Panel cleaning

[J.P.M.]

I have an Ambient Weather WS-1900A I did not clean the radiation sensor, but the weather station has only been up about a month. The weather station is about 40ft from the closest panel. Total generation is what was generated by my 19 panels that day as reported from Solaredge into PVOutput.

I don't think I have a way to compute the other numbers you're talking about but I do upload 5 minute interval values for irradiance from the weather station to PVOutput under my extended data if that helps at all

https://pvoutput.org/intraday.jsp?id=58288&sid=52056

Thank you.

If you're interested, to get array efficiency as expressed as (array electrical output/array irradiance input) requires the Plane Of Array (P.O.A.) irradiance. That efficiency is then reduced by the inverter efficiency to get overall system efficiency.
What most every weather station measures is Global Horizontal Irradiance (G.H.I.) the two (P.O,A. and G.H.I.) are only the same when the array is horizontal. The ratio of the two values is constantly changing.
Most arrays are tilted toward the equator to increase the P.O.A. irradiance and generate more power than horizontal arrays of the same size, orientation and location.

Unfortunately or otherwise, the algorithms to calculate P.O.A. irradiance are somewhat complicated.

However, there are other, probably more practical ways to measure array fouling and get estimates of losses in array output from fouling (dust, dirt, guano, etc.) while avoiding most of the weather related variation (because such weather variables all - or mostly all - wind up in both the numerator and denominator of all the efficiency equations and so cancel out).

One such way that works - because you have individual panel monitoring - is to find two adjacent panels in the same array that have the closest output to one another of all the pairs of panels with at least one of them also easy to get at. Then, clean the more easily accessible panel with dish soap and tap water (but - and this is important - clean it in the morning before the sun hits the array) and wipe it reasonably dry. Then, check the output of the two panels during and/or at the end of the day and subtract the (ratio of the dirty panel's output to the clean(ed) panel's output) from 1.0 for an estimate % of performance (output) lost to dirt on the array.

If you're really interested, and since array fouling is somewhat dependent on array orientation (as well as geographical and other factors) you can do the same procedure for each array.

You can get instantaneous fouling or daily or any period overall fouling and rates of fouling by time period as you desire, but most folks probably aren't that concerned about it.

FWIW, doing it the hard way, I feel safe in saying that, if it doesn't rain (in zip 92026) my array fouls at an approx. rate such that the output decreases approx. 0.6% to maybe 0.75% per week or so with the accumulated performance loss leveling off at something like about 6% or maybe 8%. I haven't been able to come up with a working theory for the leveling off of fouling but that's what I've come up with after measuring instantaneous output over 500 times on very clear days at the minute of minimum angle of beam incidence on my array in both clean and fouled conditions for the entire array over the last 8+ years. Those numbers seem to be in some general agreement with stuff on array fouling that can eb found in the open literature.

Some validity or perhaps anecdotal or practical confirmation to the leveling off of the fouling may also be found by looking at a skylight in my roof that doesn't seem to have all or even a significant portion of its light blocked even after months of no rain which, as you know, is common in So.CA from spring until late fall.

[jasonvr]

Thank you.
What type of weather station do you have ?
How far is the weather station from the array ?
Do you clean the radiation sensor when you clean the panels ?

I'd assume what you report here under Total Generation is the time integrated daily G.H.I. from the instrument.
Do you have a way to calculate or measure the time integrated daily P.O.A. irradiance ?

As someone who has done similar to what you've presented here only several hundred times more, this is not a knock. I appreciate what you've done. I also have a weather station. It's a Davis Pro II plus located about 1.2 m north of my array's N-S centerline at the same approx. elevation, this is not a knock.

It seems to me after looking at your data that it's a good example of how difficult it is to get any quantitative estimates of panel fouling or cleanliness, especially with only a few days of data. The 3 biggest variables, irradiance (especially P.O.A. irradiance), ambient temp. and wind velocity are (or can be) of about the same magnitude in terms of how they influence array output both instantaneously and over a day as does array cleanliness.

