Monofacial vs Bifacial - Are they calculated the same and how does the rear ISC impact the total ISC of 2 strings in parallel

[garybryan33]

Seg Yukon 460-watt bifacial panel specifications:

Front:
VOC: 41.50
VMP - 34.49
ISC - 13.96
IMP - 13.34

Rear:
VOC - 41.52
VMP - 34.49
ISC - 9.84
IMP - 9.34

I have 3 strings of 8 panels feeding an EG4 18K, 2 strings will go into input 1 thus paralleled, and the third string will go into input 2
The question is, What ISC value should I use to determine the amperage of the paralleled strings?

As I understand, If you have solar panels connected in series, the total current passing through the circuit is limited by the panel with the lowest current output. In this scenario, if one side of the panel produces 13.34 amps, and the other side produces 9.34 amps, the current is limited by the 9.34 amps (the lower value). This is because the current must be the same at all points in a series circuit. All that to say, which one is correct?

8 panels in series:
volts: 332.16 volts: 332.16
ISC: 13.96 OR ISC: 9.84
Watts: 3680 Watts: 3680

Paralleled
volts: 332.16
ISC: 27.92 or 19.68
Watts: 7360

Thank you

 

[sunshine_eggo]

Seg Yukon 460-watt bifacial panel specifications:

Front:
VOC: 41.50
VMP - 34.49
ISC - 13.96
IMP - 13.34

Rear:
VOC - 41.52
VMP - 34.49
ISC - 9.84
IMP - 9.34

I have 3 strings of 8 panels feeding an EG4 18K, 2 strings will go into input 1 thus paralleled, and the third string will go into input 2
The question is, What ISC value should I use to determine the amperage of the paralleled strings?

As I understand, If you have solar panels connected in series, the total current passing through the circuit is limited by the panel with the lowest current output. In this scenario, if one side of the panel produces 13.34 amps, and the other side produces 9.34 amps, the current is limited by the 9.34 amps (the lower value). This is because the current must be the same at all points in a series circuit. All that to say, which one is correct?

Front and back should be in parallel with each other, so their currents would add.

Those ratings are if you flip them around so the back is perpendicular to the sun. You won't get anything near that in front-facing-the-sun orientation.

Fantastic bifacial performance is about 130% of the front side rating. This is very rare. It requires at least 1m of clearance between the back face and the closest surface, a steep angle, and you need a highly reflective background (like snow).

A realistic worst case Isc for your panels is 13.96*1.3 = 18.1A

 

[garybryan33]

Thank you for your reply.

The MPPT input 1 on the 18k recommends 25 amps, and a max short circuit input current of 34 amps. Should I reconsider supplying 2 strings in parallel? The current installation is roof mounted with approx 5-inch space between the roof and panel, it is a light-colored metal roof.

 

[zanydroid]

I would bet that adds tiny bifacial gain.

Note that the shading/blocking calculations for monofacial applies to bifacial. If you only have partial boosted light on the back, the current across the panel will not match (it will be choked to the lowest in the series, and I’m sure your MPPT and optimizers will do nothing for it)

IOW visualize the reflection as an equivalent shadow

 

[sunshine_eggo]

Thank you for your reply.

The MPPT input 1 on the 18k recommends 25 amps, and a max short circuit input current of 34 amps. Should I reconsider supplying 2 strings in parallel? The current installation is roof mounted with approx 5-inch space between the roof and panel, it is a light-colored metal roof.

You are likely going to get very very little benefit from the back side. I would regard 2P on a 34A max tracker fine.

 

[garybryan33]

I would bet that adds tiny bifacial gain.

Note that the shading/blocking calculations for monofacial applies to bifacial. If you only have partial boosted light on the back, the current across the panel will not match (it will be choked to the lowest in the series, and I’m sure your MPPT and optimizers will do nothing for it)

IOW visualize the reflection as an equivalent shadow

Can you elaborate on the shade/blocking? I understand that there will be minimal gain from the backside considering they are sitting close to the roof, but I did not consider how that would impact the front. The roof is exposed to full sun no shade, I went with Tigo RSDs and not optimizers because of that.

 

[garybryan33]

You are likely going to get very very little benefit from the back side. I would regard 2P on a 34A max tracker fine.

Thank you for your time.

