Considering that solar panels are close to perfectly black, they will also close to perfectly absorb the remaining 80%. The 20% of solar electricity that is then used for AC, will be released as heat as well.
You've just made a ~100% efficient heat capturing machine. The 98.1% reflecting paint will be much more efficient, with the added bonus of not being dependent on the grid.
Are PV cells really that black for photon energies below their bandgap?
It would be nice if the cells were highly reflective at a range of wavelengths between the range they can actually use and the range where they radiate waste heat.
Yes they are very black in the visible spectrum. Like the parent said a highly reflecting white paint for rooftops would certainly be the smarter choice.
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.
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.
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. (Actually it's a bit more complex, because the atmosphere emits IR in some bands.)
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.
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.
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.
My point was that a carbon black shingle would result in a much higher amount of work required to cool the inside of the house because you'd be baking yourself alive. The standoffs for solar prevent that. Which at 20% efficiency panels, back of the envelope says that could end up cancelling out the benefit of the panels entirely, for more than half the year.
I wondered this a long time ago too (if solar panels cause more heating by absorbing more), but actually they still reflect a fair portion of the suns energy. [This article](https://www.treehugger.com/ask-pablo-do-solar-panels-contrib...) says earths average reflection coefficient is ~0.35, and that cities are more in the range of ~0.1. Solar panels are ~0.3, so much better than many existing roof materials and of cities in general.
You've just made a ~100% efficient heat capturing machine. The 98.1% reflecting paint will be much more efficient, with the added bonus of not being dependent on the grid.