Bifacial Modules for Dark Asphalt Single Roof?

[SolarfortheOuachitas]

I recently requested an estimate for solar installation on a residential home that has conventional dark asphalt shingles. The return estimate included bi-facial solar modules for the East and West side of my dark composition asphalt shingle roof.

Does anybody have any insight on why they would work or not work in this application or why the installer would recommend them?

 

[Steve_S]

You will not gain anything with a dark background. Bi-facial depends on a surface that will reflect light back, so the lighter the colour the more it reflects the solar radiation. Optimally, Pure White reflects the most light & radiation. Dark roofs are best served by MONO-FACIAL Panels that also provide ample airspace between roof surface & panel for enough airflow to be a partial thermal break, otherwise the shingles can get cooked, reducing their lifespan drastically..

 

[palehorse]

Typically you will not get any (or minimal) bifacial gain on a roof. They are intended for ground mounts. I have 32 480w bifacials on my shop roof rather than traditional panels. The reason I have that is because my solar company does a great deal of commercial work and they were able to get these cheaper than a single face panel. So not only did i get them cheaper but i get a minimal gain from any reflectivity etc.

Can I measure the difference? No. But its better than nothing as all is how i see it. And commercial panels from what see have a 5 year longer warranty. 30 vs 25.

Edit. Agree with Steve_S that having the black shingles would not likely help as you need a brighter background etc.

 

[Steve_S]

The higher the SRI the more Solar Reflection you get.
In many parts of the world (hot zones) there is no such thing as a "black roof" simply because it's not very bright to invite heat into your building. Think Northern Africa, the Mediterranean and the tropics of course.

 

[Steve777]

If (and it's a big if) the installer was planning on painting the roof white, and if the panels were going to be mounted angled up such that they could get reflected light off the roof to their back early and late in the day, then perhaps bifacials might be of some use. But if it is a typical roof mount (just a short distance off the roof) seems like a waste.

 

[G00SE]

A lot of the higher wattage panels I’ve seen seem to only be bifacial. Maybe it’s due to availability or looking to achieve a certain panel wattage (even without bifacial numbers added)

 

[Steve_S]

Everyone should look at the panels which are over 400W.. Bi-Facial sizing compared to Mono-facial hi watt. They can get HUGE physically and the number of cells is commensurate.

 

[SolarfortheOuachitas]

Everyone should look at the panels which are over 400W.. Bi-Facial sizing compared to Mono-facial hi watt. They can get HUGE physically and the number of cells is commensurate.

 

[SolarfortheOuachitas]

Thank you all for your comments and insights. After reviewing all of the responses and doing my own research on the NREL (National Renewable Energy Laboratory) website, it seems like the bifacials are better suited for ground mount installations or in some rare cases on white or lighter colored roofs.

Thanks!

 

[hwy17]

I know this thread has concluded, but since I just did quite a bit of research on asphalt shingle reflectivity I'll throw in my findings.

Rough numbers for reflectivity:

Black standard: 5%
Grey/Brown standard: 10%
White standard: 30%
Grey/Brown Solar Reflectivity Enhanced: 20%

The notable fact here is that due to the way the surface granules bounce light sideways into each other, all asphalt shingles have horrible reflectivity. Even white asphalt shingles reflect horribly compared to white metal or any other flat white surface.

 

[Watts Happening]

I can attest to the benefits of bifacial. Yesterday was pretty thick cloud cover, this specific screenshot wasn’t the highest for the day, but this is from a 6.6kw array. Pretty amazing if you ask me.

A dark roof clearly isn’t ideal, but I’d be quite surprised if there was ZERO benefit, obviously some photons will pass through and reflect back. If you’re not spending much more, why not have a better panel?