I wasn't talking about the visible spectrum, I was talking about near IR below the bandgap (which is just below visible for Si cells.) This light does not contribute to the output of the cells, so reflecting it back into space would reduce heat generation at no loss of efficiency. If anything, by keeping the cells cooler it would slightly boost efficiency.

There were HN threads about this (?)

https://news.ycombinator.com/item?id=27782370 ("Passive radiative cooling below ambient airtemperature under direct sun")

You can do both, typical rooftop solar installations typically don't cover the majority of the roof. Make the rest reflective and/or make it a space for people. Rooftop area has some of the greatest views in any city. Kinda crazy that we don't use it as space for humans and greenery in most cases.

I want to point out that to optimize for temperature reduction, you want to minimize absorption while maximizing thermal radiation. This says you want the surface to be reflective at wavelengths below some cutoff, but perfectly emissive (that is, black) above that wavelength.

Some work on surfaces of this kind have shown promising results, being able to fall below ambient temperature even in full sunlight.

Rooftops are great, we have a 100m2 rooftop terrace and kids love it. But I had no idea how hot it gets. To the point where I don’t even step outside during those heatwaves. When the offical temperature was 36C we had in fullsun above 46C and the concrete panel was too hut to wall barefoot. So my take is that in dry/hot areas rooftops are just unliveable.

Definitely depends on the area but there are flooring options that stay cool, and if really hot weather is a common issue, a misting system can keep things surprisingly cool at the cost of some water use.

They are usually reflective out of band, for unifacial (ito on semiconductor on a metallic backing), so ~40% of the radiation is heading back to space, vs 10-20% for a typical roof.

However... that means they don't radiate well at night (they have low emissivity and are not well thermally coupled to the roof). Painting the roof white is ideal.

Hmm. I wonder if the glass can have a coating that would make them more emissive in the atmospheric transparent bands of the mid- and far-IR.

It is easy to tell that they are black in the visible spectrum because they look black.

I wonder, would that make for some interesting (or potentially problematic) visual side effects in an urban environment? With the variety of roof angles and sun angles, suddenly all manner of observation points (e.g. on roads) would potentially be subject to blinding reflections at different times of day and year.

These concerns already come up with things like glass and metal clad buildings - plenty of urban zoning codes already account for the light that reflects off new or modified structures (or at least try to).

It tends to be newsworthy when it goes wrong, because the results can be things like buildings cooking nearby objects at certain times of day. Spicy stuff.

https://www.upi.com/Odd_News/2013/09/03/Skyscraper-melting-c...

Fair point. In hindsight, my mind had wandered off unsupervised, and was picturing the unadulterated chaos of an instant universal conversion of all existing rooftops to super-reflective.

"All will be simultaneously unveiled at 12 Midday tomorrow, citizens, so, ...take care out there!".

You seem to be assuming the surfaces will reflect like mirrors and not as if they are painted white. There is no reason the reflection would have to be specular.

Roof reflectivity higher than 40% is illegal by zoning code in my city.

White roofs are being pushed in NY City.

(shared link, readable even without a sub)

https://www.nytimes.com/2021/11/10/realestate/cool-roofs-cli...

Folks really ought to be free to make their roof whatever color they want. I personally would choose white if I had sufficient rights to do so.