Any downside to bifacial panels?

[fafrd]

I got a dark roof that is not going to reflect and like back up to the bottom d a bifacial panel, but because the cost of bifacial panels is better than monofacial panel, I’m considering using them and want to make sure there is not some downside I’m overlooking.

For example, does the lack of white vinyl backing on a bifacial pane mean that some from-side irradiance that otherwise would be reflected back by the white vinyl and generates some power under STC will instead be wasted by passing through a bifacial panel where it will be wasted heating a non-reflective roof?

Can the power output under STC of a bifacial panel mounted on a dark non-reflective roof be lower than datasheet specifications?

Are there any other reasons to shy away from the use of bifacial panels mounted on a non-reflective roof I am overlooking?

 

[fafrd]

No experience on the Forum with bifacial panels?

 

[fafrd]

I’ve discovered at least one downside to bifacial panels: since they can be rated for up to +25% backside production, all wires, fuses, etc need to be sized for that higher max current level even through it will never be needed on a non-reflective roof…

Another alternative could be to limit current through the use of an SCC, but you’d need to be careful to assure that higher current levels cannot be delivered in any possibly scenario…

 

[Snowynorth]

I have no answer but am interested, in your questions. I was planning a ground mount with perk, 1/2 cut cells. I don't anticipate a lot of back reflection, but am thinking about using up some 4x8 SS sheeting, that I have sitting here, and connecting to the rear anchor posts at an angle reflecting back. I'm not sure if there is real world benefit either. I can buy non perc for less, and safe the expensive SS if the return is minimal.
I'm going to follow this fafrd. I'm sure you will get some good impute.

 

[fafrd]

I have no answer but am interested, in your questions. I was planning a ground mount with perk, 1/2 cut cells. I don't anticipate a lot of back reflection, but am thinking about using up some 4x8 SS sheeting, that I have sitting here, and connecting to the rear anchor posts at an angle reflecting back. I'm not sure if there is real world benefit either. I can buy non perc for less, and safe the expensive SS if the return is minimal.
I'm going to follow this fafrd. I'm sure you will get some good impute.

Is SS stainless steel?

Bifacial panels are rare in this country - a small sliver of total panel volume.

But they don’t cost much more expensive hat their monofacial brethren in terms of manufacturing cost and because they are currently exempted from the 18% tariff on solar panels, they are cheaper for us here (if you can find them).

Any backside production is a bonus and I frankly have no interest in chasing after that (though when I reroof, I will probably shift from dark red asphalt shingles to white ).

How bifacial are mounted and racking systems for them is one concern (especially if you are getting plans permitted). The ‘Simple Plan’ we use here in California almost certainly can’t be used if the panels are bifacial and urban building departments have likely never even encountered them before.

And the other concern is wire sizing for max current levels. Typically a bifacial will show ~125% of front-side Isc when max backside production is included. So as I already stated, either you need to oversize all wiring and OCPD by 25% (which will easily be approved) or you will need to assure currents can never reach that level by ‘overpanelling’ so that the SCC begins throttling current if any backside generation kicks-in (in which case you need to be careful with any possible partial-shading issues).

I’m not ready to dismiss the idea yet but I’m thinking that if you are aiming for a permitted install, bifacial panels are probably not worth the trouble…

On the plus side, the backside glass apparently makes bifacial panels significantly stronger and more robust that monofacial panels (at the cost of some added weight).

 

[svetz]

Bifacials are typically better sealed than regular panels, so they should give superior durability for increased weight. To get power out of the backside of the panel, there is often an issue as they need to be elevated making them susceptible to wind.

I got a dark roof ...

Dark might cost you some efficiency. You might want to paint the section of the roof where the panels will go with a reflective paint, I suspect it'll help more than a dark roof. Henry's 887 is not only very reflective but as it's silicone will provide another layer to prevent roof leaks where the penetrations are (My roof is coated in it to reduce AC loads, it's always amazing to go up on the roof in July and the roof is cool to the touch).

 

[MrM1]

@svetz does Henry's bond well to metal?

 

[fafrd]

Bifacials are typically better sealed than regular panels, so they should give superior durability for increased weight. To get power out of the backside of the panel, there is often an issue as they need to be elevated making them susceptible to wind.

Understand (and absolutely no worth the trouble to me).

