Panel cleaning

[J.P.M.]

Around my area - San Diego county, I look at PV arrays as I'm driving around the area and walking in my neighborhood. The last measureable rain at my house was 06/06/16, and that was only 0.03" - just enough to cake the dust on my panels. Many/most arrays I see are getting a nice and quite noticeable bathtub ring with the remainder of the panel getting pretty dusty looking.

From a bunch of measurements I've done and continue with, as best as I can give an educated guess, my array's performance degrades approx. somewhere between 0.75% and 1.0% per week as a result of dirt buildup on the panels if they are not washed. Without rain or washing, I believe that rate starts to get asymptotic after about 6-8 weeks or so in some way I can't yet estimate, quantify or model.

A decent rain or hosing the array down seems to restore about 1/2 to 2/3 or so of the clean array performance. So, if my array is fouled to the point of, say, a 6 % decline in performance due to dirt,
hosing it off will probably improve the array's performance by about 3 or 4 %.

A mostly south facing array around here will produce about 150 - 175 kWh/month per nameplate kW this time of year. 3% of that is ~ 5 kWh per nameplate (DC) kW. If in tier 4, or prime time T.O.U. that's ~ $0.35 or so per kWh --->>> approx. savings per installed kW ~ = $1.75/mo. So, for those with arrays that have seen no moisture in a few months, a few minutes with a hose and probably what amounts to less than 50 gal. of H2O, someone with a 5 kW array might well reduce their current electric bill by something like ($1.75) X (5) = ~ $8 or $9. per month or ~ 30 day billing period.

Caution: before you run out with a hose: ONLY HOSE AN ARRAY WHEN THE SUN IS NOT ON IT, PREFERABLY IN THE MORNING, AND NEVER AFTER ABOUT 8 A.M. AT THE LATEST.

REASON: THE PANEL GLAZING MAY CRACK FROM THERMAL SHOCK.

Just sayin'. No guarantees expressed or implied. Your mileage may vary.

Take what you want of the above. scrap the rest.

[Ian S]

We had heavy monsoon storms last week so I expect my panels were cleaned off to an extent. Still a lot of bird poop along the top edge of the highest row of panels. I still plan to install a row of bird spikes but it's just been too hot.

[Crowbar]

I read recently that dirty panels efficiency can be reduced from between 20 to 40%. Looking around online I've seen tools for cleaning the panels. One that I like is a brush that has super soft bristles, like the brush I have for washing my RV, and you can connect a hose to it so water runs through the brush and so you are gently washing your panels to remove all dirt. However, I think the catch is you need to used distilled water or de-ionized water so you don't leave any mineral deposits. Just a good rain won't wash away all the dirt and or bird poop so I think the brush idea is a good one. Do you have a solar panel cleaning service in your area that you can sign up with? maybe get your panels professionally cleaned about 3 to 4 times per year?? Obviously, if someone has invested in getting solar panels, you want them to be operating as efficiently as possible.

[bcroe]

My advice, is don't wash them unless its dark; electricity and water are very dangerous. Bruce Roe

[DanKegel]

Dew is as good as distilled water. The one time I tried, there seemed to be enough morning dew at dawn to clean a panel with a good sponge/squeegee without needing extra water. Has anyone else tried that?

[J.P.M.]

I read recently that dirty panels efficiency can be reduced from between 20 to 40%. Looking around online I've seen tools for cleaning the panels. One that I like is a brush that has super soft bristles, like the brush I have for washing my RV, and you can connect a hose to it so water runs through the brush and so you are gently washing your panels to remove all dirt. However, I think the catch is you need to used distilled water or de-ionized water so you don't leave any mineral deposits. Just a good rain won't wash away all the dirt and or bird poop so I think the brush idea is a good one. Do you have a solar panel cleaning service in your area that you can sign up with? maybe get your panels professionally cleaned about 3 to 4 times per year?? Obviously, if someone has invested in getting solar panels, you want them to be operating as efficiently as possible.

If the 20-40% claim were true, I suspect skylights would show a lot less transmittance than they do. Always possible, but my guess is more advertising hype than reality. I'd SWAG 10%. or so max.sThe folks in Death Valley, although they do not monitor input very well, do not see much of a decrease in output and they only clean their 1 MW array every 2 years. Thjey do not get a lot of rain.

There are panel cleaning services for ~ $1-$2/panel, but after 3-4 weeks the array will probably be 3% or so fouled. I hit the array with a hose and restore about 1/2 to 2/3 of the clean performance, save the money and don't break as many tiles as a cleaning service.

I cannot detect a difference from cleaning and then a distilled H2O rinse and a plain hose rinse. The hard water spots do not seem to impair performance in any way I've been able to measure. The eye is not the best discriminator of transmittance of solar radiation. There is no catch to D.I. or distilled H2O. IMO, it's not necessary or worth the hassle. I'd like to see hard numbers showing something else. More hype IMO.

[azdave]

I vote for installing one or two more panels to begin with and then staying off the roof. Far easier than regular cleanings, smashed rain gutters and wear and tear on your roof surface. Bonus is no ladders, no dragging hoses and no risk to your health.

[J.P.M.]

I vote for installing one or two more panels to begin with and then staying off the roof. Far easier than regular cleanings, smashed rain gutters and wear and tear on your roof surface. Bonus is no ladders, no dragging hoses and no risk to your health.

That's one way to do it. Opinions vary. I've suggested about 3-5 % oversizing for such opinions and live with the built in cost ineffectiveness. I'm still trying to get a better handle on how much and in what ways rain reduces the fouling and whether or not the rate of fouling becomes asymptotic and if so , in what ways and time frames.

[BakerPV]

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 100 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

[J.P.M.]

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

Thank you for the info.

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'.

[BakerPV]

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.
Kirk

[J.P.M.]

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.
Kirk

Hosing things off before the sun rises too far can't hurt.

[azdave]

Hosing things off before the sun rises too far can't hurt.

Oh but it can. Medical bills from falling off your roof or ladder can erase all solar savings for your entire lifetime. I'll wait on the rain.

[J.P.M.]

Oh but it can. Medical bills from falling off your roof or ladder can erase all solar savings for your entire lifetime. I'll wait on the rain.

But the benefit to society will be an improvement to the gene pool. Or, live in an area that gets 40-50 cc annual precip. Or, as I and others suggest, oversize the array by a few % to make up for fouling and consider the degree of added cost ineffectiveness as the insurance premium against gravity induced accidents.

[Ward L]

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.