01/13/ and 01/14/2014 were about as close to identical and cloudless as may be possible to get. I washed my array 01/14/2014 at about 0715 hrs. with a soft cloth, water and a 10ft. pole. the job took about 30 min. The array was assembled 10/02-10/03/2014. There was some mist and fog, but no rain between installation and 1st cleaning. The array was dusty w/ more dirt along the bottom 9-12 in. of the panels when cleaned. the array is 4 X 4 in landscape.
There is a Davis Instruments Pro 2 Plus weather station located about 3 ft. north of the centerline of the array about 1 ft. higher than the highest point of the array. The Davis instrument records all the usual weather variables including wind speed and also global horizontal irradiance. Weather variables are recorded at 1 min. intervals on a continuous basis. The solar array output is measured and recorded at 5 min. intervals, with linear interpolation at 1 min. intervals.
Between the hours of 0930 and 1430 P.S.T. for the 13th (dirty) the irradiance in the plane of the array was 97,201 Watts. The array output to the grid was 17,542 Watts.
Between the hours of 0930 and 1430 P.S.T. for the 14th (clean) the irradiance in the plane of the array was 97,608 Watts. The array output to the grid was 17,642 Watts.
If adjusted for ambient temp. and the effects of wind on the array which it is assumed changes the array temp. and hence output, the output on the 13th would have been about 17,465 Watts. The 14th was warmer and less windy, thus the array would likely have been a bit warmer with correspondingly lower array output if the temp. and wind profiles on the 13th had been those of the 14th.
Using the adjusted output on the 13th, the dirty system efficiency was : 17465/97201 = .1797.
Using the Unadjusted output on the 14th, the system efficiency was : 17642/97608 = .1807.
From the above I estimate that cleaning the array (I believe thoroughly) , resulted in an increase in system efficiency of approx. : (.1807/.1797) -1 = .0056, or roughly somewhere between ~~ .5% and 1.0% penalty for an array with 3+ months of dirt on it.
There's a lot more to this and anyone who wants to know more can send me a P.M. I just don't want to bore folks w/gory details.
I'm not claiming this is typical or predictive. It's simply what I got as described above. The whole issue of array "fouling" is one of those things that is not now and may never be amenable to analysis. The array next door to me may well be and probably is different in how quickly and the mechanisms by which it fouls.
I plan on doing this about every quarter or so. I'll advise results.
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Bando,
Thanks for update the orientation of your arrays. So we know they are 24 deg tilt, large array facing SW249, smaller array facing SE159. Your installer's estimation of annual power generation 18200kWh seems to be a reasonable estimate. (some think your installer under estimate your production.... for some reason.....regardless of either SAM or PVWatt the estimation is fair). Your daily output of 50kWh is super!! My 6.63KW Bosch facing SW245 with 23 tilt only producing around 24kWh daily (I have a hill above my house from SW240 all the way up to the NW which kill the late afternoon production).
I'm using the softener water and seems to does the trick. I look at my panels next day and haven't see any water spot as using hard water. It looks like the the panels love collecting dust for just one night, I'll try not to think about the dirt for while........Leave a comment:
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forgot to add that Row 1 and Row 2 show the same amount of caked on dirt. so it doesn't appear that Row 1 is making Row 2 worse.
also, the larger array faces more to the west that the backyard array, and the larger one appears to be significantly dirtier. so that probably has something to do with wind directions and patterns too ...Leave a comment:
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i'm sorry, i'm very bad with these things. all i know is that the tilt is 24 degrees and the azimuths are 159 and 249. i don't know which array is which azimuth.
it's a pretty standard pitch, nothing too crazy but nothing flat either. part of the reason we didn't utilize more of the backyard roof is because there are multiple roof "sections" with varying pitches so (1) it would look bad and (2) the lower sections of the roof don't have ideal pitches.
yes, the larger array is two rows of 18 panels across, however there is a very small gap between the two rows, the same gap that exists between each panel side by side. so condensation that runs off the top row just drips underneath, unless the water flow is great enough that it "jumps the gap" coming down (which is the case when you hose off Row 1). i don't know much about various mounting differences. we have Unirac TileTrac mounting if that helps.
it could be the area i live in is just kind of dusty - there are some newer developments within 1-2 miles in all directions of me (literally west, east, north and south), even though my neighborhood has been established for quite some time. i imagine dust and dirt can travel pretty far, esp with the occasional santa ana wind conditions. i just didn't know it until i had solar panels on there.Leave a comment:
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I'm curious on what the pitch of your roof is? Maybe if your roof is not pitched steep enough, and the rain is not hard enough and long enough, then the dirt on the top part of the panel simply gets moved to the bottom part of the panel.
Also, if I remember from your picture, you do have panels lined up in 2 rows one on top of the other, right? Do you have caked on dirt on the top row panels, too? Or just on the bottom row panels? Just curious to see whether it's a problem of too many stacked panels resulting in dirt from the top row panels accumulating at the bottom row panels, or whether it's just a panel by panel issue.Leave a comment:
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i would be very interested in seeing the results of your study! i requested access to the slideshow...
your photo also shows exactly what is happening on mine, however on mine the caked-on dirt is a straight line covering the bottom 5-6" of the panels, not just the corners. the caked on dirt just looks terrible. it has been WEEKS here without any drop of rain, and the panels are pretty dusty again. we are planning some spring cleaning ourselves with some pressure washing and window cleaning and will probably have them do the panels just for our own research's sake this time.Leave a comment:
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Flashing back to the original thread purpose
Interesting discussion both on the cleaning and the performance of various equipment.