Thanks again.

I have an Ambient Weather WS-1900A I did not clean the radiation sensor, but the weather station has only been up about a month. The weather station is about 40ft from the closest panel. Total generation is what was generated by my 19 panels that day as reported from Solaredge into PVOutput.

I don't think I have a way to compute the other numbers you're talking about but I do upload 5 minute interval values for irradiance from the weather station to PVOutput under my extended data if that helps at all

https://pvoutput.org/intraday.jsp?id=58288&sid=52056

[J.P.M.]

I do now have a local weather station on my roof with a solar radiation sensor. Some readings, including temp

Date	Max Solar Radiation	Max Temp	Total Generation	Peak Generation	Weather Notes
8/18	764.830W/m2	87.980°F	34.888kWh	4,711W
8/19	752.368W/m2	87.260°F	34.970kWh	4,638W
8/20	756.101W/m2	83.300°F	37.072kWh	4,963W
8/21	797.632W/m2	83.120°F	28.060kWh	5,628W	Cloudy until after 1pm
8/22	748.043W/m2	87.440°F	32.945kWh	4,950W	Cloudy until 11am
8/23	749.834W/m2	88.160°F	36.524kWh	4,934W

But yes, panels cleaned early morning 8/20

Thank you.
What type of weather station do you have ?
How far is the weather station from the array ?
Do you clean the radiation sensor when you clean the panels ?

I'd assume what you report here under Total Generation is the time integrated daily G.H.I. from the instrument.
Do you have a way to calculate or measure the time integrated daily P.O.A. irradiance ?

As someone who has done similar to what you've presented here only several hundred times more, this is not a knock. I appreciate what you've done. I also have a weather station. It's a Davis Pro II plus located about 1.2 m north of my array's N-S centerline at the same approx. elevation, this is not a knock.

It seems to me after looking at your data that it's a good example of how difficult it is to get any quantitative estimates of panel fouling or cleanliness, especially with only a few days of data. The 3 biggest variables, irradiance (especially P.O.A. irradiance), ambient temp. and wind velocity are (or can be) of about the same magnitude in terms of how they influence array output both instantaneously and over a day as does array cleanliness.

Thanks again.

[jasonvr]

Nothing firm without more weather information including P.O.A. irradiance to be able to filter out/account for how the weather variables changed over the same time period.
But if I had to guess, I'd SWAG a cleaning took place on the 20th.

How did the daylong outputs compare before and after cleaning ?
Also, how did the daily outputs compare with the daylight ambient air temps. for the same days ?

I do now have a local weather station on my roof with a solar radiation sensor. Some readings, including temp

Date	Max Solar Radiation	Max Temp	Total Generation	Peak Generation	Weather Notes
8/18	764.830W/m2	87.980°F	34.888kWh	4,711W
8/19	752.368W/m2	87.260°F	34.970kWh	4,638W
8/20	756.101W/m2	83.300°F	37.072kWh	4,963W
8/21	797.632W/m2	83.120°F	28.060kWh	5,628W	Cloudy until after 1pm
8/22	748.043W/m2	87.440°F	32.945kWh	4,950W	Cloudy until 11am
8/23	749.834W/m2	88.160°F	36.524kWh	4,934W

But yes, panels cleaned early morning 8/20

[J.P.M.]

Another panel cleaning and another nice improvement day over day. Can you tell which day I cleaned on?

PanelCleaning-08-2022.jpg

Nothing firm without more weather information including P.O.A. irradiance to be able to filter out/account for how the weather variables changed over the same time period.
But if I had to guess, I'd SWAG a cleaning took place on the 20th.

How did the daylong outputs compare before and after cleaning ?
Also, how did the daily outputs compare with the daylight ambient air temps. for the same days ?

[jasonvr]

Another panel cleaning and another nice improvement day over day. Can you tell which day I cleaned on?

