For an arbitrary body emitting and absorbing thermal radiation in thermodynamic equilibrium, the emissivity is equal to the absorptivity. Is it in ideal case?
the equality of emissivity and absorptivity often does not hold when the material of the body is not in thermodynamic equilibrium.
This law is a function of frequency such that at a particular frequency Kirkhoff's Law is true. In the example cited by Evangelos above, in the frequency range that the emmisivity is 0.9 the absorbtivity is also 0.9 (visible and near IR). In long wave radiation the absorbtivity and emmisivity are equal and very small. This allows the surface to collect sunlight while inhibiting re-emmision which significantly improves it's collection efficiency.
If you simulate in steady state conditions, may you can take these values eqaula to each other.
However, there is a special example. The selective solar collector. The absorber in thsi case has absorbance of 0.9 and the emmitance is close to 0.1 due to the special TINOX material
This law is a function of frequency such that at a particular frequency Kirkhoff's Law is true. In the example cited by Evangelos above, in the frequency range that the emmisivity is 0.9 the absorbtivity is also 0.9 (visible and near IR). In long wave radiation the absorbtivity and emmisivity are equal and very small. This allows the surface to collect sunlight while inhibiting re-emmision which significantly improves it's collection efficiency.
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