SolarRoof Help

[SolarRoof2020]

Hi TMC Folks,

Been lurking here for about 3 months and learning what I can from the experiences and expertise in this forum. I'm finally at the stage in my solar roof order where I've gotten a site plan and now I have some questions that I hope some of you can answer.

The current plan has ~145 modules or ~8.5kW divided into three planes North, East , South. I'm in the USA (~45.512 N latitude). So obviously the south plane will get generate the most power per module (ignoring shading due to trees and other obstructions).

The original design had ~3kW on the south, 4kW on the North and 1.5KW on the east. The east is also heavily shaded by trees. North and south have minimal (basically none on south) shading due to trees. This was already less than ideal. The layout seemed to have space for another 10 or so modules on the south side. I asked them several times to change the layout and they kept saying "design changes are impossible at this point".

They never showed me any schematics or design proposals other than the layout up till that point.

Then I got the site plan and things got even worse....

They selected a single inverter with 3 MPPT ports. With a maximum of two strings per port and 35 modules per string. This made it basically impossible to isolate all the planes. But worse they decided to not isolate the south plane at all. And even more worse they cut the south size by 50% and moved it to the north (so much for design changes being impossible).

This is all pretty disheartening and I'm on the verge of cancelling my order.

My questions or I guess more a plea for advise are:

A) Is the design changes in the site plan as bad as I think they are? Connecting a north and south string and a north and east string together must cause a major drop in efficiency of the array.
B) How would you approach Tesla to try and get more modules moved to the south side? Everything I've tried up till this point has failed.

Even before the design change it looked like there was room for another 10 modules on the south side. It seems like they could either reduce the total number of active modules, fully populate AND isolate the south string(s) and still produce the same amount of energy. OR produce more energy by fully populating AND isolating the south string(s) but keep the same number of modules.

Thanks!
Solar Roof 2020 (maybe?)

 

[woferry]

The balancing of the strings is definitely a challenge. I'd detailed my configuration on my thread in post #73. It seemed at least with mine that they tried not to put multiple planes on one string, but one inverter channel does carry two planes facing different directions. A v3 design I saw to the same level of detail definitely took some PV Modules from one plane to another (facing differently) to balance things or stay within their max string voltage limit. Tesla did not enable the MPPT feature on my inverters, and as I understand it a minimum 200V is required for it to work on my Solivias, and 3 of the 4 strings basically spend little to no time above 200V, so I don't think it would have helped any anyway. I know they depend on their Diode Trunk Harness to deal with differences in shading and the like within a string, but I must admit that while I'm an EE I don't understand just what's in the DTH or exactly how to model a solar panel well enough to understand exactly how well the DTH works (how much it affects efficiency).

About the only thing I could think of, if you really cared that much and told them that you were willing to pay for more inverters to maximize the output on certain planes, perhaps they'd be willing to play ball. I suspect they're trying to maximize the output while minimizing the overall system costs, and personally I'd have good confidence that they understand things well enough to do that. But if you care more about production then price perhaps they can optimize things differently.

During v2 (or at least my v2) they were providing a production guarantee. I heard they aren't doing that anymore (even for their panel systems I believe), but do they even give you an indication of their estimated yearly production?

 

[SolarRoof2020]

Thanks woferry,

They actually did give me a production guarantee. But its about 40% less than what PVWatts Calculator says I'd get if each plane was fully isolated. Now I know that solar tiles are less efficient and that the web site doesn't know about my east shade. But 40% less is quite a lot. I assume the reason they were able to move so many tiles from the south to the north without changing that guarantee is because they put in a huge margin of error.

The inverter they picked for me is a delta M8-TL-US. Peak efficiency is >380V (about 42 modules at Vmp adjusted). Being that they have a max of 35 modules, it seems we will never hit the peak efficiency of the inverter at Vmp adjusted, ~8.9V. I have no way to model the Vmp adjusted over time/dates. Given Vmp@25c is 10.99, min number of modules to hit 380V is ~35. So 35 string max is also pretty close to the ideal MPPT size. If they moved the modules I asked to the front, we would get a single string of 35 modules. I'm sure I'm missing something there.

The other two ports would need to be ~27 modules per string ((145-35)/4 = 27.5)), which would mean a loss of a few modules. Or they could do two strings of 28, two of 27 and the one of 35. Really seems like that would work. Except the east one is 26 modules. They would need to find away to squeeze in one more module there or move more to north and do two of 30 and two of 25. Lots of options .

Thanks!
Solar Roof 2020

 

[jboy210]

Don't forget about clearance issues at ridge lines and vents. I believe they must keep active panels 3 feet from the top of the ridge to all fire crew access. You can see the were trying to maintain ridge and vent clearance in our plan. Solarglass Roof - V3 - any customers?

Also, the V3 active tiles are one size 45"W X 17"H and are never cut. The inactive come in 5 widths and are 17" H. This reduces the flexibility of placement.

 

[SolarRoof2020]

Hi jboy210,

I spent many hours contacting different building code divisions both at state and county level. After them getting tried of me they finally declared that there were no 3 feet pathway requirements for the tesla solar tiles. This is because those pathways are for fire fighters to walk between solar panels. As the solar tiles are at the same height as the non-solar ones, there is no need for a pathway. However, even if you add in the 3ft ridge requirement there is still room for more tiles. The tiles they moved from south to north were not near the ridge. They had >45 in the original layout. Current layout has < 30. I'm just trying to get them to put at least 5 to 10 more on the south side.

 

[SolarRoof2020]

Hi woferry,

I've studied the inverter manual some more. There is no mention of away to turn off the MPP-Tracking. I do see graphs and other data that suggest the "Full Power MPPT operating range" is 200V to 480V. That would give a min voltage of 5.71, which is below even Teslas adjusted Vmp of ~6V. I take that to mean a 35 module string should be able to operate in "Full Power MPPT" over a large range of temperatures.