How hot can sunshine heat up an object?

[Chronos]

If you try to heat up an object at the earth’s surface, you’ll get lower than the 342K they list because of absorption of radiation by the atmosphere.

Solar hot water heaters will heat water up to about 60-85C (140-185F), which is enough to scald you.

Double-check your work for internal consistency: You just said that you’ll get lower than 342K, then stated that 358K was possible.

[CalMeacham]

@Chronos

Double-check your work for internal consistency: You just said that you’ll get lower than 342K, then stated that 358K was possible.

They’re only rough numbers – I’m not bothered by a 10K difference

[Chronos]

I’m not bothered by a 16K difference either, but I am bothered by the sign of that difference. Or rather, I would be, if the explanation hadn’t already been posted. The Stephann-Boltzman law applies only to blackbodies, or as a limit to bodies of uniform albedo. It does not apply to bodies of spectrally nonuniform albedo. Make an object that’s “white” in the infrared but “black” in visible light, and it’ll be heated more than S-B says is possible.

[Nefario]

@Velocity

Say you leave a black, non-reflective, light-absorbent metal plate out in the hot sunshine of, say, Death Valley. No lenses or focusing equipment; just plain unfiltered sunshine.

Hot enough to be dangerous but I don’t know the temperature.

In college about 1980 I was around some Ag Engineers that were trying solar collectors for grain drying. These were made of styrofoam with a metal absorber and a plain glass cover and about 15’ long. Air was sucked through them and was hot enough to burn coming out; I know tissue is killed at 140 F. So the metal was at least 140 but not hot enough to melt styrofoam…

[Kevbo]

Kitplanes magazine did some tests. It is desirable to keep composite airplanes as cool as possible when setting on the ramp. But plain white airplanes are boring, so how much difference does color make?

Polished aluminum got the hottest. Significantly hotter than flat black. The black absorbs more heat, but also radiates it well, so the heat doesn’t build up. Titanium Oxide based white paint stays the coolest. It radiates well in the infrared, but absorbs poorly in the visible.

[scr4]

@Francis_Vaughan

Where you can gain is in noting that sunlight doesn’t have a uniform energy/wavelength distribution. If you have a material that adsorbs energy at those wavelengths that are strong in sunlight, but has low emissivity in the longer wavelengths that you would expect the peak of your object’s emission to be at the temperatures you hope to get to, you can do better than a black body.

Exactly!

This is the basis of various magic proprietary coatings used in solar hot water heaters. A quick Google turned up this site. And we see materials that when used in sunlight have a ratio of up to 11. So make your box out of stainless steel which has had been heated to oxidise the nickel content. It would appear that could get very toasty.

I tried using these values for the nickel oxide (emissivity=0.08, absorptivity=0.92). If there was a piece of it in vacuum, exposed to full sunlight on one side and 300K ambient radiation from the other, it should reach a temperature of 613K (340 C). Though this assumes the emissivity value is still valid at this temperature, which I doubt.

[DrDeth]

A really good solar oven gets to 360 to 400 degrees F.

[Velocity]

@DrDeth

A really good solar oven gets to 360 to 400 degrees F.

Yes, but it is focusing or manipulating sunlight rays. I am referring to unfocused sunlight.

[DrDeth]

@Velocity

Yes, but it is focusing or manipulating sunlight rays. I am referring to unfocused sunlight.

Yes, but I am thinking that it couldnt get hotter than that. That would be the top end.