Black vs Silver panels?

[bob-n]

Black panels seem to be popular and sell for a premium.

Esthetics aside, my gut tells me that black panels will absorb more sunlight and run warmer than silver panels, so could be less desirable. But if they are more effective at changing light into electricity, then they could be more desirable.

Have you seen data either way?

Thank you.

[solarix]

Yep, Black panels are not as productive, but they look so much better and sell better (black is beautiful). Around here, the local city officials have mandated black frame solar panels only. You'll notice that in the high-efficiency panels (the ones on the bleeding edge of efficiency bragging rights), they are all aluminum framed.

[J.P.M.]

I seem to recall looking at mfg. data sheets that showed STC efficiencies for all black panels being slightly lower than identical panels that are not all black. That will make them less efficient and less productive and so less cost effective, but not necessarily less desireable

Since the published/claimed penalty in STC efficiency I've seen is of the order of ~ 0.4 to 0.5% or so, and most folks are clueless about what that may mean, I suspect, esthetic considerations will sell more product than efficiency.

If you're a peddler, put aesthetics aside at your peril. It's easier for a peddler to leverage customer vanity than explain anything involving math. The probability of closing a sale is also higher using vanity than rubbing a customers nose in their math phobia.

On efficiencies: I remember seeing Sunpower data sheets that had that 0.5 % lower STC efficiencies or so (21.0 vs. 21.5 STC eff. or so) for all black panels w/monolithic frontsides vs silver frames and busbars on front, but I have no independent data to confirm that. Data sheets seem to play with numbers a bit and some mfg. processes/procedures and testing protocol do change slightly with time, but (21.5-21.0)/21.5 seems like a believable % increase in a panel's temp. diff. with the environment ( w/bottomline temp. increase maybe ~ 3-5 C. or so difference, black to silver) when operating at close to STC conditions.

From a mfg./production standpoint, I can't see where two otherwise identical panels except for color and/or maybe if the wiring is moved from the frontside to the backside of the panel would mandate different cell characteristics. I'd also think black vs. white backsheets on the all black panels don't do much to lower the equilibrium temp. of a panel (but probably not a lot +/-, at least for a roof mount), s, my $$'s are on cells in either panel being identical. If some of that is a correct description of how the game is run, "blacker" panels will be less effective or less effective at changing sunlight into electricity, but they may be more marketable due to appearance, and customer vanity is an easy thing to take advantage of.

From a scientific standpoint, while there may be some mitigating factors that may need consideration(s), thermal radiation basics, etc. will confirm that objects with greater absorbance of solar radiation (things that are more "black " will get hotter than objects with lower absorbance of solar radiation.

There are no white solar thermal panels for a reason.

[bcroe]

How is the black coating applied to an aluminum frame, and will it actually
last over 2 decades? Bruce Roe

[J.P.M.]

How is the black coating applied to an aluminum frame, and will it actually
last over 2 decades? Bruce Roe

Depends on the coating, how it's applied and, for one set of considerations, how fit the coating is for prolonged exposure to the elements, including solar radiation.
Spray painting with rustoleum on unprimed and unoxidized aluminum most likely has a lower probability of lasting as long as the useful production life of a PV panel.

My guess is most colored frames are painted - e.g., see LG product hype - but it's possible to have the anodized aluminum colored during the anodizing process. Don't know how common that is, if at all, for PV framing applications. FWIW, some types of anodizing make some types of aluminum more brittle. That may/may not be a consideration for PV framing applications. Powder coating may also be an option, which, for all I know, may be common.

I'd also guess packaging and handling during erection would also be a consideration with respect to surface durability.

[bob-n]

This is very interesting and helpful. Thank you all.

I just did some quick web searching. There are a few articles claiming that indeed black panels are less effecient (is that the right word) than "normal". Reading about black panels also links to many discussions about poly vs mono, but that's another topic. It's hard to separate claims for black panels from claims for mono panels.

Silicon will naturally oxidize, just like aluminum, and is protected by intentionally grown and/or deposited oxide. The interesting difference is that silicon dioxide (or silicon nitride) will have a different color depending on thickness, due to the wavelengths of light at different colors. I haven't yet found how they make some cells intentionally appear blacker than others.

