Originally posted by sensij
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Yesterday's standard type measurements as I do on my array resulted in an estimate of my array's fouling to cause a 4.9 % performance penalty compared to what my array for that day compared to what it would output if it were completely clean.
The prior day's result was a 5.5 % penalty.
The day before that was 3.94 % penalty.
Keeping in mind that results of my fouling measurements are probably good to +/- ~ 0.75 % to 1.0%, that is, if I measure fouling at, say, 3.0%, the actual fouling will probably be something like 2.00 and 4%. or thereabouts, maybe a bit tighter than for a clean array, with a confidence level somewhere between 90% and 99 % or so. More on that variation in a minute.
Once/yr., when the incidence angle on my array at the daily time of minimum incid. angle (when the cos(incid. angle) is >0.99 or so, from early April to June as it turns out), I clean my array every morning, rain or shine. The first cleaning in April being as thorough as possible w/ lots of water, soap, soft cloth brushing, rinsing, squeegeeing, wiping and windexing. That "defines" clean for me ( "As clean as is reasonably possible"). The subsequent every day cleanings at ~ 0630 hrs.are not as severe, consisting of a rinse, a brush with a soft cloth and another rinse, followed by a drip dry. As previously describes, I believe that method gets an array as clean as other more thorough methods to the limits of my method's ability to measure, which is why I believe that gets most all of the prior 24 hrs. accumulation off the array ( as I hope and define). Then, as with every other day with cloudless skies when I'm home and available, I measure and record my array's output as previously described. If I do that every year for enough days to get 35 or so sets of instantaneous results, and I keep everything in calibration as best as I can, I'll also get a dart throw on other things like annual performance degradation, but that's another topic. More slightly off topic:To be clear, for the rest of the year, no cleaning except rain is done. That's how I estimate the effects that rain has on array output.
Among the many things I measure and record, for the last set of clean measurements between 04/04 and 06/21/17 I got following results:
For array cleaned on 85 consecutive days, rain, clouds or shine.
Cloudless skies at least 2 hours (+/- 1 hr.) around time of minimum incidence angle on 40 of those 85 days. Most of those measurement days being completely cloudless.
Average array fouling based on measured output vs calculated clean output under the measured conditions : 0.0006, min.: -0.0079, max. 0.0084, population std. dev.:0.0038
That is how I define a clean array, and what the subsequent year's performance is measured against..
That negative min. fouling data is one indication of the variation or precision limits in my methods, as is the relatively high max. value of 0.0084. I also appreciate that 3 sig. figures is about all I can claim, so I carry 4 or 5 in the calcs and on the spreadsheet and truncate to 3 in the results. I show more here for information purposes only. I also report honestly, warts, embarrassments, good stuff and all, and I don't cook data - the only person I'd fool doing so is me.
Anyway, that's a roundabout way of explaining why I believe my data for fouling penalty as a % of clean performance is no better than ~ +/-0.75% to 1.0%%. Some of that variation (And I think it's a small part), may well be day/day variation caused by dry dust that comes & goes and other things I'm not measuring, or that I am measuring but not wringing/calcing as much out of all the data as I might.
And, overall, I believe Sensij's result so far at least, seems reasonable.
Now, Sensij and I are about 20-25 miles apart. I'm about 20 miles from the coast. I believe he's closer, but don't know and don't necessarily want/need to. Point is, while day/day weather at either site will vary some, one to the other, (but probably not a whole lot, at least most of the time), the climates will be, and are, believe me, quite similar, at least similar enough that long term effects of climate, long term being a couple of months or so for this discussion, will be essentially the same.
Sensij's panels have been on his roof since mid June. I'd assume they went up there new and clean. My last cleaning as was described above at was about the same time, 06/21/17. Given the closeness of the dates when arrays were known to be, or defined as clean, and given the similarity in climates and recent weather, I'd suggest that both array's were are about equally clean on 06/21. If so, and given the results I've noticed - that fouling, even on arrays very close to one another - a few hundred meters or so - can vary, and also from the variations in measurement I've observed in my method, and the possibly smaller but still likely variation in Sensij's method, I'd suggest that a difference between his latest measurement and my recent measurements of (~~0.05 - .035) = ~ 0.015 is within measurement limits for accuracy and precision. It also seems to jibe, perhaps coincidentally, with my variation of ~ +/- 0.75% to 1.0% for my method.
While I'd respectfully suggest his results will show some of the same day/day variation mine seem to show, I'd expect his fouling estimates may be tighter in terms of day/day variation because he can get results by measuring fewer variables, that is 2, and I measure and record about 6 directly, and others indirectly, adding data scatter, and also because his method measures actual values, while my method compares actual output to output calculated as clean under the same conditions using published values that I then modify 1X/yr. during my cleaning period readings. Although my calculated values are adjusted to measured, "clean" conditions once /yr., I can't and don't expect the same precision I could get if I was able to use single panel data as would be possible if I had, for example, a SolarEdge type system. I get/measure more data to estimate more array parameters that probably have reasonable/acceptable accuracy and precision. His method gets what's probably more accuracy and precision for a lot less work by measuring 1 variable for 2 different panels, and that's not a knock or praise for either method, just a description of a the difference of 2 fit-for-purpose methods.
I look forward to more results from Sensij. Maybe other PVOput users who have individual panel monitoring capability and access to easy cleaning of one panel will pick up the gauntlet I throw down here.
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