Wednesday, May 25, 2011

Measuring Dirt

A friend asked me if I could quantify the dirt on my solar panels from the previous post... o_O

I'd never tried to measure the effect of dirt before, but the conditions were right today to give it a go.  Not completely scientific (I am a bodger after all), but here's what I did:

Firstly, there was the pair of panels from the pictures in the previous post.  Now, they were very dirty, and I'd cleaned half of them to show the dirt.  These modules are easy to measure, as they have a chocblock in the link between the pair.  So, I crawled under them (taking the opportunity to clear out the weeds growing under them!), and inserted an Ammeter in the circuit.

I manually recorded some values for the current at 15 second intervals; before cleaning, whilst cleaning, and post cleaning.  Obviously, while cleaning the things, I didn't actually record any data in my scrap of paper, so missing values are just the last known value repeated (to fill the gap - Excel wouldn't draw a line unless every cell had a number in it...). The modules were still wired into the array, and the battery was at bulk mode, so was drawing all the current it could.

Afterwards, I took a copy of the logger file that had the time and other solar system data (like the solar sensor data that tells you how strong the sun was as a percentage of the maximum it could be), and input the manual data against the same time records.  A bit of fiddling in Excel chart maker and...

If you click on the graph, it will open up bigger.
You can see that when I washed the pair of modules (half of which had been washed yesterday for the photos), the output current jumped up.  The solar input power remained within 1% of flat the whole time, so it was definitely the effect of the dirt on the module that made the difference.  I've noted on the chart where the modules were wet, and I dried them with a cloth, and then you can see the panels output drop off a bit, as they warm up again in the sun.  PV cell power drops off a bit, as they get hot.  I used warm water to wash them, so as to prevent any thermal shock to the glass, and not cool them down too much.

Then I thought, that seeing as I'd got all set up for washing these panels, I might as well do some of the others too.  The trace below shows the power output of the TriStar array controller while I cleaned most of the panels attached to it.  This time, I just let the modules dry in the sun.

I excluded the power figures from the SunSaver array, as those panels are completely separate, and I didn't clean them.  I also ran out of water before being able to clean the bunch of panels at the front of the garage.  In truth, I couldn't be bothered to do them, and I was supposed to be doing some work anyway :D.

Again, you can see a marked improvement in output from not having a load of grunge on the array...

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