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Parallel solar panels, shading and blocking diodes?

 
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mikepepler
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Joined: 24 Nov 2005
Posts: 2868
Location: Rye, UK

PostPosted: Thu Jan 30, 2014 10:11 pm    Post subject: Parallel solar panels, shading and blocking diodes? Reply with quote

We currently have 3 parallel 250W PV panels feeding into our off-grid system. They all have identical shading (none till late afternoon, then all the same), so I've not worried about blocking diodes.

However, I've just picked up two more identical panels for a good price on ebay (new surplus stock from an installer). I can certainly get one on the roof with similar shading to the existing three, but if I add the second one then it will get some shading from early afternoon onwards from the chimney. If I only fit one, then I'll re-sell the other, so no problem if that is the best course of action.

My question is IF I do fit both, is this the time to install blocking diodes in the junction box, one per panel? From what I've read, if one panel gets shaded, the others can start shoving power backwards through it - at best causing a loss in power, and at worst overheating the shaded panel and damaging it. (the panels have *bypass* diodes built in AFAIK, but not *blocking* diodes)

The diodes are cheap, so cost isn't an issue. The loss they imply is also small - 0.3V drop, so about 1% of the power produced. I just want to make sure the system is safe and effective!

Any advice on this?
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ujoni08



Joined: 03 Oct 2009
Posts: 880
Location: Stroud Gloucestershire

PostPosted: Tue Feb 04, 2014 4:29 pm    Post subject: Reply with quote

I'd be interested in the answer to this, as I have two panels which catch mainly the morning sun, and two which catch mainly the afternoon sun. The system, which I designed and installed myself, operates through a charge controller and stores energy in two 100A-h deep cycle batteries. It all seems to be working well, with the LEDs on the charge controller shining when they should be, etc. so I can't tell if the effect you mention is present or not.
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mikepepler
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Joined: 24 Nov 2005
Posts: 2868
Location: Rye, UK

PostPosted: Thu Feb 06, 2014 2:55 pm    Post subject: Reply with quote

I've asked a colleague at work, and their view is that yes, I should add blocking diodes if panels will be shaded at different times of day. My guess is that the avoided loss in power during shading will offset the loss by using the diodes, and the bonus is I won't be worrying about melting the shaded panel!
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"Deal with reality or reality will deal with you"
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http://peplers.blogspot.com
http://peakoilupdate.blogspot.com
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