I posted this a while back and may not be applicable do to our lack of rain but is a study done by Google
about their solar arrays and their cleaning experience. I've been MIA for a while. JPM did you ever buy panels?
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I appreciate what you are writing, perhaps more than most. I also appreciate the limitations of the paucity of data I'm hanging my hat on, but I do find it interesting. I'm a bit surprised, but only a bit, that the data from 2 systems with different equipment and orientation probably 20 miles apart, seem as consistent as they are. Still, caution that this is all little more than anecdotal, at least at this time is probably appropriate.Leave a comment:
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Very localized conditions can really change things - ask Mike the moderator about his farm location and early morning fog.Leave a comment:
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I'd respectfully suggest Bando's data may show how vendors underpredict performance more than so. CA has a lot of sun. - we do, but I'd suggest that Dec. at my house, while good, was not that far off "average" - whatever that means. the 2d half being somewhat "sunnier" than the 1st half.
My system's output for Dec., 2013 was 590 kWhrs. SAM output for the same period using TMY data was 582 kWhrs. I've got other stuff dealing with something called "clearness indices" that show about the same thing.
Comparing:
Bando Dec. = 1256 kWhrs.
J.P.M. Dec. = 590 kWhrs.
Bando/J.P.M. = 2.13
Bando Jan., mo.to.date = 910 kWhrs.
J.P.M. " " " " = 433 kWhrs.
Bando/J.P.M = 910/433 = 2.10
Bando system size = 11760 W
J.P.M. system size = 5232 W
Bando/J.P.M. = 11760/5232 = 2.25
Bando's bigger, but with different orientation and equipment. See my prior post on this thread.
I don't want to get too many irons in the fire here, or put too fine a point on it. But, to me it looks like based on recent, consecutive, nearly identical clear day measured output comparison between Bando and J.P.M. further back on this thread, the above data and SAM estimates for my system, that Bando's estimate from some vendor of 943 for Dec. was a bit low and may perhaps have been better represented by something like 1200-1250 kWhrs.
As a yearly "estimate" - SAM's estimate for my (J.P.M.) system is about 9479 kWhrs./yr. That seems high, but it's for a new system (with 1% soiling, more on that later), pretty good orientation and not too much shade. It also seems to match system output when compared to many hourly data I've taken over the last 3 months when adjusted for actual vs. TMY weather and irradiance data.
Anyway, if Bando's output really is about 2.1 times J.P.M. output, as seems possible given the above, then perhaps long term, reasonably clean, trouble free output might be something like 2.1 X 9479 =~~ 19,900 kWhrs./yr.
If someone predicted 18,200 kWhrs./yr., that estimate doesn't seem too far off - about 8 or 9% low. But still low.
At the end of the day, this is all a crap shoot anyway. The reality may very well be something like output can be guessed +/- 20% half the time on a good day, maybe. It just seems to me that many people believe the stuff that vendors often spew and get oversized systems from vendors who underpredict performance - one result being a happy customer who thinks they got more than they paid for because performance exceeds a overly conservative estimate when actually they may have paid too much for a system that is not as cost effective as it might otherwise be if properly sized.
I might still get an oversized system, and that's my choice. But I'd suggest I'd be better making that choice on data that better predicts probable, long term performance than some dour overly conservative estimate from someone with skin in the game. No one I know of ever got fired for selling too much. And, sort of analogous to the movie business which is in the end all about putting asses in seats, vendors make money putting equipment on roofs - not saving customers money per se.
Caveat emptor.Leave a comment:
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PV Watts is based on 20 year averages - more than likely it will never be correct at any given time.Leave a comment:
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Without putting words on Russ' keyboard (or mouth if he uses speech recognition), The CSI estimate is just that - an estimate, not a prediction. It is a sizing tool. No more. Actual weather for every hour, day, and year that the model uses and which controls how the array will perform will be different than the actual weather conditions that array is exposed to. Models like PV Watts and others used for sizing often use one of two "Typical Meteorological Year" data sets which are designed to be representative of "average" weather, by combining 12 months - each month being the most "typical" by some reasonably logical criteria, giving some extra attention to the solar irradiance.The expected value were exat same number from CSI calculater
[ATTACH=CONFIG]3612[/ATTACH][ATTACH=CONFIG]3613[/ATTACH]
It's probably good as used for performance estimates for sizing, but about as useless for predicting future short term array performance month to month as long range weather forecasts are for planning vacations. Over many years a well designed system sized using TMY data may well produce the expected output - short term useless.
The idea that the "expected" values on the graphs seem to be copied from PV Watts is something I would call misleading - and perhaps lead the ignorant to infer some incorrect and misleadingly wrong conclusions without some clarification, which clarification as it appears to me, seems to be missing.
The CSI data is no more than a dart throw of long term performance. To predict what a system will do in any short term based on the TMY data is a misapplication of that data. Caveat Emptor.Leave a comment:
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