Selective surface for radiation

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In certain applications, it is desired to minimize solar absorption while maximizing radiative loss from a surface. This is the case in hot climates, where rooftops may be painted white to minimize solar absorption. The effect can be augmented by the use of a selective surface with low spectral absorptivity up to a cutoff wavelength followed by a high absorptivity (and thus emissivity) at the longer wavelengths where most surface emission occurs. Such a surface will radiate strongly to the night sky to provide overnight cooling, while minimizing solar gain during the day. Coatings with these characteristics are particularly useful to minimize boil-off from large storage tanks for cryogenics or hydrocarbons. Many materials that appear to be white to the eye are actually good selective surfaces. Common bright white paint uses titanium dioxide as the pigment, and has a solar absorptivity of about 0.12, yet its room temperature emissivity is above 0.92.

References

Faghri, A., Zhang, Y., and Howell, J. R., 2010, Advanced Heat and Mass Transfer, Global Digital Press, Columbia, MO.

Further Reading

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