I also found that both black anodized frames and black powder-coated frames are used, depending on the manufacturer. I also read that all black anodized coatings fade when exposed to long-term sunlight,

This proves one thing about solar power: no matter what aspect you delve into, there are so many details.

[J.P.M.]

This is very interesting and helpful. Thank you all.

This proves one thing about solar power: no matter what aspect you delve into, there are so many details.

Just like most applied technologies, you can go as deep as you want. But unlike pure science, the knowledge depth of most applied technology is often limited by economics and application.

[Mike90250]

I think all frames are anodized, and that's why only specific sets of hardware are rated to pierce the hard anodized finish. And from my experience with powder coating, it's only good for a couple years before the UV cooks the plastic and it crazes and starts peeling off. Sheltered, it lasts a long time, on a PV frame or rack I'd guess 5 years tops.

[nwdiver]

Black panels seem to be popular and sell for a premium.

Esthetics aside, my gut tells me that black panels will absorb more sunlight and run warmer than silver panels, so could be less desirable. But if they are more effective at changing light into electricity, then they could be more desirable.

Have you seen data either way?

Thank you.

I tried to find 320w 60 cell panels with silver frames. Looks like Residential panels only come in black now.

[PVAndy]

I tried to find 320w 60 cell panels with silver frames. Looks like Residential panels only come in black now.

I think some are confusing black frames with black back sheets. I don.t think the black verses silver frame matters much, the white back sheet does.

Andy

[J.P.M.]

I think some are confusing black frames with black back sheets. I don.t think the black verses silver frame matters much, the white back sheet does.

Andy

I didn't get that impression. I thought folks were referring to monolithic looking "all black" panels with black frames and no lines visible lines on the front face behind the glazing.

As for the color of the back sheet, since probably most applications folks here may be talking about are residential rooftop applications with backsheets mostly hidden from view as well as having close to zero irradiance on them, why and how do you think a white back sheet matters that much with respect to either aesthetics or cell temp./panel efficiency ?

[nwdiver]

why and how do you think a white back sheet matters that much with respect to either aesthetics or cell temp./panel efficiency ?

The white vs black back sheet definitely has an effect but it's small. I got a tour of the Mission Solar factory with their CTO. He mentioned that the 315 and 320w panels had identical cells. The only difference was the 315s used a black back sheet while the 320s were white. The cooler temperature from the increased albedo (and maybe a little more light scatter IIIRC) gave it an extra 5w. So < 2% difference. Pretty negligible and IMHO the improved aesthetics is worth such a small price.... probably why almost all residential panels are black/black now.

[littleharbor]

On monocrystalline panels the white backsheet gives you the typical white diamonds at the corner junction of all cells. while I don't mind the look I do think the black backsheet, with its uniform look on the face of the panels looks better.

[J.P.M.]

The white vs black back sheet definitely has an effect but it's small. I got a tour of the Mission Solar factory with their CTO. He mentioned that the 315 and 320w panels had identical cells. The only difference was the 315s used a black back sheet while the 320s were white. The cooler temperature from the increased albedo (and maybe a little more light scatter IIIRC) gave it an extra 5w. So &amp;amp;lt; 2% difference. Pretty negligible and IMHO the improved aesthetics is worth such a small price.... probably why almost all residential panels are black/black now.

The albedo an object or a surface receives is not a function of the color of that surface as your statement seems to infer.
In the same location and physical environment, a white surface sees just as much irradiation due to albedo from the surroundings as does a black surface. The albedo is f(surroundings), not the properties of the receiving object.

For reasons explained below, the backside of a nearly flush or small roof to panel gap roof mounted array will see little, if any, albedo. In any case, albedo (reflected irradiance) coming off an object will not reduce its temperature. Depending mostly on the properties of the surface of the reflecting object, that albedo may have the effect of making the object's temp. lower than if it was a blackbody (or as may seem counterintuitive higher, depending on the surface characteristics such as any wavelength dependent emittance that makes % absorbance > % emittance), but that's not the same as a temp. reduction, and that's not semantics or separating fly crap from pepper. So, to say "The cooler temperature from increased albedo ..." makes no sense. Emittance and albedo are not the same.