PanelCleaning-08-2022.jpg

[jasonvr]

I recently purchased a pole mounted brush for my panels and have used it twice. The first time was just with water and I didn't see a huge improvement. However, that was after about a year of not cleaning with any mechanical means (I had done a hose spray a couple of times). I think I incurred a pretty significant penalty due to ash from local brush fires, one of which caused me to evacuate (side note, monitoring my solar production remotely let me know that my house had not burned down). When looking at the panels after they dried I could definitely see that the panels were still dirty.

My latest clean was a couple of weeks ago and I dragged a bucket with some Dawn up on the roof with me and used that to scrub the panels down and did see a decent increase in production as a result. It looks like my peak production went up about 250W with very similar solar radiation readings. Total production went up a little over 2kWh on the day of cleaning (on my 6.27kW system). The panels looked much cleaner and definitely had a shinier appearance as compared to using just water.

Roughly a year later and a pretty similar result on my array. I have cleaned it once or twice in between then and now, but I just cleaned my array this morning and saw a peak production increase of about 200W (5 minute value from SE to PVOutput) and a total production increase from the previous day of about 1.8kWh on my 6.27kW system.

Basically, I periodically look up at my array and if it starts to looks pretty non-reflective, I choose a weekend morning to climb up on the roof with the brush and bucket before the sun really starts shining. Takes less than 45 minutes to finish. I end up doing this 2-3 times per year. Being in SoCal, I don't get a lot of rain for most of the year to keep the panels "naturally" clean. This time around I did discover there is a little squeegee on the back of the brush that lets me get some of the small, hard stuff off as well since it's a pretty soft brush.

[J.P.M.]

Here is the numbers (kWh) -
Sunday: 113.4 -
Monday: 113.0
Tuesday: 112.8
Wed: 113.6
Thurs: 123,7 Cleaned in the morning. More high cirrus clouds in the afternoon then the prior days.

Temps did drop a bit, Thurs was 3F cooler then Wed, and Wed was 7F cooler than Tuesday. Sunday was the hottest day but skies were a tad clearer with minimal high cirrus clouds.

A tad under 9% (and 3F cooler temps). So I would say that is a worst case of fouling I would ever expect - ash from two large fires combined with about 4 months of dust/pollen/bird droppings/pine needles (minimal) and what looks like very small caked on spots of dried sap. Most of it came off from the simply spray, I'll look at it more closely this weekend in the early morning.

What this tells me is after this unusual ashy summer it was a good use of time to hose them off, and after future fires it may be worth the time to hit them once the air clears. Previously (prior to the broken water line) I was trying to hit them one or twice every two months but did not sweat it since we were in rainy season.

Thank you. Unless you have more instrumentation, I wouldn't bother getting more involved. Panel efficiency calcs as f(cell temps.) are primarily driven by 3 things, Panel POA irradiance, wind vector and ambient air temp. at the array. Without a pyranometer and ways to estimate wind vector and air temp., the instantaneous efficiencies much less lay long efficiency will be a SAWG. Qualitative conclusions such as you've done are about as good as you can expect and are probably at least good enough for what you want to know which sounds like "when/how often do I want to clean the array, and how involved do I want/need that maintenance to be.

Respectful suggestion: Hose the array ~ 1X/month or so if it doesn't rain. Get the really big chunks of guano off the array if/as you think necessary and then use ~ 3/4 gal. of H2O per panel. Doing so probably holds my fouling levels to ~ 3% performance penalty. Use it as a guide or something to consider. This is not an exact science.

Don't bother with D.I. /special rinse/*chemicals. I've measured before/after cleanings a lot, and a good part of those data with/without getting ALL the hard water/mineral spotting off the array. If a benefit exits from getting my array spot free vs. normal (for my situation) hardwater spotting, I've not been able to measure it.

[tyab]

Here is the numbers (kWh) -
Sunday: 113.4 -
Monday: 113.0
Tuesday: 112.8
Wed: 113.6
Thurs: 123,7 Cleaned in the morning. More high cirrus clouds in the afternoon then the prior days.

