I read in another thread that a person had used diodes to reduce the effect of shadows on a PV array if I understood correctly. I am currently installing six 235w panels on the roof of a 43 ft motor coach. they are in 3 sets of 2 panels in series running at about 60 volts to the Classic 150. Four of the panels are pretty much away from any other roof 'stuff' that could cast shadows. One set of 2 panels is less lucky and will get more shadows from roof vents and the corner of an air conditioner. They will still be fine most of the time but I ask what I can do to minimize the effect of small shadows on parts of these 2 panels. Will the other four panels be effected when the shadows hit the 2 susceptible panels? Thanks in advance 8)
Ted N Ruthy
Classic 150
6 - 235W panels
WhizBang when installed soon
Possibly it was my thread you read, so I am not sure if I should reply here to, maybe one of the real experts should chip in for real knowledge.
But I tell you my reasoning for putting diodes:
If you put two panels (or two strings) in parallel, and one of them get only a little shadow, will drop its voltage. Not only will that reduce the panels own output, but it is even possible that the non-shaded panel starts to flow its energy back into the shaded panel. Not only loosing even more energy, but possibly even heating up and damaging the shaded panel.
Terminating the strings with diodes would prevent the flow-back of current from one unshaded panel into a shaded panel. The shaded panel would barely contribute, but not hold back the other panels.
More ideal for this setup would be a micro-optimizer per panel, and I am still considering upgrading to that, but last year the brands I found were not compatible. They had to use their own end-stage inverters, which were always grid tied. One Israely brand promised they were working on a optimizer model that could be hooked up to a generic inverter like the Classic
That is my understanding as well. You should note also that all high end produced panel have diodes in some configuration to serve this purpose to some extent if not entirely and to prevent the flow of electricity from batteries when there isn't light.However, you definitely need an expert to advise you on this before you proceed .Don't want to damage or shorten the life of those pricey components.Sometimes you can do more harm than good or, in some cases it can just be a waste of time and money.Check with the experts.
Actually, what I found is that the integrated diodes are for a different but related purpose: They are to bypass groups of shaded cells in a forward direction, but they are not to block backflow into the panel. Both are anti-shading items, but for different setups:
Bypass diodes: allow forward current to bypass a shaded cell inside a string. Most panels have 3 to 6 bypass diodes integrated in the panel.
Blocking diodes: prevent backflow of current from one string into a parallel string. I did not find any panels with blocking diodes integrated.
See also here
http://www.solar-facts.com/panels/panel-diodes.php
or look for blocking and bypass diodes elsewhere on google.
You do not need to add series blocking diodes to a string.
The only reason you might want to do that is if you have one shorted PV module. But that is what
a PV combiner box is for (breakers). And even then, you do not need breakers in the combiner
unless there are 3 or more strings of PV in parallel.
Series diodes have absolutely no effect on partial shading performance. That is what the
bypass diodes are for and are always built into any PV modules that are worth using
in this industry. Some of the older modules had a single diode bypassing the entire
module and that works fine for that single module.
As long as there aren't any "shorted" modules, (extremely rare), and the modules all are rated for
the same Voc, you will not have any backfed current and even if you did, it would only be at times
of open circuit (Voc) and would completely go away as soon as the inverter or controller draws
current from the array.
boB
Yes that's is about it nice info Cybermaus
Well there you had it from an expert. i did recall that it would be somewhat useless to put those diodes for your purpose but was not entirely sure.
Hmm, then I did a lot of wasted effort building a nice diode junction box with passive cooling.
On the other hand, it will be quite easy for me to test. Once of these days, when I have a nice blue sky with no random clouds I will hook up a few ampmeters to bypass the diodes and see what happens if I shade one. If indeed I find boB is correct, I will permanently bypass the diodes. Pity, but it would mean 1Volt less drop.
Ok, tested and I have to confirm boB's statement:
If you have healthy and matched panels, blocking diodes are not useful.
When I bypassed a diode and shaded its panel, I got no backflow at Vmp and only 50mA at Voc. It might be a tiny bit more if all diodes would have been bypassed, but not significant.
So I now have 6 useless diodes consuming 0.75V drop. That's 2.1%.
Edit: PS: while I got no backflow at Vmp, I also had zero forward flow. So at least the idea to make strings of 1 was good. Otherwise there would have been two panels not contributing.
Thank you all very much for your inputs. I will not be concerned with the small shadows I get on one of my 3 strings of panels when the sun is low in the sky and the air conditioners or a adjacent panel casts a shadow on one of the two panels in this string. The old coach had four of these panels installed with little to no shading concerns. The new coach was 5 ft longer and I added these two new panels but could not completely eliminate the shading at low sun angles. Thanks again. Ted