As for hard water stains blocking irradiance, as a subset of my array fouling estimates, I cleaned my panels on 2 consecutive "clear" days on a couple of separate occasions. The first day with tap water which does leave a residue, and the next day with distilled water, using a cloth to remove water drops for the distilled water cleaning which resulted in no visible residue. I was unable to detect a difference in the array's instantaneous performance from measurements taken at the 12 min. around the time of min. array incidence angle.That was done by measuring individual panel temps. as described in previous posts, measured GHI at the array, wind velocity at the array and air temp. at the array, converting the GHI to plane of array irradiance and calculating measured efficiency against theoretically clean efficiency. The difference in measured efficiency one cleaning method to the other - one with hard water spots and one without hard water spots was about .003, or 0.3%. Well below my instruments and methods precision of ~~ +/- 0.5% - 0.75% or so if I'm careful and lucky. If water spots impair performance, I can't measure it.
Not claiming perfection, but what I measured may me more than anecdotal.
Also, as it turns out, and depending on the nature of the light, and its frequency, the human eye is probably not a very good instrument to judge the transmittance or reflectance characteristics of various films with respect to solar radiation frequencies. In other words, hard water spots may very well look worse than they are with respect to how much they impair solar PV production. Sometimes things are not what they seem.
Not claiming perfection, but what I measured may me more than anecdotal.
Also, as it turns out, and depending on the nature of the light, and its frequency, the human eye is probably not a very good instrument to judge the transmittance or reflectance characteristics of various films with respect to solar radiation frequencies. In other words, hard water spots may very well look worse than they are with respect to how much they impair solar PV production. Sometimes things are not what they seem.
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