably is cleaner, or clean enough.

One more time: This ain't rocket science.

With commercial cleaning they will be as dirty as they started after a couple of months or so. The production penalty in $$ will probably not be as much as your cleaning bill. I'd Hose'em off.

[Bostonfan49]

The dirt and grime are not from the rain. They seem to be normal during dry times in Los Angeles - the air is just full of dust, and near busy roads, the dust may be grimier and/or there may be more of it.

JPM said when it rains, it cleans off about two thirds of the crud on his system.

I'd try hosing them off (in the early morning - BEFORE 8 A.M. or so solar time ) and see if it makes a diff. For very dirty panels, if the array looks cleaner after hosing, it probably is cleaner, or clean enough.

With commercial cleaning they will be as dirty as they started after a couple of months or so. The production penalty in $$ will probably not be as much as your cleaning bill. I'd Hose'em off.

🌞Thanks all!!! I will hose off at some point early in the AM when everything is cool. I just need to make sure that my water (after the water softener) is good to go onto the panels. Thanks again, Bill

[J.P.M.]

Thanks all!!! I will hose off at some point early in the AM when everything is cool. I just need to make sure that my water (after the water softener) is good to go onto the panels. Thanks again, Bill

For my part, you're welcome.

[Mike90250]

I use a cheap blast nozzle on the hose, and since I have soft water, no wipe needed.

You can get a Mr Clean carwash kit, and use the DI rinse filter to get a spot free rinse after the blasting.

[silversaver]

I run the softwater with this nozzle from Homedepot, works really good.

http://www.homedepot.com/p/Little-Big-Shot-Super-Nozzle-LBSR-1/202514529

[J.P.M.]

On this D.I./water spot business: Mostly opinion on my part, but based on some reading and some experience, my understanding is that the human eye is not a very good indicator of the solar radiation transmission characteristics of water spots left by minerals in water, or other such things.

Also, as I reported in the epistle/mental spoor post below from last night, my readily admitted limited investigations couldn't find a diff. or improvement using soap/D.I./squeegee ( actually distilled H2O) the day after using soap/water/air dry. I suppose if the panels "look" "nice & clean" == cleaner and better performing either by definition or wishful thinking or emotion, but things may not always be what they seem. There may well be data I've not seen that would make me change my preliminary opinion.

Extra measures probably do no harm, but I'm not sure D.I. rinse and/or other extra measure produces a measureable or longer lasting effect on array performance. To the degree that's a reflection of reality, I'd question the worth or cost effectiveness of such extra measures. The array will look spotless for a few days and that may have advantages, but it will then probably revert to the prior dusty condition, the bathtub ring at the bottom 10"- 12" or so of each panel will be back, the D.I. rinsed/squeegeed panels will look about like the hosed/air dried panels and my semi educated opinion (only), is that the array performance will be about the same using either method.

[SunEagle]

I run the softwater with this nozzle from Homedepot, works really good.

http://www.homedepot.com/p/Little-Bi...SR-1/202514529

You missed a spot at the top.

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