upper and lower panels on different MPPT

[electrofelon]

Say you have a large ground mount, at least a few 100 KW, where each row has an upper and a lower row of panels - Super common. Has anyone seen or done an analysis on the gains of segregating the upper and lowers onto different MPPTs? We have worked for one solar developer who spec'ed segregation, and another who didnt care....Just curious on what others think of this.

 

[GoldDigger]

Say you have a large ground mount, at least a few 100 KW, where each row has an upper and a lower row of panels - Super common. Has anyone seen or done an analysis on the gains of segregating the upper and lowers onto different MPPTs? We have worked for one solar developer who spec'ed segregation, and another who didnt care....Just curious on what others think of this.

Unless one row shades the other at times or there are significant differences in environmental partial shading I do not see any general reason to separate them.

 

[electrofelon]

Unless one row shades the other at times

Thats it. I always see shading at the end and beginning of the day. We always seem to work on these oct-march when the shading would be more pronounced though. Im sure its minor due to the time of day and time of year, but the question is how minor.

P.S. I should be more specific and say I am talking about 1-2 Meg systems where there are lots of rows and it isnt worth the extra real estate to have zero shading.

 

[ggunn]

Thats it. I always see shading at the end and beginning of the day. We always seem to work on these oct-march when the shading would be more pronounced though. Im sure its minor due to the time of day and time of year, but the question is how minor.

P.S. I should be more specific and say I am talking about 1-2 Meg systems where there are lots of rows and it isn't worth the extra real estate to have zero shading.

Separate MPPT's isn't as important as stringing so that lower modules aren't in the same string as upper ones.

 

[GoldDigger]

Separate MPPT's isn't as important as stringing so that lower modules aren't in the same string as upper ones.

That would depend in part on the details of the partial shading with respect to the bypass group structure of the panels.

If it reduces the available current from all modules equally, but leaves it significant in terms of production, then same MPPT input should be fine.
But if it totally shades one or more cells, dropping the string down to 2/3 or 3/4 voltage because the bypass diodes are activated, then putting the shaded string on a separate MPPT input would allow some production as long as the bypassed string voltage did not go below the minimum input voltage of the inverter.

You can also argue that if the shading is at extremely early and late hours only the amount of production affected will not be really important.
The counter argument is that every little bit of power has a value and the cost of wiring differently would be minimal. If you would have to use different inverters to get the needed MPPT inputs, then the cost calculation is more interesting.

 

[electrofelon]

Separate MPPT's isn't as important as stringing so that lower modules aren't in the same string as upper ones.

I concur. All the systems I have done are strung across a single row. This also allows for "skip stringing" which uses less wire and keeps the ends in a common location. The site we are on now, they not only didn't require uppers and lower to be on seperate mppts, they had upper and lower panels on the same string. We changed that.

 

[BillK-AZ]

A detailed study in 2011 of an older (1986) 192 kW, two row, array in Phoenix showed major differences between the rows. Basically, the rear row, close to a fence, operated about 2°C warmer and degraded more over 28 years. As a result, the Vmax degraded by a few %. Similar front to back temperature and degradation effects were found on other older arrays with multiple rows. A study of wind direction when the ambient was over 41°C/106°F (often in Phoenix) showed that the dominant wind direction was from the Southwest. The East side modules were also warmer than the west side modules and degraded more. As arrays age, separate MPPT becomes more important and needs some study.