Originally Posted by s/v Jedi
Ehrmm.. These are strings within a single
solar panel... In parallel, partial shading of a panel will take out the whole panel because it's voltage drops too far. In series you have plenty voltage so that only the shaded strings are lost
Hum...I still say not exactly.
First as you indicated there are bypass diodes on each series string of cells in the panel. For 12V panels, that's normally 2 for the two separate circuits. For 24v panels there are normally 3 diodes for the three separate circuits. This mainly as the higher voltage panels are larger and have one more string of cells.
the panel is parallel or series wired, the percent loss of the panel output from shading is the same. This as the panels are strings of series cells in 2 to 3 circuits.
Anyway, So lets take a three panel 17V system putting out oh 7.5 amps each. Two are in bright sunshine and one is in shade (due to boom, mast
or some dude standing in front of it).
In parallel your getting oh 17V's and 15 amps from the two panels assuming the other panel is fully shaded or shaded across both strings of cells.
In series with the one panel shaded your getting 7.5 amps but only 33.5 volts, Not 34V's from the two panels. The bypass diodes have a 0.5V forward voltage drop across them. So in series the bypass diodes when the panel is shaded place a drop across the other two panels of .5 volts. If you have two panels shaded then the third panel is only putting out 16V at 7.5 amps.
OK it's only a 3% or 6% loss of power but its a loss. So I still say Parallel is better. Also if the bypass diode fails say by excess heating
then you'll still loose a good part or most of the output of the series string.
Now the other issue is thermal runaway of the bypass diodes. If that happens the Diode fails and you'll most likely loose the whole sets output or a fair amount of it anyway when wired in series.
Now if you have a cheap
panels from oh say china
without bypass diodes and your wired in series, when one gets shaded then you pretty much loose most of the output of all the panels. I've actually read of a fellow on here a few years back, complaining that his panels were not working right...
Speaking of bad information on the web, A web site I visited had a diagram as shown at the bottom. I thing Jedi pulled a link from on another page from this site, so maybe he knows about it. The problem with the diagram is its not correct. The two diodes on the left the each panel don't daisy chain as shown on the left, but each bypasses a single set of cells in the panel.
While it mentions heating
and runaway thermal overload being a parallel problem, the problem is really a series issue. That as each bypass diode in parallel only sees the current
and voltage of that panels cell string. It does not see the other panels wired in parallel current
In a series set of panels all the watts of the unshaded string passes through the two shaded panels diodes. Which can cause thermal runaway. Thermal runaway will burn up the bypass diodes and then the whole output in series dies. So if someone has series wired panels and the out drops to near zero, you need to change out the fried bypass diodes.
Here's another link discussing parallel and serial Serial vs. Parallel - Shading
Plus while its possible for only one string of cells to be shaded in a panel, My experience is if there is shading on a boat, it generally crosses all sets of cells on a panel. Least wise more times then not from what I've seen.
Oh here is the link to the site with the bad diagram: http://www.pveducation.org/pvcdrom/m...ects-in-arrays
While is says the problems in parallel, its really showing panels in series. Plus it's showing the bypass on the non shaded panel, where the bypass would be on the shaded string of cells only which is on the right. Gee does anyone check this stuff.....
Ok time for me go spend time with my grumpy cat...