Also, for solar arrays sitting parallel to a roof with very small (roof to panel clearance/array length (or) width) dimensions, the relatively small contribution of albedo on the backside of an array to total solar production even in ideal situations is so small for common roof top applications that it's likely to be unmeasurable and can be ignored. Same thing goes for array temps.

Consult sources on thermal radiation heat transfer, particularly the "Handbook of Heat Transfer" by Rohsenow and Hartnett and you'll see what I'm talking about. Other authors: Siegal &amp;amp;amp; Howell, Sparrow &amp;amp;amp; Cess, Wiebelt, and others. See in particular the areas dealing with something called "view factors".

Bottom line: For most rooftop applications, or those applications with relatively small clearances between the back of panel and a parallel surface such as a roof, there will be little, if any measurable solar radiation in the form of albedo that gets to the backside of a panel or an array. To the degree that is a statement that describes an application, it's a true statement.

There will be however, a small temp. increase in panel temp. from the darker diamonds on the frontside that's the result of a black backsheet as little harbor notes.

Further, and for information if you're interested, any albedo that does sneak behind the array (through the small panel to parallel (roof) surface clearance), and then somehow manages to hit the backside of a panel in such a configuration will do so at an angle of incidence pretty close to 90 degrees making the average projection of that diffuse radiation on the backside of a panel closer to zero yet.
Further to that, just like frontside glazing reflectance increases as incidence angle increases, the reflectance of wavelengths we're talking about here by backsheet material increases as the angle of incidence increases. I don't know the reflectance of albedo as f(cos (incidence angle), wavelength) for the EVA material of most backsheets, but I'd guess that regardless of what it is, it'll be a lot closer to 1.0 than 0 for the high incidence angles we're talking about here.

All that comes down to very little of what's already a small amount of reflected irradiance that's had the additional and compounded reductions of small view factor and high incidence angle to sap what's left of the albedo on the backside of an array of any color will be so small as to be immeasuable.
.
Anecdotally, when I got my array, I also got a Davis weather station with a pyranometer that I can detach. When I designed the array, for several reasons, I made the roof to panel clearance high enough that I can get under the array - somewhere between ~ 30 and 35 or so cm. clearance/tile height/profile, depending on where the measurement is taken. I put the pyranometer in an orientation such that the sensor faced "down" on he underside of my 4 X 4 array at the NW corner of the NW "interior" panel. Under bright sun at several times of the day on a day near the spring equinox, I got readings of zero or 1 W/m^2. that seemed to confirm, or at last not contradict what I'd learned about radiation heat transfer.

On what you were told by the Mission Solar folks, that seems to be different than what they put on their datasheets, which in/of themselves seem a bit confusing, with a lot of the small differences in STC ratings due to small changes in rated currents and rated voltages as is tpical of most PV mfgs. I found, for example data sheets for 310 W and 320 W with both monolithic black and checkerboarded frontsides. Interestingly, to me anyway, both the 315 W and 320 W monolithic black panels had a published NOCT that was slightly lower than the 315 W and 320 W non all black panels.

On the backsheet color, for starters, look at how the STC output of a panel is measured (flash testing). The color of its back sheet is or ought to be, in any logical sense, irrelevant, and so have no bearing on a panels' STC rating. If that's a true statement, saying the increase in STC output of a panel is due to the color of its backsheet doesn't make sense.

Overall, I'm having trouble understanding how the color of a backsurface that sees VERY close to zero irradiance can have an effect on cell temp. when that surface sees very little to effectively no irradiance, or in most any case, calc'ed or measured, << 1% of the frontface irradiance, especially when 1.) all of that backside irradiance is diffuse, 2.) all of that small amount of incident backside irradiance has an incident angle that is, due to the geometry of most rooftop arrays, pretty close to 90 degrees.

[bob-n]

Many years ago, I measured light transmission through silicon wafers. It was quite substantial at many wavelengths, in many cases, 90%. PV cells are roughly 20% efficient. Some of the 80% lost must be going right through and either reflecting back or being absorbed. If it is being absorbed by the back sheet, then it is extra heat. The extra heat may not show up in STC measures. But if it is reflected back, it may have an additional opportunity to create electrical energy (standing waves, etc.) and then it would show up in STC.