Ive got another idea brewing!

[electrofelon]

Some of the regular users will recall my solar system. I did a little write up of it here. https://forums.mikeholt.com/threads/my-solar-system.148865/ Basically I used solar panels as the roof for an outbuilding. For some time I have been planning on building a shed roof addition off the back, the north side, at about 16 feet wide, and the length of the building, about 50', but probably with a little overhang on each end so maybe 54 feet. I had always assumed I would use metal roofing, probably for this just the cheap thru fastener stuff. Today I got thinking about it more (and distracted from the stuff I should have been doing but it was more fun to think about this). I thought about using the ribbed clear polycarbonate panels that I used on part of the north side wall of the building (pictures in the link). Metal roofing is about $1.50/ sq ft now. Those polycarb panels are only a little more at $1.66. Then I got thinking.........Why not use solar panels as the roof like I did for the main south facing roof? Ok so I already have a surplus of power, usually about 3500 KWH/year AND if you dont recall I have a 2400V step up step down electrical system using 15 KVA transformers so I cant send infinite amounts of power. I am at 14KW AC now, and I already planning to add another few rows to the south side on a 5KW inverter I already have, so lets just say it will be 19KW AC of PV generation when that is installed. The system is "over-paneled" so that is ACTUAL AC KW that the system makes from about 10:00-2:00 solar time and the transformers will see it (maybe not during hot summer days). I am ok making the transformers run at 19KW for a few hours some days, they are in the shade and Im in the northeast where its not too hot. Ok so its seeming quite absurd to add more PV right? Don't worry about that, I will find a use for the electricity - think sauna, hot tub, lumber drying kiln, heat in the shop, air conditioning.......Well Ive been looking at cheap used modules and it looks like they can be had for about 25 cents/watt delivered, or $3.40 sq/ft. So say ONLY $1500 over the polycarb panels or $1650 over metal roofing. I dont want to buy more inverters (nor would I want to push more than 19KW thru the 15 KVA transformers anyway) so I would string this potential north facing new stuff into the current 3 inverters (7+7+5 KW). They are sunny boy inverters and have 3 MPPT inputs. I havnt run any numbers yet or thought much about stringing yet. I suppose the first thing to do is see how much power north facing mostly shaded panels will produce . Thoughts? Yes I am insane, but any other thoughts?

 

[electrofelon]

I also like the idea of using used solar panels for the roof, rather than consuming more new materials.

 

[electrofelon]

So here is the boiled down problem:

Existing 70x235 panels connected to (2) 7kw inverters with 3 mppt each, so strings of 12,12,11 on each. 20x280 panels to be connected to a 5kw inverter, also with three inputs. How should I connect 50 north facing mostly shaded panels in? This gets to I think the same situation @rev_polarity has in the other thread. Can I or should I just parallel connect five strings of 10 and connect it to the third mppt on that 5kw inverter? It seems like for maximum gain I should distribute the north facing panels amongst the three inverters but there doesn't really seem to be a good way to do that. I likely have to play with it and see what kind of power output I get from the shaded panels.

 

[gadfly56]

NO FAIR! I thought this was going to be about BEER!!

 

[electrofelon]

NO FAIR! I thought this was going to be about BEER!!

Well some beer can be enjoyed while building this

 

[rainwater01]

Have you ran the numbers? I was thinking you loose up to 40% of your production when the modules are facing north.

I’m not sure about how your service is set up to this building? Are you receiving power from that transformer that your feeding your solar to so you’re actually using some solar production at this building and the remainder goes to wherever the transformer is feeding?

I haven’t installed any sunny boys so I can’t speak to that yet. I might look it up for giggles.

Interesting project though.


Sent from my iPad using Tapatalk

 

[ramsy]

They are sunny boy inverters and have 3 MPPT inputs. ..Yes I am insane, but any other thoughts?

If your inverters shut down during power outages, having some panels off grid might keep the lights on, especially if charging a switched SDS.

 

[GoldDigger]

So here is the boiled down problem:

Existing 70x235 panels connected to (2) 7kw inverters with 3 mppt each, so strings of 12,12,11 on each. 20x280 panels to be connected to a 5kw inverter, also with three inputs. How should I connect 50 north facing mostly shaded panels in? This gets to I think the same situation @rev_polarity has in the other thread. Can I or should I just parallel connect five strings of 10 and connect it to the third mppt on that 5kw inverter? It seems like for maximum gain I should distribute the north facing panels amongst the three inverters but there doesn't really seem to be a good way to do that. I likely have to play with it and see what kind of power output I get from the shaded panels.

When you are using string inverters (without optimizers or micro inverters) it is essential that all of the panels in a given string have the same orientation and shading characteristics. Othewise the lowest current output panel will drag down the output of the string. Even with bypass diodes allowing other panels to deliver full current, at best you lose the entire power output of the low current panel.
You can parallel string(s) of panels of one orientation with string(s) of a different orientation into the same MPPT input with little power loss. But you cannot parallel strings of different shading characteristics because they will have radically different Vmp values.
A mixed orientation system which cannot be divided into strings of matched orientation and a situation with mixed variable shading is exactly the system that should be designed based on optimizers or micro inverters. Period.

