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February 6

Should backs of solar panels sometimes be black?

I noticed that the backs of solar panels tend to be silver or white but I was thinking, in the UK, where the climate is cold most of the time, it would be useful if solar panels were radiating as much heat as possible into the building they are attached to. Is a black coating better for this? Are passive daytime radiative cooling coatings relevant given that reflection isn't needed? It would be useful to passively cool the panels and heat the building. Doing that actively is not viable in the UK. 92.7.46.126 (talk) 00:32, 6 February 2024 (UTC)[reply]

Says here that the optimal temp for solar panels is 25°C, and hotter is bad. Abductive (reasoning) 05:20, 6 February 2024 (UTC)[reply]

Heat can only passively flow hot to cold, so you can only radiate net heat from the solar panels into the building if the solar panels are hotter than the interior of the building. That won't happen when you want to heat the building. Cooling the panels is useful, but radiative cooling isn't very important. Most cooling happens by conduction into the air, which then takes the heat away with convection. Liquid (evaporative or run-off) cooling can be effective too. Solar panels tend to perform best in unstable spring weather here in Europe: not too hot, bright sunshine, alternating with short showers to cool the panels. I don't think a black back coating will have any significant effect.

There are solar panels that are both PV and heat collectors. As most mixed devices, they're not the best at either of them: they have to be designed to run hotter than ordinary PV panels. PiusImpavidus (talk) 11:58, 6 February 2024 (UTC)[reply]

"idiopathic?" kidney damage and date rape drugs

What is known about the number of criminal convictions for using date rape drugs that have caused kidney damage to the victims?Rich (talk) 02:28, 6 February 2024 (UTC)[reply]

If kidney damage is known to have been caused by a drug, it is (by definition) not idiopathic. While the use of MDMA can lead to acute kidney failure (PMC 4220747), I suspect that the incidence of such convictions (where I assume the defendant stands accused of administering the drug surreptitiously to a victim) is too low to warrant keeping statistics. --Lambiam 11:35, 6 February 2024 (UTC)[reply]

I wonder whether you're referring to ketamine in particular? It doesn't cause kidney injury injury, but rather cystitis (bladder injury) and it has been used as a date-rape drug. Klbrain (talk) 20:58, 20 February 2024 (UTC)[reply]

Nitrogen microwave heating

Nitrogen (N2) molecules are non-polar and have no molecular dipole moment. Microwave radiation induces rotational motion in molecules with a dipole moment, thereby increasing their kinetic energy and temperature. It seems that nitrogen would not be susceptible to microwave heating, yet nitrogen is heated with microwaves in many practical applications: [1], [2], [3]. How is this possible? 2600:1700:6D30:49A0:89F5:4D2B:8BCD:3F6C (talk) 11:20, 6 February 2024 (UTC)[reply]

All molecules (and atoms) have a non-zero "instantaneous" dipole moment. Although the electron cloud is symmetrically distributed, it undergoes spontaneous quantum fluctuations that break the perfect symmetry. See also London dispersion force. --Lambiam 11:48, 6 February 2024 (UTC)[reply]

Thanks for your response. So are molecules with no permanent dipole moment heatable with microwaves in an efficient fashion in general? Or is nitrogen specifically susceptible? 2600:1700:6D30:49A0:89F5:4D2B:8BCD:3F6C (talk) 15:06, 6 February 2024 (UTC)[reply]

I don't think "efficient" (measured in power consumption) is the right term, but it is an effective method that is very easy to control. --Lambiam 20:19, 6 February 2024 (UTC)[reply]