Am I missing something? Silly question?

[David Essam]

After much reading on this (exellent) board, I have either learned or had reinforced the following things which I was aware of before:

Solar panels lose efficiency when hot
Panels are usually mounted "stood off" from roofs to promote circulation under them for cooling

Why then is it that most commercial panels I've seen have the underside finished in silver or semi-gloss white? Surely finishing the underside in matt black would promote heat dissipation into the air gap under them by improved radiation and thereby cause them to run fractionally cooler?
I realise any gain would be marginal, but even a very small gain would add up over the 20 year plus life iof a panel in terms of energy harvested.

Without expensive test equipment it would be hard to determine accurately how much difference this small mod would make, if measurable, but a "quick and dirty" comparison could be made by mounting a pair of panels in a warm sunny location, and measuring the output of both individually, then refinishing the reverse of one of them in matt black, replacing it beside the second, and seeing if the relative output chanced with respect to the unmodified panel.

This would confirm if there was a useful gain to be had while eliminating variations in the conditions between the two tests, and also eliminating variations between panels which could mask an effect if two random panels were selected without as it were "calibrating" the test.

Or is there some other reason the undersides of PV panels aren't black that I'm unaware of? Is it a carry-over from solar water heating perhaps, where you obviously would want to eliminate as much heat loss through the panel as possible?

[Rob Beckers]

Hi David,

There are in fact panels with black undersides: I've installed "black-on-black" panels, that have black frames, and black tedlar for the backing (so the spaces between cells when viewed from the front look black as well). When dark mono-silicon cells are used those panels look deep-black when viewed from the front. People like the aesthetics of them, that is why they are installed.

The downsides of these panels are that they cost more, and having the black tedlar on the back means no light is reflected off the spaces between cells on the front. That reflected light would for the most part bounce of the front glass and onto the cells (the glass has an anti-reflection coating to keep as much light in as possible). That gains about 2 Watt per panel. So having a black backing means a loss of 2 Watt vs. those with a white tedlar backing.

From this it would seem that any gain from extra cooling due to a black backing is dwarfed by the additional losses because less light is available for conversion to electricity..

-RoB-

[David Essam]

Interesting, and manufacturers will always charge extra for "looking pretty", however, I wasn't really thinking of altering the construction, merely applying a few microns of black paint to the reverse of the entire assembly or using a stiffening board that was black that side during construction, to improve radiation into the gap betwixt panel and roof. No need to change what's directly behind the cells, merely the colour of the external surface on the "bread" on the underside of what is in effect a multi-layer sandwich. This would be cheap to do, if there's a gain to be had, with no downside re trapped heat that I can see inside the assembly.

You could probably do even better with a finned "heatsink" arrangement on the reverse, but that would add enormously to the cost, very doubtful it would pay back, simple black paint is cheap though, wouldn't even need to be all that good quality paint either, since it lives in a sheltered environment. Looked like a simple gain to be had for little effort or cost, for any panels constructed with a white/silver reverse side, as many I've seen are.

I'm unlikely to be in a position to test my theory, since the only solar project in the near to mid term is likely to be a panel on the garden shed roof to run some lighting in there off some worn-out (but still good enough for that purpose) wheelchair batteries, since that's only going to require one PV panel, I shan't be able to do the side by side comparison I mentioned to confirm if there's a measurable gain or otherwise.

Whole project should cost less done that way than just the armoured cable to run down the garden, even with an "overkill" panel and small charge controller, not to mention no need to dig a trench... Might paint the panel back black anyway, when that happens, since I can't see any way it could hurt, unless you know one?

[Rob Beckers]

David,

I would think that will all the black panels out there it would be clear by now if there's any significant gain from making the back black. To the best of my knowledge that hasn't panned out.

The backside of all panels is made of tedlar. I see that's a brand-name from Dupont, it's actually polyvinylfluoride, something not all that different from PVC. The difference is that it does not burn well (not unimportant on a roof) and it does not age like PVC (no cracking and hardening over time).

Adding paint to the mix means you're engaging in a chemistry experiment. With panel life times measured in decades for a good panel the question is what the paint will do to that...

-RoB-

[David Essam]

An experiment was exactly what I was proposing, nice to know what the backing material is, that tells me something, namely any paint suitable for a PVC substrate should be OK. If I'm remembering my high school chemistry correctly, then in any family of halide compounds, the fluoride is more stable than the chloride, is more stable than the bromide, is more stable than the iodide, due to the strength of the chemical bonds reducing as the atomic weight rises, anything that won't degrade PVC won't degrade PVF either, though the reverse might not be true.

... having the black tedlar on the back means no light is reflected off the spaces between cells on the front. That reflected light would for the most part bounce of the front glass and onto the cells (the glass has an anti-reflection coating to keep as much light in as possible).

Worse, with solid black Tedlar the non reflected energy between cells converts directly to heat, aggravating the temperature problem, and cancelling the effect for a roughly equivalent or greater area of the black rear radiating better, since it now has more heat to dissipate.

This effect would disappear with an applied coating rather than uniform black Tedlar would it not? Experience and knowing your panels will tell you the overall net effect of black Tedlar outweighs the improved cooling, but doesn't separate the effect at the front of the panel from that at the rear on a warm sunny day, just that the combined effect is overall negative by you stated 2 Watts.

Ok so if the panel loses 3 watts because of the front face effect and gains 1 watt back from the rear cooling improving this would describe the effect, but so would a 10 watt loss due to the front effect and an 8 watt gain back from the rear, only experimentation or a much better understanding of the materials involved will tell me if the effect at the rear is 1 watt or 8 in the invented example. Without being privvy to the manufacturer's development data, assuming they even tested this effect, I don't actually know how much effect each surface has, only that the effect at the sunny side is 2 watts more than the one at the shady side if solid black Tedlar is used, since you've told me the overall effect for the case where the Tedlar is black both sides.

Not having actual data offends my engineering sensibilities. Having something white in shade where natural cooling of that thing is a priority also offends them.

Hence proposed experimentation.

[Rob Beckers]

David, let us know what you find out! :cool:

My engineering nose tells me there's nothing to gain here, but great inventions have come from ignoring those naysayers (great failures too, but let's assume the glass is half full)...

-RoB-