Would like to know if I have the following correct.perhaps arrange the strings to match the shading.
Portrait orientation will cost you more in racking costs for ground mount.Would like to know if I have the following correct.
With a rectangular array of panels, and in the afternoon a sharp N-S shading line moving across the array from west to east, then I understand the relevant issues with a string inverter are:
1) It will be marginally more efficient to use "portrait" orientation. A panel in this orientation has 6 columns of cells and uses 1 bypass diode per 2 columns. So as part of the western column get shaded, the first two columns will drop out, leaving the panel at 2/3 power output, etc. While in landscape orientation, with 6 rows, when the shade first hits the panel the whole panel will drop out.
2) If you have one series string, the power output will step down in increments per the above until the number of unshaded (one-third) panels is too low to meet the string inverter's minimum DC input. No significant efficiency gain for optimizers/microinverters.
3) If you have multiple series strings, each one on a separate MPPT input to the inverter, then it's the same situation as (2), just in parallel. With respect to the minimum DC input level, I think it might be a little more efficient to spread the shading evenly across all strings, rather than the opposite, but I'm not certain. [The idea being that by the time all the strings are shaded enough to cause them all to drop out, the sun angle of incidence will be lower, so the power lost is less. Of course, if late afternoon power is more valuable, doing the opposite would be better.]
4) If you have multiple series strings and need to parallel two of them on the same MPPT input, it's important they have identical shading vs time profiles. Paralleling a string with all panels on the west edge of the array with a string with all panels on the east edge of the array would perform poorly (haven't thought out the exact behavior, but the shaded string would preclude the insolated string from producing.)
I obsessed about this for a while when stringing my array. In my case, I have some early morning and late day shading and it's not a clean north south shading line on the array, plus it's different for the morning shading and the evening shading. There was no perfect way to do it. If you have a clear view of each horizon, then I think yes stringing in north-south lines would be the way to go. Either way, I think you are looking at quite small gains.So is that a yes, my analysis about how a hard N-S shadow line moving west to east across a small array affects string inverters is correct? No real upside to microinverters/optimizers, assuming you properly divide the array into strings for the string inverter?
If by that you mean some strings would have more of the western panels, and some would have more of the eastern panels, I would think that would only make sense if each string is on a separate input on the inverter. Otherwise, if you need to parallel strings on one input, you'd want equivalent shading on each paralleled string, so for an E-W moving shadow line, you'd do the opposite.If you have a clear view of each horizon, then I think yes stringing in north-south lines would be the way to go.
Yes assume there is no paralleling of strings. Paralleling is actually rarely done in residential systems. I guess the exception would be if one used one of the large fronius inverters (15kw) that only have two MPPT's. If using an SMA, their largest is 7.7 kw and has three. Of course this is assuming the philosophy that if you have the MPPTs then you might as well use them.If by that you mean some strings would have more of the western panels, and some would have more of the eastern panels, I would think that would only make sense if each string is on a separate input on the inverter. Otherwise, if you need to parallel strings on one input, you'd want equivalent shading on each paralleled string, so for an E-W moving shadow line, you'd do the opposite.
Actually I really haven't found that to be the case. I've done three systems in the 700-800 foot range recently and running strings back wasn't worth it over just running large aluminum conductors and bringing a 230.40 exception 3 service out to the array. Even priced it with fronius inverters and their 1000v strings. Still not worth it.Its almost always better to run long distances with 600Vdc than it is 240Vac. If you don't have the ability to put a string inverter outside, you'll need to add a rapid shutdown disconnect to the DC feeders entering the house.