There is no need for blocking diodes with parallel panels or strings subject to partial shading. As I mentioned above, a shaded panel is cooler, so the voltage drop of its equivalent diodes is higher. It turns out that the power lost due to the small current that it does shunt from parallel unshaded panels is about the same as what would be lost due to the voltage drop of blocking diodes. So it's better to omit the blocking diodes and eliminate another source of failure.T1 Terry wrote:My thoughts were to arrange the panels in 2 rows of 9 panels in parallel, series linked at one end or both ends with blocking diodes on the positive output from each panel. This way all the unshaded panels in the A parallel row will feed into the common conductor but the blocking diodes will prevent the shaded panels from draining any of the current back into themselves causing cell damage and wasting the energy generated. This then feeds into the common conductor connected to the negative of the B parallel row. The positives from the B row also have blocking diodes and feed into the common positive conductor linked to the solar controller.
Right. Which is why I figure it is better to place all the panels of a string as close together as possible (i.e. side-by-side in the same row), so that advancing shade takes out the whole string before it starts on the next one, so at least one, and often two, strings are unshaded at any given time.Looking at the image in the photo, the shade does not move squarely across the roof so a series string of 2 cells bottom to top would be effected by partial shade on the lower panel but full shade on the upper panel rendering zero output from that series string.
The controller method you have chosen to use senses the average voltage and current from all the series strings and makes a calculation based on this averaged reading, not individual reading from each series string. By using 3 panels in series the strings in full sun will call the shots so to speak, the series strings with partial shade will not have the voltage or current output to compete and their output will be effectively blocked.
It's true that the unshaded strings will call the shots voltage-wise and therefore any string with even one fully-shaded cell (and I do mean "cell", not panel) will contribute zero. But this goes just as much for strings of 2 as it does for strings of 3, except that in the former case a single shaded cell means only losing the output of 2 panels instead of 3. So that is one benefit of your suggestion.
With strings of 3, if only one panel was shaded in three out of the six strings then the MPPT could reduce the array voltage and get 2/3 of the array power instead of only half. However I admit that such a shading pattern is fantastically unlikely. So strings of 3 do not help with the shading issue as I mistakenly stated earlier. They just reduce the amount of cable required and the number of circuit breakers required (from 5 to 3).
Unfortunately I cannot physically lay the panels out in two rows of 9. But even if I could, and if I were to connect them as 9 strings of 2 with cross ties (or two parallel-blocks-of-9 in series) as you suggest, I do not see any advantage in putting one parallel block in one row and the other parallel block in the other row as the north row will be far more prone to shading than the south row, since the trees are to the northeast and northwest.
I worry that, although you know better, you may still be thinking of solar panels as voltage sources like batteries, instead of current sources with a string of (non-ideal) diodes shunting them. I know it was a hard habit for me to break.
From an earlier post of yours:
Sure, the voltage will still be there, but in the example below (where ">" means shaded), current will still be limited to that of one panel, and hence power will be limited to that of one string, whether the cross-ties are there or not. Do you agree?This way any part of any panel in the "A" paralleled group can have shade across it but the output from the unshaded panels still feeds into the parallel cabling so any of the "B" unshaded panels can utilise the input from the paralleled 24v nom. supply to produce the required 48v nom. to charge the batteries.
A B 1 +-[>]-+-[_]-+ ----| | |---- 2 +-[>]-+-[_]-+ | | | 3 +-[_]-+-[_]-+BTW It probably isn't necessary for you to quote my very long posts in their entirety for me to know what you are responding to. I worry that others may find it annoying to have to scroll through my ravings all over again.