Radiation: Processes and Properties Environmental Radiation

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Radiation Processes and Properties-
Environmental Radiation -

• Chapter 12
• Section 12.8

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Solar Radiation
Solar Radiation

• The sun is a nearly spherical source of
  radiation whose outer diameter is
• 1.39 x 109 m and whose emissive power
  approximates that of a blackbody at 5800K.

• The distance from the center of the sun to the
  center of the earth varies with time
• of year from a minimum of 1.471 x 1011 m to a
  maximum of 1.521 x 1011 m, with
• an annual average of 1.496 x 1011 m.

• Due to the large sun-to-earth distance, the
  suns rays
• are nearly parallel at the outer edge of the
  earths
• atmosphere, and the corresponding radiation
  flux is

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Solar Radiation (cont).

• Interaction of solar radiation with earths
  atmosphere

• Absorption by aerosols over the entire spectrum.

• Scattering by gas molecules and aerosols.

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Solar Radiation (cont).

• Effect of Atmosphere on Spectral Distribution
  of Solar Radiation

• Attenuation over the entire spectrum but more
  pronounced in spectral bands
• associated with polar molecules.

• Why is the assumption of graybody behavior
  often inappropriate for
• surfaces experiencing solar irradiation?

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Solar Radiation (cont).

• Effect of Atmosphere on Directional
  Distribution of Solar Radiation

• Rayleigh scattering is approximately uniform in
  all directions (isotropic
• scattering), while Mie scattering is
  primarily in the direction of the suns
• rays (forward peaked).

• Directional distribution of radiation
• at the earths surface has two components.

• Diffuse radiation Scattered radiation
• strongly peaked in the forward direction.

• Calculation of solar irradiation for a
• horizontal surface often presumes the
• scattered component to be isotropic.

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Terrestrial Radiation
Terrestrial Radiation

• Emission by Earths Surface

• Emissivities are typically large. For example,
  from Table A.11

• Atmospheric Emission

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Terrestrial Radiation (cont).

• Although far from exhibiting the spectral
  characteristics of blackbody emission,
• earth irradiation due to atmospheric emission
  is often approximated by a
• blackbody emissive power of the form

• Can water in the natural environment freeze if
  the ambient air temperature
• exceeds 273K?

If so, what
environmental conditions (wind and sky) favor ice
formation?
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Surface Properties
Surface Radiative Properties
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Surface Properties (cont).
Rejection
Collection
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Problem Heat Load on Food Delivery Truck
Problem 12.119 Determination of preferred roof
coating (Parsons Black, Acrylic White, or Zinc
Oxide White) and corresponding heat load for
prescribed operating conditions.
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Problem Heat Load on Food Delivery Truck (cont)
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Problem Heat Load on Food Delivery Truck (cont)
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Problem Heat Load on Food Delivery Truck (cont)