Hi Folks,
Quote:
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Originally Posted by gbeal
Black, and don't leave it outside during the day!!
Not sure of the science, but it is my understanding black dissipates the heat better, and allows better cooling.
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On the rare occasion, I may leave it out during the day for a short period of time...
Anyway, how cool something remains depends on the absorbtivity, the emissivity and the reflectivity of an object, along with the incident radiation.
Light falling on an object is absorbed (or transmitted) and reflected. A black surface such as carbon fibre/epoxy would absorb about 90% of the incident light and reflect about 10% while a white (titanium dioxide/epoxy) surface would absorb about 5% and reflect about 95%.
The absorbed light is degraded to heat and then re-radiated at a lower wavelength - in the IR. Some of the heat is also lost by convection - the air passing by carrying of the heat.
The amount of re-radiated energy is dependent upon the emissivity of the surface - the higher the emissivity, the more heat is reradiated and the cooling the object will be.
A black surface in the sun (~1000 W/m-2) will absorb about 900 W/m-2 and need to get rid of this by re-radiation and convection. The equilibrium point -where the heat gain equal the heat loss - will be 50 - 100 C or even more.
A white surface in the sun will only absorb about 50 W/m-2 and so only need to reradiate or lose this by convection.
In general, a ‘black’ surface will reradiate better than a ‘white’ surface. However a black surface will also absorb much more than a white surface. All things being equal, a black surface will become much hotter than a white surface. However all things and not equal and a titanium dioxide surface actually has an extremely high emissivity in the IR bands and so will stay very cool. About the only thing that beats it is a Zinc Sulphide surface.