 

[zanydroid]

Can you elaborate on the shade/blocking? I understand that there will be minimal gain from the backside considering they are sitting close to the roof, but I did not consider how that would impact the front. The roof is exposed to full sun no shade, I went with Tigo RSDs and not optimizers because of that.

The extra light you get on the back can be modeled as extra light on the front, electrically speaking. If you have similar unevenness at the front, the light from the bright spots is not available as electricity.

You won’t LOSE anything, but you’ll gain even less than you might think, compared to if solar panels had all cells wired in parallel. I doubt it will put extra stress on the solar panels in a meaningful way.

 

[shadowmaker]

Once again, but maybe this video can shed some light on your roof install.

Short story even shorter: Elevated (4-8") and loosely installed (~4" spacing in every direction) bifacials with reflective roofing material can give you something like 20-25% more yield.

I'm a Bi-fanboy, so my writings should be taken with a grain of salt.

 

[sunshine_eggo]

Once again, but maybe this video can shed some light on your roof install.

Short story even shorter: Elevated (4-8") and loosely installed (~4" spacing in every direction) bifacials with reflective roofing material can give you something like 20-25% more yield.

I'm a Bi-fanboy, so my writings should be taken with a grain of salt.

11% increase but slightly less W/m^2 than standard panels. Where are you getting 20-25% more yield?

 

[shadowmaker]

11% increase but slightly less W/m^2 than standard panels. Where are you getting 20-25% more yield?

Actually 16% increase on the edge panel letting more light under it.

Why would W/m2 matter if panels have different efficiencies to begin with? To compare you'd need identical panels (efficiency, temp coef) just that the other one is bi and the other is not, which could be hard to do. I bet you could easily find bi panel with better than that W/m2 if you just went with some newer panel type, but I can't see the point here.

To get 20-25% bifacial gain, you need more light under the panel than in the video. That should be achieved by elevating them more (I suggested 4-8") and spacing them more "loosely" (I suggested 4" in every direction). Of course it depends how reflective roof is (plating/paint) and I'm sure roof profile has an effect too.

 

[garybryan33]

Once again, but maybe this video can shed some light on your roof install.

Short story even shorter: Elevated (4-8") and loosely installed (~4" spacing in every direction) bifacials with reflective roofing material can give you something like 20-25% more yield.

I'm a Bi-fanboy, so my writings should be taken with a grain of salt.

I have seen Dave's video, thank you. The Gist is there are benefits and drawbacks and I am okay with that. The issue that's got me a bit confused is the amp output with the gain, as negligible as it may be. The goal is to stay as close to the recommended input amperage of 25 amp for input 1.

 

[shadowmaker]

The goal is to stay as close to the recommended input amperage of 25 amp for input 1.

It's safe to go over and it's called overpaneling. If there's too much amperage the inverter starts "clipping" and just doesn't receive the excess. This way you can stay at the peak performance longer and when conditions are far from ideal (which they usually are). It's the string voltage you should not exceed as that's dangerous for the hardware.

 

[garybryan33]

On the voltage side, we are humming along at about 332V, calculating the temp coefficient 358V. Specs are 230 - 500-volt. The nominal MPPT tracker voltage is 360V

 

[Cmiller]

Just some interesting data on bi-facial vs non bi-facial production on a ground mounted scenario. Here is a screenshot of an install where we installed 3- 6 panel pole mounts and then added 3 more a year later. The rear 3 are the exact same brand and watt panel, only they are bi-facial. This screenshot was January through sometime in April of 2022. It gives a good picture of bi-facial production benefits.

I don't have any newer screenshots, as the SolarEdge inverter had failed for the 3rd time (in about 2 years time) at that point, so we swapped it out for a Fronius and haven't been back out to that job since then....



A quick guess on average for each set looks to be about 145kWh for the non bi-facial and 165kWh for the bi-facial ones. That comes out to 13% gain in production.

Back to the original question. As others have stated rear gain is in parallel to front production, and being flat mounted on a roof will yield fairly low gains. The mppt will "clip" amperage to its rating. You shouldn't be hurting anything even if the panels do hit that "clipping" amperage limit, however if you parallel 2 strings you will be over the amp limit before even having any bi-facial gains! If I were in your situation I would utilize all 3 mppt inputs to avoid clipping, as you will possible do a bit of clipping!

Here in Ohio we need all the power that we can get in the winter because heating systems are on and using power, days are shorter, and most days are cloudy. So if we have a sunny day, we don't want to waste any available power!