Temps did drop a bit, Thurs was 3F cooler then Wed, and Wed was 7F cooler than Tuesday. Sunday was the hottest day but skies were a tad clearer with minimal high cirrus clouds.

A tad under 9% (and 3F cooler temps). So I would say that is a worst case of fouling I would ever expect - ash from two large fires combined with about 4 months of dust/pollen/bird droppings/pine needles (minimal) and what looks like very small caked on spots of dried sap. Most of it came off from the simply spray, I'll look at it more closely this weekend in the early morning.

What this tells me is after this unusual ashy summer it was a good use of time to hose them off, and after future fires it may be worth the time to hit them once the air clears. Previously (prior to the broken water line) I was trying to hit them one or twice every two months but did not sweat it since we were in rainy season.

[J.P.M.]

JPM has done lots of write-ups on this topic and my case is more of a worst case example. The water line that I ran the 400' down to the solar system is also on a water line used for irrigation and I had a cracked pipe near the pump house that force me to turn off that water line after the last snow in early May. So starting in mid May I have not hosed down the four arrays. No rain since early May, and we had two massive fires close enough to cover everything with ash - the Ferguson fire was the worst. Low humidity and no dew left me with an amazing layer of gunk on the panels. I finally fixed that water line and just this morning hosed it all down. Wish I had my phone with me - what an amazing video of brown water it would have been. No scrubbing - just hosing. Really want to be careful with that ash - it does a number on car paint if you rub it before getting the majority gone from a water spray so I'm just hosing only.

Initial numbers for today compared to Mon-Wed with same clear skies and similar temps is just a tad under 9.5% higher. Today is the same temp as Wed but a bit cooler than Mon and Tues. When the day is done I will see where I end up. I really can't imagine those panels being more dirty than they were, so this seems to be in line with the lower numbers JPM has so carefully reported. At least for my system, I feel that I can safely say that dust/ash/pollen gunk is under 10% in a worst case and matches the under 5% ish I has seen previous when more regular hosing.

Thank you for the info.

IMO, you did the right thing by hosing as much of the fouling layer(s) off before any abrasion or rubbing.

Just to reiterate, never get an array wet when the array is warmer than the air temp. or when the difference between the panel temp. and the water temp. is more than maybe 20 F. or so.

Don't hit a hot panel with cold water. (or a cold panel with hot water)

As for how much your performance suffered from fouling, every application/location will most likely be different, even from the one next door. That the ~ 10% figure you report seems in line with what I might SWAG seems reasonable makes little or no difference to reality, but still that does seem in line with my experience, particularly if some asymptotic character of the rate of increase as f(time) has any relevance to your situation/location.

One thing I've noticed with my array: dew points close to the ambient temp. can be either good or bad for my array's fouling. If the array is very clean, a heavy dew, or marine layer that causes a lot of condensation to from on the array will tend to do little that will affect performance, maybe a bit to the good. However, if the array is less than clean, I believe a general characteristic I've found is that a high dew point or air that's close to saturation will most often tend to cause any dust/stuff on the array to mud up and then tend to run down the panels (somewhat) with the result that a bathtub ring forms on the bottom 12" or so of each row on panels, and what doesn't run down the panels tends to stay put and cake up all the way up the panel, blocking sunlite (more fouling_) and also acting as a wick that traps more moisture and more dust.

I'm still hanging my hat on an approx. fouling rate that results in a 1%/week deterioration in output over the clean condition for my array. Yours and other arrays will behave differently, but I' not be surprised if that 1%/week number was pretty common for suburb/rural locations.