 

[ggunn]

If your inverters shut down during power outages, having some panels off grid might keep the lights on, especially if charging a switched SDS.

Nearly all inverters without batteries shut down during a power outage and cannot be run off grid.

 

[electrofelon]

Nearly all inverters without batteries shut down during a power outage and cannot be run off grid.

You no doubt know gunny, but for others, the SMA inverters do have a 20 amp 120 circuit available for use off grid, PV power permitting of course.

 

[electrofelon]

My responses and evolving thoughts so far:

If I want to play around with this without buying any new inverters, it seems about the best and only option is to put a string of 10 of the 280s on each of two mppt inputs on the 5kwi nverter. That leaves the 3rd mppt available for all these North facing panels. I'm limited to 13, maybe 14 panels per string so I would need to parallel like four strings. That gets beyond the current SMA says I can have on an input, but it's not clear to me the inverter will destroy itself or that these North facing panels will even produce that much peak anyway.

On a different note:. I've been digging into cheap used panels. Sun electronics has some that are real cheap, but they are only selling and container quantities unfortunately. They have some more expensive used ones, 35 cents/watt, that they sell in pallet quantity but that is getting too expensive and more than I want to pay over regular roofing. I'm willing to pay about $1,500 to 2,000 over with the roofing materials would be. There are some cheap ones on eBay about .18 a watt that have cracking in the vinyl back sheet and some others that have snail trails but they say are working fine.....

 

[wwhitney]

If I want to play around with this without buying any new inverters, it seems about the best and only option is to put a string of 10 of the 280s on each of two mppt inputs on the 5kwi nverter. That leaves the 3rd mppt available for all these North facing panels. I'm limited to 13, maybe 14 panels per string so I would need to parallel like four strings. That gets beyond the current SMA says I can have on an input, but it's not clear to me the inverter will destroy itself or that these North facing panels will even produce that much peak anyway.

Have you plugged the details into PVWatts to see how the north facing panels perform vs the south facing panels? You may be underestimating the north facing panel performance.

Also, if the primary function is roofing, you have the option to install the north facing panels and string them appropriately but start by only hooking up, say, 2 parallel strings. Then you'll be able to gather some performance data, and see what you can do with the 3rd and 4th string.

Cheers, Wayne

 

[electrofelon]

Have you plugged the details into PVWatts to see how the north facing panels perform vs the south facing panels? You may be underestimating the north facing panel performance.

Also, if the primary function is roofing, you have the option to install the north facing panels and string them appropriately but start by only hooking up, say, 2 parallel strings. Then you'll be able to gather some performance data, and see what you can do with the 3rd and 4th string.

Cheers, Wayne

Yeah I may just have to play with it. It's a bit tricky to model the output, I'm not quite sure how to do it honestly. Because these panels are not only North facing, but are also shaded as this is a shed roof behind the main structure. The direct sun will hit the northernmost row or two but the South Rows will never ever see direct sun.

 

[gadfly56]

Yeah I may just have to play with it. It's a bit tricky to model the output, I'm not quite sure how to do it honestly. Because these panels are not only North facing, but are also shaded as this is a shed roof behind the main structure. The direct sun will hit the northernmost row or two but the South Rows will never ever see direct sun.

Maybe uncomplicate your life by skipping those?

 

[ggunn]

You no doubt know gunny, but for others, the SMA inverters do have a 20 amp 120 circuit available for use off grid, PV power permitting of course.

Of course; that's why I said "nearly all".

 

[electrofelon]

Maybe uncomplicate your life by skipping those?

That's actually a reasonable idea and perhaps a good compromise. Put 8 ft of metal roofing on the Southern shaded half, and then 8 ft of panels on the Northern outer part, where the sun will reach....

 

[electrofelon]

Just found some 54 cell 205 watt panels for $25 each. Thats $1.50/sq ft, the same price as metal roofing!

 

[electrofelon]

Just been thinking more about this general idea of using cheap(er) used PV panels as roofing. I really like the idea of using a product that already exists instead of using more virgin metal or plastic.

The market price of used panels seems to be around $45-$50 per panel, which is about $2.60/sq foot. Like I said, I found some for $25 a panel but that is unusually cheap. The cheap galvanized thru fastener roofing is $1.50/SQ ft. Standing seam metal is quite a bit more. There are some "balance of system" materials which are more likely to be less for the PV roof. For example with metal, you typically would need 2x4 purlins every 2 feet, and you could likely skip those with the PV which could be a substantial cost savings. PV would require some caulk, foil tape, or flashings depending upon the sealing method chosen (I would probably just use outdoor rated foil tape). Both methods would require fasteners of course let's just call that a wash both ways.

 

[ggunn]

Does your metal roof have standing seams? If so, you don't have to worry about penetrating or hitting purlins if you use S-5 clamps.

 

[electrofelon]

Does your metal roof have standing seams? If so, you don't have to worry about penetrating or hitting purlins if you use S-5 clamps.

I would be using the PV panels AS or INSTEAD OF The metal roofing.