Marcin at 22:47, 6 May 2019

2019-05-06T22:47:22Z

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			Concept: max T of a solar heated object is determined by radiation only. There are other losses which reduce the max T, but this is the upper limit. Point: the amount of energy radiated solely by blackbody radiation is significant. Below - 1 kW is radiated by a one square meter body at 90C.

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Marcin at 22:42, 6 May 2019

2019-05-06T22:42:23Z

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			Power is sigma T to the fourth.

			https://physics.stackexchange.com/questions/186910/how-hot-can-metal-get-in-sunlight

			While the answer for your particular case depends on a lot of factors, you can estimate how hot an object gets that is facing the sun (and whose back side is insulated) using the Stefan-Boltzmann law. Let us assume that the material is perfectly black (emissivity of 1.0) and facing the sun which delivers approximately 1 kW of power per square meter at the surface of the Earth (a bit more at the top of the atmosphere).

			The black surface will heat up until it loses that kW as fast as it is coming in. It will do so by a combination of radiation and convection.

			We all know that things inside a car can get very hot - this is because the windows reflect more radiation at long wavelengths (the "greenhouse" effect). But for this calculation I will initially ignore this effect although it does play a role.

			Stefan-Boltzmann:
			𝐽=𝜎𝑇4
			To lose 1 kW over 1 m2 requires a temperature of
			𝑇=10005.67⋅10−8‾‾‾‾‾‾‾‾‾‾‾√4≈364𝐾
			This assumes only the surface facing the sun loses heat by radiation: in other words this is only valid for a black surface mounted on a good insulator. It is an upper limit when we ignore convection and greenhouse effect (which are opposite - convection will cause the material to be cooler, and greenhouse effect will make it hotter). It is clearly not enough to melt your lettering.

			Incidentally there are some interesting links on the temperature of objects in the sun. I quite liked this one describing measurements on cars. It shows that things left in the sun can get significantly hotter than the surrounding air (although there are some issues with the method used, the conclusions are mostly valid),

			One final note - how hot things are is not the same as how hot they seem to be. Touching a good conductor (metal) will exaggerate the apparent temperature (hot feels hotter and cold feels colder) when compared to touching thermally insulating materials; and using non contact thermometers can lead to errors when comparing surfaces of different colors (emissivity) - but not enough to explain the reading of 160 F mentioned in the above article.

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	*[[DIY_Solar_Concentrator#Calculations]]		*[[DIY_Solar_Concentrator#Calculations]]

Marcin: Created page with "=Links= *DIY_Solar_Concentrator#Calculations"

2019-05-06T22:41:18Z

Created page with "=Links= *DIY_Solar_Concentrator#Calculations"

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Blackbody Radiation - Revision history

Marcin at 22:47, 6 May 2019

Marcin at 22:42, 6 May 2019

Marcin: Created page with "=Links= *DIY_Solar_Concentrator#Calculations"