Dark might cost you some efficiency. You might want to paint the section of the roof where the panels will go with a reflective paint, I suspect it'll help more than a dark roof.

I assume you meant that reflective paint might help more than a white roof, correct?

I’ve got to reroof sometime in the next two years anyway, and both of my neighbors are already using white asphalt shingles, so that’s a no-cost, no hassle option for me (and better to keep the house cool as well).

I’ll have all the power I need, so I have no motivation to spend more money or go to additional trouble to try to squeeze out more.

My only interest in bifacial panels is if they are cheaper than monofacial panels or if they deliver superior durability for equal cost.

Any additional power production they are able to (easily) deliver is a bonus.

Henry's 887 is not only very reflective but as it's silicone will provide another layer to prevent roof leaks where the penetrations are (My roof is coated in it to reduce AC loads, it's always amazing to go up on the roof in July and the roof is cool to the touch).

Have you installed bifacials and if so, can you comment on whether you were able to use a standard racking solution (such as IronRidge) or if anything special was required?

 

[OzSolar]

No experience on the Forum with bifacial panels?

I have experience. I replied to another question on a different thread but being a noob to this forum I don't know how to tell you to get there.

 

[fafrd]

I have experience. I replied to another question on a different thread but being a noob to this forum I don't know how to tell you to get there.

Copying the url to that thread or your specific post and pasting the link here would be the typical way to point to another thread/post…

 

[Zwy]

I’ve discovered at least one downside to bifacial panels: since they can be rated for up to +25% backside production, all wires, fuses, etc need to be sized for that higher max current level even through it will never be needed on a non-reflective roof…

Another alternative could be to limit current through the use of an SCC, but you’d need to be careful to assure that higher current levels cannot be delivered in any possibly scenario…

That is one major downside, the number one is VOC can make series connections much lower. Instead of 4 or 5S, you might only be able to have 3S.

 

[OzSolar]

That is one major downside, the number one is VOC can make series connections much lower. Instead of 4 or 5S, you might only be able to have 3S.

Actually from what I've seen neither Voc or Vmp increase as the bifacial gain increases so no difference in how many modules you can put in series. It's happening on the Imp on the Isc side which you should factor into wire sizing of course.

Here's a snip from a typical spec sheet. https://www.invertersupply.com/media/data/CS3W-450MB-AG_Data_Sheet.pdf

 

[fafrd]

Actually from what I've seen neither Voc or Vmp increase as the bifacial gain increases so no difference in how many modules you can put in series. It's happening on the Imp on the Isc side which you should factor into wire sizing of course.

Here's a snip from a typical spec sheet. https://www.invertersupply.com/media/data/CS3W-450MB-AG_Data_Sheet.pdf

View attachment 85023

Yes, the impact of backside production is primarily on increased current (Isc, Imp), not voltage (Voc, Vmp).

 

[fafrd]

I have experience. I replied to another question on a different thread but being a noob to this forum I don't know how to tell you to get there.

Here is a link to the post I believe you were referring to: https://diysolarforum.com/threads/bifacial-power-claims.35790/#post-449948

If BiFacial panels are enough heavier to invalidate more of the ‘simple plans’ that avoid the need for engineering review, they are definitely not worth he trouble (even if cheaper).

Believe this puts this subject to bed (at least for me).

 

[svetz]

@svetz does Henry's bond well to metal?

I have one aluminum panel on my roof coated with it, I can see "runs" in it. It's sealed, so just an aesthetic thing. I suspect it was coated while it was hot originally, causing the silicone to cure at different rates. So, for coating metal I'd say do it very early in the day.

Oh, check your local energy savings incentives...I got an credit for my doing mine.

I assume you meant that reflective paint might help more than a white roof, correct?

White is a reflective color, my reflective roof is white. Henry's 887 is 90% reflective. Although I see from the Cool-Roof database that AcryShield A590 is 92% (new stuff is always coming out) and I seem to recall seeing something higher mentioned on the forums (although that might not have been durable enough for a roof).

I’ve got to reroof sometime in the next two years anyway, and both of my neighbors are already using white asphalt shingles, so that’s a no-cost, no hassle option for me (and better to keep the house cool as well).

Definitely good to do any roof work now rather than need to take the panels off in a decade. Henry's 887 has a "lifetime" warranty, but I'll be thrilled if I get 25 years out of it.

