solar power calculation for bifacial panels

[xcentric]

It should be simple, but.....

I have some Renesolar Bifacial Double Sided panels specced as 425W front 90W rear - so when I'm looking at sizing my array so as not to overload my inverters max input, what figure do I use? 425 + 90 = 515W? But as they are on a dark green roof, not a white one, and half are mounted at 20 degrees and the other 40, will it be less. But how much?

My 5kW inverter can take 6500W in, so assuming 12 panels at 5100 front and 1080 rear (max) = 6180 will be fine - but could I put 14 panels on there?
front = 5950 W front and 1260W rear = 7210W notionally, but as suboptimal angle for front half will lose ~10% (making ~5600 front) and much less efficient at rear (say 50% less efficient overall - low panels at front and low albedo ~600W rear) totalling 6200W so within inverter range.

Any suggestions as to what will work?

 

[millsan1]

The watts isn't a problem. You could in theory put 10,000 watts on there, the inverter draws the power and will cap.

What you should think about is volts. That is a hard limit. Without knowing panels and inverter, can't tell you, but guessing that VOC will be around 40, 14*40 = 560 volts.

 

[xcentric]

VOC is 38.5 v
inverter is Sunsynk 5kw - 125-500v
but plan is 2 strings so 2 x 269.5 v
(I assume inverter voltage is per string not total)

 

[millsan1]

According to these specs:

5kw Hybrid Inverter | Sunsynk

www.sunsynk.org

Solar Input
Max power input – 6500W
Max V DC input – 370V (100V~500V)
Max current input – 13A + 13A
MPPTs – 2
MPPT Range – 125~425V

You either have a 500 or 425 or 370 V limit. Kind of hard to decipher.

But yes, 2 strings of 7 would seem to be in the safe realm.

 

[shadowmaker]

500V max. so 12 in one string is doable if outside temp never goes under 0C. If your place can have lower temps then 11 panels on one string.

 

[xcentric]

We're not Finland but it does go below 0. 2 strings are most likely for ppt tracking issues.

I get the VDC input limit, but am still wondering about the power - if it's not relevant, why is it quoted, and if the bifacilas give full power, won't I be over that (and if so what's the consequence? - some internet info says overload, overheating and shutdown, but I'm less sure).

 

[shadowmaker]

You are not getting much bifacial gain with dark green roof, but you can optimize backside production by installing panels (rails) as high from the roof as possible and leaving gaps between adjacent panels to get more light under those panels.

Power isn't that important, it won't break anything to have more panel power than your Sunsynk can handle. It is a very common thing to do and we call it overpaneling.

 

[xcentric]

I know it's overpaneling - it's the spec limit I'm at (the inverter is officially rated for 5kW) - so it's pushing the overpaneling limit!

 

[shadowmaker]

Oh and seller saying it can produce max 90W from the backside is kind of funny way to describe bifaciality as I'm pretty sure it can make at least 300W from the backside only (bifaciality factor is most probably over 70% on those panels). It all depends on the amount of light you can get to that backside. With dark green roof install I bet you can get only something like 3-5% (13-21W) backside gain max and with ultra white roof maybe 25% (106W) in close to perfect conditions. As conditions rarely are perfect overpaneling some 10-20% will help.

 

[shadowmaker]

I know it's overpaneling - it's the spec limit I'm at (the inverter is officially rated for 5kW) - so it's pushing the overpaneling limit!

It's safe to overpanel some 10-20% over rated power. It's the voltage you shouldn't exceed at any time.

 

[shadowmaker]

Oh, and inverter won't produce more than it's rated output even overpaneled in perfect conditions. It's called MPPT clipping. It's just that those perfect conditions are so rare that there is usually room in MPPTs for more panels than their nominal rating suggests. Panels are so cheap now that it usually pays off to overpanel if one just has room for it.

 

[millsan1]

I know it's overpaneling - it's the spec limit I'm at (the inverter is officially rated for 5kW) - so it's pushing the overpaneling limit!

Again, that max is what the inverter can pull from the panels. Panels do not push energy. So if all the inverter can pull is 5k, you could have 7 or 8 or whatever of panels, the inverter will just pull a max of 5.

 

[zanydroid]

There are some fault modes where the MPPT will pull more, so some attention needs to be paid to Isc limits on MPPT

But if you don’t stack bifacial gain on top of tickling the dragon levels of overpaneling you should be fine.

 

[xcentric]

But if you don’t stack bifacial gain on top of tickling the dragon levels of overpaneling you should be fine.

I like the evocative language, but have no idea what you mean!

 

[zanydroid]

I like the evocative language, but have no idea what you mean!

Usually a MPPT has overpaneling margin. Suppose you stayed at the nameplate power before accounting for bifacial gain. The 30% of bifacial gain generally will be within the overpaneling margin.

Now if you were overpaneled 100% then adding 30% potential on top of that is a risk.