Surface Area and Thermal Transfer
Name: Paul I.
Date: November 2003
How does surface area affect solar heat absorption and cooling rates?
In general heat exchange is proportional to surface area for smooth surfaces. For convoluted,
porous, or rough surfaces the effective surface area may be quite different (larger) than the
'geometric' surface area however.
The solar heat absorbed by a flat surface depends on the area of the surface, the cosine of the
angle between the direction of the solar rays, and the absorptivity of the surface. Doubling
the area doubles the rate of heat absorption. Rays normal to the surface produce the most heat
(cos (0 degrees) = 1. Rays parallel to the surface produce no heat absorption (cos (90 degrees)
= 0. Black surfaces absorb the most and white surfaces the least. A perfect mirror reflects
all the light and none is absorbed.
Cooling follows the same laws except for the angular dependence. Heat is radiated in all
directions. Notice that a painted wall illuminated by a diffuse light source looks equally
bright no matter what angle you observe it at. Black areas emit the most and white the least.
Again, the amount of heat radiated depends linearly on the area (doubling the area doubles the
rate of heat radiation).
Best, Dick Plano...
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Update: June 2012