The only downside is the silicone has a slight tack (not sticky, more like anti-slip grip) so dirt starts to stick to it. When it rains it's very slick (even on a flat roof it's dangerous to walk on) and washes off...so might not be good for dusty places where it rains lightly once a year. They make acrylic products that are very hard and dust just blows off them, but my understanding is they need to be recoated every decade.

I’ll have all the power I need, so I have no motivation to spend more money or go to additional trouble to try to squeeze out more. You might write the manufacturer and ask. Most panels have reflective white back-sheets built in just to get that extra percent of power. Bifacials don't have those. That's why I was thinking putting them over a reflective surface might get you the "base" wattage out of them. When Will tested them, he laid them flat against concrete, but the concrete was white.

Have you installed bifacials and if so, can you comment on whether you were able to use a standard racking solution (such as IronRidge) or if anything special was required?

I don't have bifacials. Like you, I was thinking about them for durability. At the time the costs were about the same, but the variable power output made them wild-cards. That is a 350 watt panel with +30% could generate 455 watts, so without knowing how to calculate the true maximum output I would have had to upsize all the other components to handle it; that drove the costs way up. In the end, I fell into a great deal on my LG Neons and said screw it, the LG warranty is long enough.

 

[MrM1]

@svetz does Henry's mold or mildew on a slope (that is so it don't pool)

 

[svetz]

@svetz does Henry's mold or mildew on a slope (that is so it don't pool)

It's a hydrophobic surface, if you've a slope water is going to run off like Usain Bolt. My roof is amazingly dangerous when wet and it's practically flat.

The nice thing is mold and mildew don't do well against the Sun's UV and typically that's what the roof sees. But, if you've mold or mildew now from some permanently shaded spot it might persist. I'd definitely bleach out any areas like that before coating.

I've had mine for a few years now. I have a spot where water pools and there is a slight discoloration there, it's some sort of bio-film that comes off if I run my thumb across it when wet. Typically it dries, cracks, and blows off leaving a slight discoloration (pollen?). My roof is practically flat though. The roof also has "creases" (see image) and "black" stuff (dirt) accumulates there making unsightly lines (although they're very thin you can't really see them unless you're on the roof looking for them). Just an FYI, I posted more on my roof (including IR images) here.

 

[Snowynorth]

Is SS stainless steel?

Bifacial panels are rare in this country - a small sliver of total panel volume.

But they don’t cost much more expensive hat their monofacial brethren in terms of manufacturing cost and because they are currently exempted from the 18% tariff on solar panels, they are cheaper for us here (if you can find them).

Any backside production is a bonus and I frankly have no interest in chasing after that (though when I reroof, I will probably shift from dark red asphalt shingles to white ).

How bifacial are mounted and racking systems for them is one concern (especially if you are getting plans permitted). The ‘Simple Plan’ we use here in California almost certainly can’t be used if the panels are bifacial and urban building departments have likely never even encountered them before.

And the other concern is wire sizing for max current levels. Typically a bifacial will show ~125% of front-side Isc when max backside production is included. So as I already stated, either you need to oversize all wiring and OCPD by 25% (which will easily be approved) or you will need to assure currents can never reach that level by ‘overpanelling’ so that the SCC begins throttling current if any backside generation kicks-in (in which case you need to be careful with any possible partial-shading issues).

I’m not ready to dismiss the idea yet but I’m thinking that if you are aiming for a permitted install, bifacial panels are probably not worth the trouble…

On the plus side, the backside glass apparently makes bifacial panels significantly stronger and more robust that monofacial panels (at the cost of some added weight).

Yes the SS is stainless steel. I horse traded 18 sheets for some other stuff I didn't need.

 

[Snowynorth]

@svetz does Henry's bond well to metal?

If you are talking about caulking, look at quad, big stretch and similar. They don't dry out and crack. Quad is better, but big stretch is easier to work with. I use big stretch inside and out. Haven't seen a crack in 10 years around my windows, doors, counter top or bathtub, and it's still pliable. Try that with standard bathtub silicone.

 

[Zwy]

Actually from what I've seen neither Voc or Vmp increase as the bifacial gain increases so no difference in how many modules you can put in series. It's happening on the Imp on the Isc side which you should factor into wire sizing of course.

Here's a snip from a typical spec sheet. https://www.invertersupply.com/media/data/CS3W-450MB-AG_Data_Sheet.pdf

View attachment 85023

Interesting.