[tyab]

JPM has done lots of write-ups on this topic and my case is more of a worst case example. The water line that I ran the 400' down to the solar system is also on a water line used for irrigation and I had a cracked pipe near the pump house that force me to turn off that water line after the last snow in early May. So starting in mid May I have not hosed down the four arrays. No rain since early May, and we had two massive fires close enough to cover everything with ash - the Ferguson fire was the worst. Low humidity and no dew left me with an amazing layer of gunk on the panels. I finally fixed that water line and just this morning hosed it all down. Wish I had my phone with me - what an amazing video of brown water it would have been. No scrubbing - just hosing. Really want to be careful with that ash - it does a number on car paint if you rub it before getting the majority gone from a water spray so I'm just hosing only.

Initial numbers for today compared to Mon-Wed with same clear skies and similar temps is just a tad under 9.5% higher. Today is the same temp as Wed but a bit cooler than Mon and Tues. When the day is done I will see where I end up. I really can't imagine those panels being more dirty than they were, so this seems to be in line with the lower numbers JPM has so carefully reported. At least for my system, I feel that I can safely say that dust/ash/pollen gunk is under 10% in a worst case and matches the under 5% ish I has seen previous when more regular hosing.

[J.P.M.]

I'm not 100% sure on the irradiance since I don't own the weather station, I just know it is close to my home. My gut says they have some shading issues in the morning as evidenced by the sharp increase of the light green line each morning.

I have two orientations on my panels. Site plan is below. The glitches in my production (dark green line) in the morning are due to cloudiness/marine layer burning off.

SitePlan.JPG

Thank you. All this is a bit off topic, but if those irradiance measurements are from some instrument that uses conventional reporting methods, then - and nothing to do with you other than a caution on using them for anything - they're screwed up. Irradiance is usually reported on a horizontal plane (GHI, or Global Horizontal Irradiance) and then, if irradiance in another orientation (such as the plane of the array, or P.O.A. irradiance), that's conventionally found by available and mostly agreed upon methods.

Around So.CA on those dates the solar noon GHI was running ~ 850 to maybe 920 W/m^2 or so, and not as flat a curve as shown and not apparently as shaded as shown. Mine Davis instrument measured 907 W/m^2 on 08/16 and 904 W/m^2 on 08/17 at solar noon on those days (12:52 P.M, P.D.T. both days @ my location). Hence my questioning of the graph numbers, but not your output numbers.

[jasonvr]

I'm not 100% sure on the irradiance since I don't own the weather station, I just know it is close to my home. My gut says they have some shading issues in the morning as evidenced by the sharp increase of the light green line each morning.

I have two orientations on my panels. Site plan is below. The glitches in my production (dark green line) in the morning are due to cloudiness/marine layer burning off.

SitePlan.JPG

[J.P.M.]

Reading days were consecutive - Aug 16 and Aug 17 (panels cleaned this day, early in the morning before any significant sun). Irradiance was determined based on a local weather station reporting through Weather Underground. It's not perfect, but is a decent approximation. Peak production is based on the 5 minute values reported from SolarEdge thru PVO

August 16 - 1:10pm
Peak production ~4640 W
Peak irradiance ~464 W/m2
Aug16.JPG


August 17 - 1:15pm
Peak production ~4930 W
Peak irradiance ~469 W/m2
Aug17.JPG

Week
Week.JPG

Thank you. Looks like you have a west of south orientation and some morning shading ? Aside from the idea that they seem to confirm that cleaning improved output significantly, I'm not sure I understand what those two top graphs are saying. Unless you've got a somewhat northerly array orientation, the irradiance numbers seem quite low. Are they POA irradiance numbers or what ?

[jasonvr]

Reading days were consecutive - Aug 16 and Aug 17 (panels cleaned this day, early in the morning before any significant sun). Irradiance was determined based on a local weather station reporting through Weather Underground. It's not perfect, but is a decent approximation. Peak production is based on the 5 minute values reported from SolarEdge thru PVO

August 16 - 1:10pm
Peak production ~4640 W
Peak irradiance ~464 W/m2
Aug16.JPG


August 17 - 1:15pm
Peak production ~4930 W
Peak irradiance ~469 W/m2
Aug17.JPG

Week
Week.JPG