For a reference on rear gains in terms of amps, my ground mounted HT 450W bi-facial panels are rated 10.91A (ISC) and the other day on a cold (35F) day with full direct sun, I saw output amps at 12.9A. That is ~18% over front rated ISC. In your scenario with an ISC of 13.96, if you were to gain 18% (won't happen if you are roof mounted) you could hit 16.67A pr string!

 

[zanydroid]

So one thing I missed in my analysis is that I assumed a narrower scatter angle for the light coming off the roof than there probably is.

So it'll be higher than zero b/c there will be SOME scatter that reaches every module.

And the farther the module is from the portion of the under-solar panel roof that gets light, the less scatter light it gets. If you poke your head (in imagination if that's all you have) under the panel and look at the darkest part, you can probably guesstimate the gain.

 

[shadowmaker]

if you parallel 2 strings you will be over the amp limit before even having any bi-facial gains! If I were in your situation I would utilize all 3 mppt inputs to avoid clipping, as you will possible do a bit of clipping!

THIS!

On the voltage side, we are humming along at about 332V, calculating the temp coefficient 358V. Specs are 230 - 500-volt. The nominal MPPT tracker voltage is 360V

I'd also add 2-3 panels on each string if you have room for them. This is to overpanel and get voltages higher. Looking from the EG 18kpv specs max Voc is 600V, so voltagewise you are safe. Overpaneling won't add much on a good day, but all days aren't good. Panels are cheap, EG 18kpv isn't.

 

[garybryan33]

Just some interesting data on bi-facial vs non bi-facial production on a ground mounted scenario. Here is a screenshot of an install where we installed 3- 6 panel pole mounts and then added 3 more a year later. The rear 3 are the exact same brand and watt panel, only they are bi-facial. This screenshot was January through sometime in April of 2022. It gives a good picture of bi-facial production benefits.

I don't have any newer screenshots, as the SolarEdge inverter had failed for the 3rd time (in about 2 years time) at that point, so we swapped it out for a Fronius and haven't been back out to that job since then....

View attachment 192882

A quick guess on average for each set looks to be about 145kWh for the non bi-facial and 165kWh for the bi-facial ones. That comes out to 13% gain in production.

Back to the original question. As others have stated rear gain is in parallel to front production, and being flat mounted on a roof will yield fairly low gains. The mppt will "clip" amperage to its rating. You shouldn't be hurting anything even if the panels do hit that "clipping" amperage limit, however if you parallel 2 strings you will be over the amp limit before even having any bi-facial gains! If I were in your situation I would utilize all 3 mppt inputs to avoid clipping, as you will possible do a bit of clipping!

Here in Ohio we need all the power that we can get in the winter because heating systems are on and using power, days are shorter, and most days are cloudy. So if we have a sunny day, we don't want to waste any available power!

For a reference on rear gains in terms of amps, my ground mounted HT 450W bi-facial panels are rated 10.91A (ISC) and the other day on a cold (35F) day with full direct sun, I saw output amps at 12.9A. That is ~18% over front rated ISC. In your scenario with an ISC of 13.96, if you were to gain 18% (won't happen if you are roof mounted) you could hit 16.67A pr string!

Very interesting information. Thank you.

As I process all this information I am leaning towards using all 3 MPPT inputs and monitoring the system for 6 - 12 months. With that data, I should know if connecting 2 strings to input 1 is a viable option. I agree with you, just going parallel puts me over the recommended 25 amps yet within the head space that's allowed, but that does not account for bi-facial gain. The 4th ground mounted string will have to wait until the data comes back. At the end of the day, I think these panels are not a perfect fit for the 18K

 

[garybryan33]

THIS!


I'd also add 2-3 panels on each string if you have room for them. This is to overpanel and get voltages higher. Looking from the EG 18kpv specs max Voc is 600V, so voltagewise you are safe. Overpaneling won't add much on a good day, but all days aren't good. Panels are cheap, EG 18kpv isn't.

Yes, I agree. I do have room for extra panels.

 

[MattiFin]

On the voltage side, we are humming along at about 332V, calculating the temp coefficient 358V. Specs are 230 - 500-volt. The nominal MPPT tracker voltage is 360V

Instead of 3x 8S strings set your panels to 2 strings of 12 panels. Still within 18K voltage limits and you have the input 1 mppt free for future use?